Optics III by yaoyufang


                                           Optics III
                                     Lecture 13 – Sept. 28th

Just a note: New ANSI standards
        Old|: -0.25 cyl  7 degrees and -0.50 cyl  5 degrees
        New: -0.25 cyl  14 degrees and -0.50 cyl  7 degrees

MyVu – projects picture
     Go to this site for more info: http://www.myvu.com/intro.html

Power of Lenses – Part 2 (cont.)

Review of slides from last time. See lecture 12. Specifically reviewed effects of panto, what
lenses you need to use panto on, how you need to make panto work (slides on page 7), and rule-
of-thumb for placing level PRP.

8/2    First sentence isn’t true b/c PALs are fit with the PRP at the center of the pupil.
       Measuring the level PRP is recommended for high index/high power and aspherics.
       Dropping down the OC compensates for the effect of tilt on off-axis optics. Need split
       PDs for: PALs, high index/high power, and aspherics. Why not high index/low power?
       B/c with low power you don’t get CA has to do with prismatic effects. For the
       Hyperindex 166 Aspheric (made by Optima) you must use split PDs in order to put the
       optical axis through CR horizontally (level PRP puts it there vertically). Who do we do
       split PDs on? PALs, aspherics, and high index/high power lenses. Why do we use split
       PDs in PALs? If you don’t match view with lenses, you narrow the reading view. In
       picture, the angle of inclination is the same as pantoscopic tilt. Why don’t we do panto
       on everybody? The measurements are a pain and you have to train people to be able to
       take the correct measurements.

8-1    most frames  OC at ½ B makes it pretty close to the proper position
       only use level PRPs when absolutely necessary = high power/high index and aspherics.

9-1    What effect does moving the OC up or down from the geometrical center have on vertical
       prism? None if patient is not an anisometrope.

9-2    Cross = pupil center
       Dot = prism dot, if no prism could can it OC or level PRP
       Dot is about 3-5mm below cross. Why is prism dot there instead of center of pupil? To
       give the lens proper optics and better off-axis optics.
       Seg height = bottom of box to center of pupil
       Side note: Varilux Liberty is used to help patients transition from D-segs to PALs (but
       patients don’t really have a problem doing that).

9-3    Panto can alter the apparent seg height of a lens; say a patient is bothered by seeing the
       seg line. By increasing panto you are moving the lenses closer to your face and
       decreasing the apparent seg height (appears lower in your field). This keeps you from
       having to remake the lens (doesn’t work for PALs though). What’s the problem with
       increasing panto? Cosmetic appearance.
       Other ways to drop seg height: push apart the nose pads (glasses drop down but could
       cause problems if fit too low) or decrease vertex distance (difficult to do). You can’t
       really do either of these on plastic glasses (can really only use panto).

10-1   Panto can alter visibility of reflections.

10-2   Reflections are a relatively common cause of patient complaints so the first thing you
       should try is to change the panto and then you might change the face-form. What if you
       tilt the lenses and it doesn’t fix the reflection problem? Decrease reflections by using
       ARCs or light tints. If you get reflections off your computer screen will ARC on your
       glasses help? No, it should be put on the computer screen.

10-3   Lighting reflections from behind especially when reading. Could also happen if you are
       wearing really dark sunglasses b/c of increase in contrast. Get reflection off the back of
       the lens.

11-1   Example of reflections from overhead lighting causing a ghost image. How would you
       fix this? Use an ARC or maybe a light tint. But ARC is more expensive and can’t add
       the new coating so have to remake lenses. You can also get reflections from street lights.

11-2   Why does a light tint work? Cuts down on reflections within the lens. What advantage
       does it have over ARCs? Cheaper and you wouldn’t have to remake the lens. Usually
       you can’t put an ARC on a patient’s lenses after they have the lens because you could
       compromise the impact resistance. Now if that person got hurt you would be liable
       instead of the original manufacturer. So if you add coatings you dec. impact resistance
       which equals bad news for you legally.
                                           Optics III
                                     Lecture 14 – Sept. 29th

Power of Lenses – Part 2 (cont.)

Review of reflections: if patient complains of them change panto first. Overhead lighting causes
problems. Tints will help decrease reflections only when you have multiple reflections inside the
lens. ARC vs. tint  ARCs are more expensive and would have to remake the lens.

11/3   Indian test used to see where reflections are coming from. If patient complaining of
       reflections under fluorescent lights and goes away when they put the hand overhead then
       your problem is coming from lights above and not say, their computer screen. If
       reflections are coming from the computer this test won’t get rid of the reflections but if
       you put a piece of paper over the computer screen and they go away you know your
       problem is the computer screen not the overhead lighting.

12-1   Headlight ―glare‖. Classically an ARC will work great to get rid of this. If older people
       complain about this especially at night, what causes headlight glare? Media opacities in
       the eye so ARC will not solve problem. Sunglasses are not a solution but they do work
       b/c they cut down the brightness of headlights but then you can’t see other stuff (i.e. like
       people running on the road). Real solution? Don’t drive at night. There are night
       driving glasses ―blue blockers‖ (yellow tints) as seen in airline magazines; don’t ever tell
       a patient to use these to help solve the problem—very bad idea.

12-3   What problems occur if a patient needs to add an ARC to their new glasses? You
       have to remake the lens (remember: if you add coatings after the fact you dec. impact
       What problems do patients have with ARCs? They seem to get dirty pretty easily (b/c
       they show dirt more) and you have to treat/handle them very gently.

13-1   Other solutions to reflection problems? Change the level PRPs or the BC. What is the
       problem with doing this? You will have to remake the lenses.

13-2   Pantoscopic Tilt and PALs. There is more panto on a PAL then there is on a regular
       pair of glasses (i.e. single vision). Why? B/c when you  panto you  FOV through near
       and intermediate portions of PAL. Why does this work? Move closer to face give wider
       FOV. Also recommend face-form to avoid swimming sensation/blurry vision.

13-3   Panto can alter the effective power of a lens. This is the reason why we don’t put a lot
       of panto or face-form in glasses b/c when you start tilting the frame it starts to effect your

14-1   Equations to know for boards. We don’t need to know this for his exam to do
       calculations but he does want us to know what they do (what power changes do to your
       vision). Can’t prove from basic geometrical optics but can see it if you tilt your glasses.
       Use these equations for frames/lenses with a lot of power or face-form. Slide 14-2
       explains equations. The new sphere and cyl power that you are calculating is what the
       patient is actually looking through when you tilt the lens. Panto adds axis 180 cyl and
       face-form adds axis 090 cyl.
14-3   If -6.00 lens you will pick up more minus sphere and more minus cyl. If plus Rx you
       would pick up more plus sphere and more plus cyl. What happened to the SE? It went
       more minus. You have to have a fairly high Rx for this to be a problem.

12-1   Myopic patient states that he sees better if he tilts his glasses. What does that mean?
       He’s picking up more minus power. Why do hyperopes not make this statement? B/c
       they pick up more plus but they can still accommodate so it just allows them to relax their

15-2   Refractions are done with no panto, but patient’s lenses have panto. Why is this not
       usually a problem?
              1. most Rx are low power
              2. if optical axis goes through CR the power changes don’t happen; so it doesn’t
       Most cases are not an issue. Problem where you have to deal with this? Aphakes.

15/3   Problems with aphakes? Really high plus Rx. Aphakes are sensitive to vertex distance
       error, panto and face-form. Aphakes glasses have to be positioned just right. You
       should not set a level PRP for these patients. Why not? B/c center thickness is an issue.
       If you increase CT then you increase the magnification.
                                          Optics III
                                   Lecture 15 – October 2nd

Optics Exam II is now on Wednesday and Pharm is on Thursday.
Extra lab exam info sheets w/test times is down in lab.

Power of Lenses – Part 2 (cont.)

16-1   Effects of Face-Form. Usually we prescribe a small amount of positive face-form
       (wraps toward the face). Why? So the frame looks good on a patient. Most people’s
       glasses have a small amt of face-form. Why don’t we prescribe a lot of face-form even
       though it can look cool in some frames? 1. Most frames don’t look good with a lot of
       face-form and 2. lots of face-form changes the prescription (changes the power the
       patient looks through). What does it take to correct for the power effects of face-form?
       How did we correct it for panto? Moved OC up or down with no prism effects. So, for
       face-form you move OC back and forth horizontally but b/c the prism doesn’t cancel out.
       See slides 16-2 through 17-1.

16-2   Figure shows frame with no face-form so the OC separation is equal to the PD and the
       optical axis goes through the center of rotation (CR) of the eye.

16-3   Adding face-form to the frame w/o changing anything else. The OC separation is still
       equal to the PD but the optical axis no longer goes through the center of rotation.

17-1   To get the optical axis to go through the CR you must increase the OC separation.
       However, now if you look through the center of these lenses you will have prism effects.

17-2   What problem do we create if we move the optical centers so that the optical axis
       passes through the CR of each eye? You get prism when looking straight ahead. What
       does this say about the amount of face-form that we can use? You don’t want much
       b/c we can’t do much about the effects. If we compensate for power effects by changing
       OC separation, we get prism effects so we’re stuck. So what can we do instead if want
       lots of face-form? Change the power of the lenses.

17-3   Face-form affects lens power just like panto (eqn on slide 14-1) except that the axis of
       cylinder created is 90 degrees. (panto = axis 180 degrees)

18-1   Face-form and PALs - the one place you can get away with putting a lot of face-form
       regularly and routinely without it being an odd Rx. What are some adaptation problems
       with PALs? FOV (help this with panto) and peripheral blur or swimming (when turn
       their head). Increasing face-form will usually solve the problem of peripheral blur and
       swimming. So, in general PALs have more face-form and panto than normal frames (i.e.
       single vision).

18-2   Highly wrapped Prescriptions (aka Eight base lenses) – have a lot of face-form
       - usually sunglasses (doesn’t have to be)
       - more available these days b/c of free-form surfacing
       - usually only available in a +8.00 BC
       - narrow range of Rxs (-4.00 to +2.00, up to 2.00 cyl)
       Problem? BC doesn’t match power. Remember: +4.00 lens  +8.00 BC, pl lens 
       +6.00 BC, -4.00 lens  +4.00 BC. So, lenses must be made aspheric or atoric to provide
       good off-axis optics

18/3   Why do wraps? (1) look cool – fashion trend (2) protects eye from side – good for
       outdoor activities like fishing, golf, etc. (3) larger FOV – maybe good for sports

19-1   The highly wrapped prescriptions you get from the lab may be different than what you
       ordered. Why does the lab (usually not local lab) alter the Rx? B/c the power changed
       when the lens was wrapped and they are trying to get best optics. There are very few
       situations where you get a different power lens back than what you ordered: (1)
       highly wrapped Rx (2) some PALs b/c of compensating for face-form and panto. Ex. you
       order a PAL with -4.00 lenses but you get back the lenses and the new power is -3.92 -
       0.38 X090. What about PALs and panto with low power Rx, will there be a difference?
       NO. (3) bifocal CL (one type). Position of wear lenses = lenses with a modified Rx
       (usually referring to compensation for panto).

19-2   Who has wraps? Nike, Sola (bought by Zeiss), Bolle, Oakley (started the trend?), Maui
       Jim. Plano sunglasses if you wrap ―the heck‖ out of them will get power changes. Side
       note: Sports eyewear standards  not ANSI instead ASTM.

19-3   Spazio wraps (by Sola) ***know trade name. Patient looks through -4.00 but lensometer
       doesn’t read that (but lab will let you know what it should read). For conventional
       wrapped lens, lensometer would read -4.00 but patient is looking through different Rx.

20-1a What happens if a patient wants his Rx in a highly wrapped frame from a company that
      doesn’t modify the Rx? Solution = for straight ahead problem you can do the calculation
      yourself with a Rx compensator (can’t use eqn from slide 14-1). However, this will not
      solve the off-axis problem caused by the wrong BC. Source: Darryl Meister
      opticampus.com Dr. Stephens will send us the Rx compensator program.

20-1   PALs in highly wrapped Rxes (memorize these): Shamir Attitude and AO b’Active
       (don’t know how long this AO lens will remain on the market).
20-2   example of wrapped lens
20-3   ―tilted guy‖ lenses look a little different b/c of the steep base curve.
21-1   With highly wrapped Rxes, edging the lenses can be difficult (b/c of big thick edges) and
       may sometimes require hand edging.
21-2   Spazio wrap lens edge. Lighter gray is actual lens. Darker gray shows where the
       conventional edge thickness would be.
21-3   Look at the differences in the steeper wrap bevel and the conventional hide-a-bevel.
22-1   The different edging designs possible.

22-2   Another option for highly wrapped Rxes? Inserts (sometimes called shields b/c the
       wrapped front is one piece). Ex: Rudy Project sunglasses. Put patient’s Rx in a lens
       holder behind the highly curved front which eliminates the need for special optics b/c the
       lens itself is not wrapped. Might create other problems though  eyelash clearance,
       fogging, FOV, etc. Another problem: if shield is too highly wrapped, the inserts might
       have to have some face-form too and then you have power change issues again.

23-1   Shows where you would put the patient’s Rx (behind the shield).
                                            Optics III
                                       Lecture 16 – Oct. 5th

Problems on tilt: 78, 80  predict answers/trends don’t actually solve

New Packet: Spectacle Lens Aberrations

2-1    Correction change the plus signs to minus signs
3-1    Transverse (aka lateral but don’t really call it that)

3-2    Dispersion = variation in index w/ wavelength
       index of refraction is not constant
        n
       we usually specify index around the middle (yellow/green area)
       nd = 589 (in US) sodium light  difference between 1.66 and 1.67 nothing really
       nf = blue
       nc = red

3/3    Abbe Number (aka Nu Value)
       - as you increase the index difference you decrease the Abbe #
       - big Abbe # is good
       - low Abbe # is bad

4-1    ***As you inc. index you inc. difference between red and blue. As you inc. the index
       you inc. dispersion. On the right side of the graph you should note that the bottom curve
       should not curve up.

4-2    This is the traditional way of plotting the index and the Abbe #. Good Abbe #s are on the
       left and bad Abbe # on the right (associated with high index). Glass: high index low
       Abbe #  lead crystal, diamonds. Why do diamonds sparkle? n reflected light and
       color aberrations.

4-3    General Rule: n Abbe #. Which lens material has just about the worst Abbe # of all
       plastic lens materials? Polycarb. B/c polycarb has the lowest Abbe# it has the potential
       for a lot of chromatic aberrations. Where do you run into this problem? High index,
       HIGH power.

5-1    LCA = longitudinal chromatic aberrations
       Notice blue wavelength is bent the most followed by yellow then red.

5-2    How is LCA used clinically? Bichrome test (red/green)
       How much LCA does the eye have? 2D
       Why do we not see LCA usually?
        1. peak sensitivity at 555nm***
        2. adaptation
       What is the LCA of a mirror? None. B/c n sin = n’sin’   = ’

5-3    LCA = F/v (nu)  (nu) = Abbe #
       Notice that the eye has about the same as polycarb
6-1    TCA (aka Chromatic Difference in Magnification)
             - off-axis objects
             - get 3 different images all different sizes

6-2    What would happen if you put a screen at the yellow image? Where all wavelengths
       combine & the images overlap you would have white image. You would also have an
       out-of-focus image where the red wavelength is; this is a color fringe.

7-1    TCA = hF/v
           power TCA
          Abbe # (v)  TCA
          h TCA
       What is the numerator (hF)? Prentice’s Rule

7-2    So, for a spectacle lens, what does it take for TCA to be a problem for a patient?
       1. high powered lens
       2. only see when look off-axis
       3. material w/ low Abbe#
       Clinically we see this with high index material. How do you get rid of TCA? Go to a
       lower index of material; however, then you get a cosmetic issue.

7-3    Correcting/eliminating chromatic aberrations
           Requires a doublet
           Not practical for spectacle lenses

8-1    Just a note we tend to (generally) refer to high power lens being >  5.00 D

8-1    Dealing w/TCA.
           Proactive solution  prescribe split PDs and level PRPs to minimize
              monochromatic aberrations
           Also minimize problem: play with panto and face-form.
           If you can minimize effects of monochromatic aberrations, the patient will notice
              TCA less
           The lens performs best when the optical axis goes through the CR of the eye

8-3 to 9-2    Review from other packet - Panto

9/3    Why does this help w/TCA? If you minimize monochromatic off-axis aberrations
       patients will notice color aberrations less.

10-1   RULE-OF-THUMB: every 2 degrees of panto, optical axis should be dropped 1mm.

10-2   REVIEW
       - Most frames have the OC at ½ the B dimension.
       - Only use level PRPs when absolutely necessary: high power, high index and aspherics.
       - Use split PDs for: high power, high index; aspherics; and PALs
                                            Optics III
                                   Lecture 17 & 18 – October 6th

Spectacle Lens Aberrations (cont.)

10/3   Chromostereopsis = BV effect related to the chromatic aberration of the eye.
       Red-warm colors make ---- tones look nicer. Blue – cool colors.
       Patients often notice it using computer and using polycarb. So how do you solve? Put pt
       in lower index material lenses.

11-1   Circle on left is red; circle on right is blue.


                                                         Figure A

          White                                 R

       ***When you get to the edge of the spectrum, shows up w/no overlap so image doesn’t
       look white.

11-2   ***Dispersion – red, green, blue all bend by different amounts. Red is on temporal
       retina= crossed (close). Blue is on nasal retina = uncrossed (far). This creates a retinal

11-3   ***Patients especially those wearing high index lenses may be concerned but you should
       tell them it’s normal.
       BV disparity falling on noncorresponding retinal points.
       How do you prove it is a BV problem? Cover one eye

       Fig. B                                                  OD -6.00 BU     +1.50 Add
                                                               OS -6.00 BD

       When the patient looks down they might say it’s blurry in the left eye.

12-1   Prism in polycarb lenses can VA. Bifocal Rx and vertical prism – where is vision
       worst? If OC on top, BD (if minus lens) See Figure B.
12/3   Aberrations are usually a problem only for off-axis vision. Two variables that can be
       changed to alter lens aberrations: (1) base curve and (2) asphericity. Correcting one
       aberration usually makes another aberration worse; you cannot get rid of all the
       aberrations. Off-axis aberrations get worse as power increases. Just a note: People
       usually turn their head when looking off-axis so you don’t really have to worry about
       aberrations too far in the periphery.

13-1   Spherical Aberration (SA)
           On-axis
           Image no longer a point; refracted rays don’t meet all at one place
           Close to axis = focus further
           Far from axis = focus nearer/closer

13/3   Definition of SA = Rays farther from axis focus at a different pt than more paraxial rays.
       Spherical Aberration amt is proportional to aperture size squared and is also
       proportional to the power of the lens.

14-1   If you change the aperture from 2mm to 4mm the spherical aberration increases by a
       factor of four (b/c doubled aperture size).

14-2   ***Choosing the proper BC can minimize SA but SA is not important for spectacles and
       correction is not done. Why?
               1. pupil limits ray bundle size (not an issue for the eye); limits aperture
               2. makes something else worse

14/3   - SA may be more important for CL b/c lens surface powers are so high; CL have very
       steep surfaces.
       - SA varies from person to person
       - What corrects SA for CL? Aspheric contact lenses
       - Seems to work best when residual astigmatism is present. Less SA means less total blur
       so patient is less likely to notice astigmatic blur. ***Dr. Stephens doesn’t buy this.
       Makes more sense possibly to say that the cornea is aspheric in shape so maybe they just
       fit better; however, this is still not always best.

15-1   Basically coma is tilted spherical aberration.

15-2   Coma
             Off-axis SA
             Image not symmetrical; shaped kind of like a comet (pt with a tail)
             Coma  aperture size2 and Coma  F (lens power)
              off-axis  coma
             Can correct by changing BC but NOT DONE. Why? Same as for SA (1)
              depends on pupil and (2) makes something else worse.

15/3   Aplanatic points – conjugate pts that are free of both SA and coma. **may be on
16-1   Radial Astigmatism (RA)
           Off-axis                                     two foci = sagittal and tangential
            off-axis  radial astigmatism              get affects of astigmatism
           one of the most important                    not dependent on pupil
              monochromatic aberrations

17-2   RA is corrected by choosing the proper base curve. Tscherning ellipse = calculated base
       curve to correct RA for a given power lens.

17-3   Graph that shows the Tscherning ellipse. Basically ignore the top of the ellipse b/c there
       really isn’t a BC that gives good off-axis optics for a really high power plus lens (+10,
       +15). The T. ellipse explains how spectacle lenses are made (which BC goes with what

18-1   Tscherning ellipse
           Note: 2 BC can correct RA for any power. We use the flatter one.
              Exception: Enigma (Sola)
           as power goes (+) to (-), BC gets flatter
           BC for plano lens is about +8.00. What should the BC for pl lens be? +6.00.
              Why is it not +8.00? b/c plano base has lots of reflections.

18-2   Sola made some fit by using the top curve (the steeper BC)

18-3   For high minus powers, the BC approaches plano. Why is plano not a good BC?
       For high + powers (>+8.00; cataract lenses; aphakic) there is NO BC that corrects RA.
       For these powers, the lens front surface must be aspheric to provide good off-axis optics.

19-1   **high + lenses are the ONLY lenses that MUST BE aspheric for optical purposes**
       Lower power aspherics have another purpose = cosmetics.

19-2   The BCs used on moderate power aspheric lenses do not follow the T. ellipse. How do
       they differ? Flatter BC on aspherics.
       “Point focus” lenses = correct only for RA

20-1   Curvature of Field
       - Off-axis and as you  off-axis makes it worse
       - when RA is corrected, left with curvature of fld (aka power error)
       - Petzval surface = curved image surface created when we correct RA
       - Must be corrected
       - The other important monochromatic aberration

20-2   Fix RA: Collapse the sagittal and tangential foci down into one which creates the Petzval
       surface and leaves patient with curvature of field.
        RA acts like astigmatism
        curvature of fld acts like sphere

20-3   Correct one aberration and it makes the other worse so you have to go somewhere in
       between; you must compromise so there is no perfect lens. Reminders: don’t really want
       a plano BC and every power lens needs a different BC.
21-1   Corrected curve lenses = lenses that correct both RA and curvature of field.

21-3   Best possible correction of lens aberrations requires a different BC for each power; this is
       not practical so a stepped BC system is used. Stepped BC system = one base curve is
       used for a range of powers. A stepped BC system cuts down on inventory.
        The more BCs the better the design. Ex. we try to have lots of BC for polycarb.
        Multifocals have fewer BCs, why? Inventory control and keeps cost down.

22-3   The BC needed to correct off-axis aberrations varies with more than just power. It is also
       affected by: vertex distance (really CR distance), working distance (computer vs. reading
       book), lens material (poly vs low index), viewing angle, and panto. BC is really the only
       thing we can play with.

23-1   BC is on the x axis. Dotted lines are blur/curvature of field. Solid lines = RA
       From the graph what is the best BC for a +4.00 lens? Very complex

23/3   Example of order form from a PAL manufacturer. They will design the lens accordingly
       so you can order it for a specific vertex distance, panto, etc. This is very expensive to do.
       Is it worthwhile to do it? Only for patients with high amount of RA.

24-2   ***Far point turns with you

24-3   Far-point sphere (FPS) for a hyperope is the curve to the far right; it is behind the eye

25-1   Far-point sphere (FPS) for a myope is the curve to the far left; it is in front of the eye.

25-2   **lens designer tries to make the off-axis image fall on the FPS by optimizing the BC**
       However, you will still have some off-axis aberrations. Again if you correct RA, you
       make curvature of field worse and vice versa (as shown on slide 25-3)

25-3   power error = curvature of field
       T = tangential foci and S = sagittal foci

26-1   Graph showing that errors vary when changing the BC. Problem with astigmatic
       patients: can’t correct powers in 2 meridians with just one BC so you must use an atoric
       lens for patients with a lot of cyl.

26-2   Distortion
        Off-axis
        Aberration of SHAPE
        Occurs when lateral mag changes as you go off-axis
        Barrel distortion = DECREASING mag to periphery
        Pincushion = INCREASING mag to periphery

26/3   Place a dot on the right top corner of the object and the two distortion images. Notice
       that with the pincushion image the tip/corner is further out (b/c of inc. mag) and with the
       barrel distortion the tip is further in (b/c of dec. mag) compared to the square object.

27-1   Distortion is only a problem for high powers especially high PLUS powers. Can’t
       correct, why? B/c requires steep BC and makes other aberrations worse.
       Orthoscopic lens = a lens with no distortion.
                                           Optics III
                                    Lecture 19 – October 9th

Problems: 53 – 56

New Packet: Aspherics, Atorics, Double Aspherics, and other New Lens Designs.
1-2    Spec Mag = ratio of corrected image size to the uncorrected image size.

2-1    What would change if the lens was moved to the CL plane? h would become 3mm b/c
       the vertex distance would be zero.

2-2    We are going to talk about Spec Mag in percent magnification.

3-1    (+) value = magnification
       (-) value = minification

3/3    round percentage to nearest .1% Why is magnification significant? b/c small
       difference between the eyes can lead to BV problems (i.e. aniseikona). Who might have
       a large difference in mag between the two eyes? Aphakes, pts with trauma to the eye(s)

4-1    General Notes
        Plus lenses = mag (power factor); retinal image (RI) bigger
        Minus lenses = minification (power factor); RI smaller
              Example of the two above on slide 2-2
        Shape factor = always mag (except rare plano or concave F1)
        What happens if flatten BC? Get less magnification
        What happens if decrease thickness? Get less magnification
              Do the above two on aspheric lenses
        What happens if decrease vertex distance?
              (+) lenses = less mag (real high at CL dec. VA)
              (-) lenses = less minification (real high at CL inc. VA though if they have
                 astigmatism VA might not improve)

4-2    Can a plano power lens have magnification? Yes, all mag comes from BC and CT. The
       second part of the eqn (power factor) goes to 1 leaving all the mag to come from the
       shape factor.

4-3    Sphero-cylinders (toric lenses) = two power factors but one shape factor.

5-1    Clinical Applications
        Thinner lenses w/ flatter BC have LESS SHAPE mag
        Flatter (+) power lenses sit closer to the face (dec. vertex distance), so there will be
           LESS POWER mag (less mag of world)
        (+) lenses that sit close to face have LESS mag of eyes; this acts like a simple
           magnifier. See Figure A on next page.
                                       FIGURE A


                              Object                              observer

             When object is inside F, it is a simple magnifier. @ F you have the max mag and @ the lens
             (where principal planes H & H’ are) you have the min mag. So for plus lenses you want to fit
             them as close to the lens as possible that won’t cause problems.
         Flatter minus lens = LESS min

5-2   Clinical Apps (cont.)
       Thinner, flatter (+) lenses = LESS spec mag so world is less magnified.
       When fitting glasses, is it best to fit at a short or a long vertex distance? Short. Why?
          B/c of cosmetics. For a (+) lens you will get less mag and for a (-) lens you’ll get less
       Myopes aren’t bothered by the cosmetics as much as hyperopes.

5-3   Calculations using the entire spec mag formula are not too useful. Why? What are we
      leaving out? Whether or not the patient is an axial or refractive ammetrope. Axial
      ammetropes in the spectacle plane have the same size retinal image no matter if they are
      an emmetrope, hyperope or myope. Can you compare the two eyes with it? NO.
             Ex.     OD +4.00  corrected image size/uncorrected image size
                     OS +1.00
             Is there magnification? Maybe; don’t know if pt is axial or refractive.
             You can’t say that the uncorrected image sizes are the same size in each eye; you
             don’t know the uncorrected image size.
             Who would we worry about having this Rx? Young kids (3, 4 yo) b/c they might
             develop amblyopia.
      So where is the formula useful? Usefulness comes from how changes in Rx parameters
      affect magnification.

6-1   Moderate Power (low power) Aspherics  NOT cataract lenses
        Side note: High powered (+) lenses  always aspheric ex. cataract lenses
       Just termed aspherics
       Spectacle lenses w/rotationally symmetric aspheric front surface with flatter than
        normal BC which provides less magnification.
       Back surface is sphere or sphero-cyl so can be made by labs easily and could be kept
        in stock.

6-2   Cross off the first two names. Profile was replaced by Essilor’s Airwear
      KNOW: ASL and Hyperindex 1.60 and 1.66

7-1   **BC of aspherics are flatter than non-aspheric lenses** but the curve may also flatten or
      steepen toward the edge of the lens. See figure on slide 6-3. Compare ellipse to circle.
                                          Optics III
                                     Lecture 20 – Oct. 12th

Aspherics, Atorics, etc (cont.)

Review: Mainly in the past, aspherics were used as cataract lenses. Really high plus power
lenses don’t have good optics so they are automatically made aspheric. Pretty much today when
people refer to aspherics they are referring to the low/moderate power aspherics not the cataract
lenses (as most people wearing those are dying off). Aspherics are used on both plus and minus
lenses but there is more benefit to them when used on plus lenses.

8-1    Aspheric front surfaces are often used for low power lenses. To get the thinnest lenses
       possible aspherics are being combined with (1) high index materials, (2) thin centers (but
       not too thin b/c worried about impact resistance), and (3) small frames.

8-2    Aspherics are NOT to be confused with high index; they are separate things.

8-3    The logic behind aspherics is basically to make the lens look better—nothing to do with
       the optics of the lens. How do you make them look better? The big effect of an aspheric
       If take a plus lens and flatten the BC, the center thickness decreases. If you take a
       minus lens and flatten the BC, the edge thickness decreases.

9-1    plus lenses:  BC center thickness
       minus lenses: BC edge thickness

9-2    Why do we not just prescribe lenses with flat BC? If you flatten the BC, you screw up
       the off-axis optics. What’s the solution? Make the front surface aspheric which makes
       this AS GOOD AS NOT better than a standard corrected curve non-aspheric lens.
       Corrected curve lens lenses that correct for radial astigmatism and curvature of field.
       TAKE HOME: aspherics have as good as optics not better optics than a non-aspheric

9-3    This advertisement is misleading. To get a picture like this you would have to look
       through the lens backwards.

10-1   Misleading. Notice the distortion in the left figure.

10-2   Supposedly the view off-axis. Fake and doesn’t work this way. Regular lens on left with
       aspheric on right.

10-3   Aspherics DO NOT have better optics than a non-aspheric lens. Atorics and double
       aspherics CAN HAVE BETTER OPTICS than even the best-designed non-aspheric or
       aspheric lenses.

11-1 &
11-2 Benefits of plus power aspherics? Thinner, lighter, less mag of world, less mag or
       patient’s eyes and less bulge. Aspherics WILL NOT eliminate the magnification of the
       patient’s eyes. Instead you will have to fit the frame close to the face (small vertex
       distance). If you want zero mag (or close to it) of the eyes, put them in CL.
11-3   Aspherics have a flatter profile (less ―bulge‖) from above as shown in picture b/c of
       thinning of lens and flatter BC.

12-1   Big effect (though not that impressive) for minus lenses – decreases edge thickness.
       So if you’re going to use an aspheric you would most likely use them more on plus lenses
       than minus. For minus lenses you would probably use a high index material (your first
       option) but if you wanted the best looking lens you would make it high index, aspheric,
       1.0 center thickness (minus lenses only) and put it in a small frame.

12-2   Does the aspheric curve itself actually help to decrease the lens CT or ET? Yes.
       However, most of the effect comes from flattening the BC. CORRECTION to slide 
       change Plus aspheric curve steepens to curve flattens and change minus aspheric curve
       flattens to curve steepens. Basically switch the two. Plus aspherics are pretty easy to ID.

12-3   General rule: the more you flatten the BC the more benefits you get. So as a lens
       designer you want to make it as flat as you can get it. How flat can you make a BC on a
       plus lens? We don’t want to make them plano-convex or biconvex. Usually the standard
       is as close to plano as you can get -1.00

13-2   What is the BC of a +4.00 non-aspheric lens? +8.00 which makes the back -4.00.
       What is the BC of a +4.00 aspheric lens? +5.00. Why +5.00? B/c it makes the back
       surface -1.00.

14-1   The back of a plus aspheric lens is what’s flat. This is how you ID plus aspherics (by the
       back surface). It still takes a little practice to ID them.

14-2   What if you have cyl in the lens? What meridian controls BC? The more plus.

14-3   Problems: eyelash clearance (most common), adaptation (b/c of magnification changes),
       and reflections (flat back surface causes reflections, manufacturers recommend ARCs).

15-2   Best patients are hyperopes, but I would also always go with a high index material.
       Why? The effects are greater.

15/3   Fit aspherics using split PDs and level PRPs. Where is this also recommended to do?
       High index, high powered lenses. You want to fit aspherics close to the face; why? B/c
       of magnification issues. ARCs are also recommended b/c of the flat back surfaces.

16-2   What are the two ways to put prism in the lens? Prentice’s rule but you’re limited in the
       amount of prism you can put in and the lab grinding it in. DO NOT decenter to provide
       prism in aspheric lenses. You want the pole of the lens (see Figure B) generally centered
       on the pupil b/c otherwise the optics of the lens will be messed up. Why emphasis on
       prism? B/c aspherics are usually used for plus Rxes. Worry about with kids,
       accommodative esotropia and amblyopia (BV problems). How does the lab grind prism?
       Tilts the lens when grinding.

16-3& 17-1 Seen before many times.

17-2   What is the meaning of the term ―aspheric PAL‖? flatter BCs making lens thinner &
                                          Optics III
                                     Lecture 21 – Oct. 13th

Aspherics, Atorics, etc. (cont.)

18-1   New Designs – Atorics. Some are calling this lens the ―HD‖ lens (high definition).
       Example of an atoric PAL – Physio 360.
       Why might a patient w/cyl not se as well with his/her glasses as a patient who doesn’t
       have cyl? The optics aren’t as good when you look off-axis. Why? B/c you have two
       different powers but only 1 BC.

18-2   Example 2 – Problem: one meridian needs a BC of +6.00 and the other needs a BC of
       +4.00 since you can’t do this on a normal lens you go in between making the entire lens
       BC +5.00.

19-1   Solution to high cyl problem w/normal lens? Use a different aspheric curve on the back
       surface for each meridian. What will this do for the patient? It will give the lens the best
       off-axis optics in each principal meridian. So the back surface is now atoric (has cyl
       and asphericity).

19-2   Atorics
        Optics CAN BE BETTER than for well designed non-aspherics or aspherics
        Designed for patients with high amount of cylinder
        Expensive
        Cannot be made by local lab b/c it requires free-form surfacing
       Side note: expensive SV lenses: aspherics, high index, ARC…

20-1   Atoric Trade Names: Vizio (Sola) and Hyperindex 1.60 & 1.66 (Optima) ***Know both

20-2   Atoric PALs – front surface is PAL and back surface is atoric. Ex. Physio 360

21-1,2,3       Double Aspherics – ex. Trinity (Augen Optics)
          Lens front surface has two different aspheric curves
          Mostly using Trivex as the material for these lenses
          Though the front surface is nominally spherical there is a tiny amount of cyl on the
          Basically the front surface is atoric which the manufacturers are involved in so the
           local lab can still make these lenses for patients w/o running into the problem of not
           having free-form surfacing
          Designed for patients with high amounts of cyl
          Off-axis optics CAN BE BETTER than non-aspheric or aspheric lenses
          Front surface must align with back surface at principal cyl meridians
          ADV over atorics  can be process by local lab b/c back surface is standard toric

Page 22 - Skipped b/c we will discuss later (this semester, next semester??); not on exam 2

New packet: Lens Materials (plus material name sheet)
***mainly Dr. Stephens is interested in the trends but will mentioned some specifics that we will
need to know (i.e. highest #, lowest #, etc.)
1-2    Why use different index materials? Cosmetics and high power
       What is driving the market? Cosmetics

1-3& 2-1       CR-39 plastic
          Most commonly used material. Why? b/c it’s the cheapest
          Lowest index of ALL materials. Meaning? Makes lenses that will be the thickest
          n = 1.50, Abbe # = 60 Meaning? Good, least color aberrations (CA). n abbe CA
          densities of all plastics are about the same
          has okay impact resistance
          Even though it has a low density it’s a low index materials which means a high power
           lens will need a lot of material making it still heavier so not the way to go for making
           the lens lighter (when comparing the plastics).
          Has several other names it goes by (i.e. allyl resin, allyl carbonate, ADC, etc.)
          Long-chain polymer
          Robert Graham, O.D. (our favorite optometrist) developed CR-39 manufacturing
           process of spectacle lenses
          What drove the market toward CR-39? Weight

2-2,3 Ophthalmic Crown glass
       n = 1.523 (still low), Abbe ~60 (high # = good, low # = bad)
       density about 2 times CR-39.
       Used up through the middle of US
       Amorphous material
       5% of market or less and declining ½ of that is photochromic lenses which
        Transitions might be wiping out of the market
       BIG ADV – scratch resistance
       Probably better optics when surfaced vs. molded plastic lenses that use molds made
        of glass
       Must be TEMPERED to be considered safe to wear.

3-1    All materials other than CR-39 and crown glass are usually considered to be high index.
       Low index is CR-39. Now there is a new term: mid-index. Mid-index (MI) is between
       low index and polycarb ( 1.523 < MI < 1.586). Trivex is an example of MI. High-index
       is poly and above. Glass high-index is almost obsolete b/c as you n abbe density
       (heavy). High-index plastics are close to the high-index of glass and they are a lot lighter
       (compare the density of Thin & Lite 1.74 to SF-6)

4-2   Glass High Index Materials
       How are they made? Add oxides to basic glass material
       Titanium dioxide – standard high index glass
             o What happens to properties of glass? n abbe & density
             o High-Lite = standard titanium dioxide glass trade name
       Occasionally see with low vision patients.

4-2.1 Problem with high index glass. Though the 1.7 glass is lighter than crown glass when the
      lens has a high minus Rx it is still too heavy to wear b/c even though the lens will be
      thinner the lens is denser than crown.
                                          Optics III
                                     Lecture 22 – Oct. 16th

Lens Materials (cont.)

5-1    Why is High-Lite not used much?
         1. too heavy
         2. can get a high-index plastic is same index
       What is the highest index available in US at present? 1.80
       In other countries it is even higher (1.9, 2.0, etc.)

5-2    Flint glass is lead oxide (same stuff in lead crystal). Why use it? Xray protection. Who
       would want this? People in hospital setting like radiologist if they are in the same room
       with patient getting x-rayed. What problems are there with flint glass? (1) very very
       heavy and (2) can’t be made impact resistant. You MUST inform the patient in
       WRITING that these lenses are NOT impact resistant. Probably the only time you will
       prescribe non-impact resistant glasses to a patient.

5-3    Fused bifocals use barium crown, barium flint. Kryptok (round 22) is a flint glass
       segment but has lots of color aberrations. You can get extremely high index lenses in
       other countries but they are NOT impact resistant so you can’t get them in the US. Only
       the US has impact resistance standards.

6-1    All plastics fall into one of three categories: thermosetting plastics, thermoplastics, and
       quasi-thermosetting plastics.
       Thermosetting plastics – CR-39 and all high index EXCEPT poly and Trivex
           o Can’t be reshaped once molded
           o Index 1.60 and up are called polyurethanes
           o Brittle plastics compared to poly (less impact resistant)
           o Side note: impact resistance of glass vs. plastic  about the same but depends on
               what you hit them with

6-2    Thermoplastics – poly, frame plastics, PMMA
       o Softens when heated
       o Poly needs different setup when edging b/c of this. Poly must also be cut dry which
          is different from other lenses b/c they are cut wet.
       o Few cross links = flexible
       o Flexibility is what gives poly its incredible impact resistance.

6-3    Quasi-thermosetting plastics – Trivex
       o Properties between CR-39 and poly
       o Incredibly impact resistant
       o Processes more like CR-39 than poly

7-1    High Index Plastics
       What happens as index increases? Abbe # decreases so you get more color aberrations
       What is the highest index available in the US at present? 1.74
       What materials have the worst Abbe numbers? Poly
7-2    Poly is the most commonly used of all high index materials; first in industrial safety. It is
       more difficult to edge than other materials b/c it tends to melt (refer back to slide 6-2) so
       you must edge part of the cycle dry and you must also polish the edges when finished.

7-3    Poly is very soft so you must have a SRC on both surfaces. Remember that SRC affect
       the ability of the lens to hold tint so poly will not tint. The harder the SRC the harder it is
       to tint the lens. Poly has a poor abbe number so at higher powers patients will notice
       color aberrations.

8/1    ID poly by sound. If you drop on a table it sounds like a poker chip. Some of the older
       poly has a light gray tint to it. Often poly is sold as a more expensive lens but CR-39 +
       SRC + UV is probably more expensive. Why do they have to add SRC and UV? B/c
       poly already has that in/on the lens. Use an ARC with poly b/c poly reflects more light
       than lower index materials; why? Bigger difference in index materials results in higher
       amount of reflections.
               r = [(n’ –n)/(n’+ n)]2

8/2    Poly is extremely impact resistant. You can get down to 1.0 mm center thickness in
       minus powers. However, you DON’T want to order <2.0 mm (it used to be 3.0) CT for
       kids; why? If you hit the lens it might pop out and do other damage. Poly absorbs all
       UV radiation below 380nm (edge of UV). Why do we care? UVB (shorter wavelengths)
       said to cause cataracts. CR-39 and crown DO NOT provide complete UV protection.

       Side note: Selling Poly downstairs – Marketing strategy  bundling
       Cougar Lens – poly with ARC
       Cougar Lens Delux – poly, ARC, transitions

8/3    If the lens absorbs too far past 380 turns the lens yellow (b/c absorbs blue light).
       290-380nm  UV
       UVA = 320 and up (we don’t know long-term effects of UVA)
       UVB = 290 – 320nm

9-1    Mid-index  Trivex to poly
              -adv = don’t have as many color aberrations
              -disadv = expensive
       High-index  poly on up
       Trivex on up all absorb UV
       Crown glass and CR-39 not good UV absorbers.

What to know on extra sheet:
Plastics    -highest and lowest index
            -abbe # poly
            -inc. n you dec. abbe
            -Trivex, CR-39, poly
Glass       -highest and lowest index
            -crown glass
            -High-Lite 1.7
            -inc n you dec abbe and inc density
                                             Optics III
                                        Lecture 23 – Oct 19th
Week of Nov 6th – no lab
Nov 9th no optics
Nov 10th back to back optics

9-2       Other high index materials can usually tint okay but are less impact resistant (except for
          Trivex). Availability is POOR (check before ordering). Difficult to ID by material. For
          example can’t really tell 1.67 from 1.70. Poly is pretty reliable to ID (sounds like
          pokerchip) and a few PALs have a ―n‖ marking on the lens.

9-3       Other High Index
         Most popular other high index? 1.60 and then 1.66
         1.66 and 1.67 are the same material so why the different labels? Looks like it’s a higher
          index. US uses a yellow light standard and the rest of the world uses a green light
         Most can be 1.0mm CT if careful with coatings. Coatings can have a big affect on
          impact resistance.

10-1      Other High Index
           1.60 and up are called polyurethanes. They smell bad when edged. They may be a
             little greater impact resistance than CR-39. Really only poly and Trivex are more
             impact resistant.
           Replacing just one lens can be tricky b/c some materials have a slight tint and it’s
             hard to match them up exactly.
                  o Lenses might look different
                  o Try not to do this
                  o Don’t replace just 1 lens unless you are absolutely sure you know what lens it
                  o Glass and CR-39 are not an issue

10-2,3             Trivex
             Other names: Phoenix and Trilogy
             Extremely impact resistant (like poly)
             Low index
             BETTER (fewer) color aberrations than poly
             Great for 3-piece mountings. What other materials could you use for this? Poly and
             May replace CR-39? Only if it matches the cost. What keeps CR-39 in the market?
              It’s cheap and on a lot of insurance plans.

11/2      Dealing with CA of High Index Materials
           Only a problem for high powers b/c of transverse chromatic aberrations (TCA).
           Solution? Use split PDs and level PRPs. What determines if it is 3mm vs 5mm?
             amount of panto
           Why is this a solution? B/c it gives the lens better optics

12-2      Where else do we use pupillometers? Aspherics and PALs
          Where else do we use level PRPs? Aspherics
12-3     To show that a lens material is thinner the best way to show patients is to use a demo.
         You should also use rimless b/c it shows the edges clearly.

13/2     Why do we prefer poly in our dispensary?
          Impact resistance ** big concern b/c we see a lot of kids
          Cost – not too much more than CR-39
          Can get back from lab faster
          Better avaibility

13/3     Scratch Resistance
          Difficult to tell if lens is coated but most lens today use SRCs. Sometimes you might
            get a little color but most don’t b/c coating is too thick.
          Most SRCs are not extremely scratch resistant. WHY? b/c harder SRC don’t tint
          Most multifocals will have a SRC only on the front, why? b/c lab surfaces on the
            back and the front scratches the most. What lens has SRC on both surfaces? Poly b/c
            the material is soft and easily scratched.

14-1     Fogging
          Glass and plastic fog equally
          Anti-fog coatings available – you can apply them yourself
          Occupations where you need to worry about fogging?
            -grocery store employee
            -someone who goes in and out of cold constantly

14-2     Welder’s spatter – sparks from grinding wheels melt into surface of glass lenses. These
         sparks bounce off plastic lenses.

14-3     Right lens (on left side of pic) is glass lens with welder’s spatter. Left lens is plastic.

15-1,2           Acetone
            Commonly available
            Cleans off paint, PAL markings
            DANGER: dissolves cellulose frames. Can also melt plastic coatings on metal
            DANGER: destroys poly if it gets past SRC
            Be careful when using this chemical and keep away from lens edges.

15/3     Alcohol
          Cleans markings off PALs
          Dr. Stephens doesn’t believe alcohol causes stress cracks; he thinks they come from
             drilling the lens wrong.
          Debate = alcohol with ARC, okay to use or not? Depends on who you ask

16-1     Bleach
          Tint bath neutralizing solution (used to remove tints)
          Hot alcohol solution; powerful solvent
          Removes all PAL markings – if alcohol doesn’t get them all off
          May damage poly; not sure if it’s b/c of tint or ARC

16-2   Order of tints and coatings: ARC always put on last (so on top) b/c it seals the lens. UV
       is in a lot of lenses already. Order: UV first, then SRC, and last ARC.

16-3   COLTS – testing lab in FL; they certify products
        Windex and Formula 49  BAD for ARC
        BEST cleaner  dishwashing detergent (i.e. Dawn) or soap and water
        BEST cleaning cloth  CLEAN ARC cleaning cloth
                                            Optics III
                                       Lecture 24 – Oct 20th

Exam on Wednesday at 10am
Know from lens material sheet: plastic trends (inc n dec abbe), CR-39, Thin & lite 1.74, n of
poly, n of Trivex, Spectralite

New Packet: Impact Resistance

1-2    Dress lenses
           Everyday use
           ANSI Z80.1
           Regulated by FDA
       Industrial lenses
           Occupational and educational protection
           ANSI Z87.1
           Regulated by OSHA (who are pretty paranoid)
        know differences between the two

FDA Requirements
1-3  #1 ALL lenses prescribed MUST be impact resistant
          TEST: drop ball test = 5/8 inch steel ball dropped 50 inches
               o Is this a tough test of impact resistance? NO, it’s the minimal standard.
                   Do we need this standard? It’s a judgment call. In US YES. Market is
                   driven by lawsuits.
          Glass lenses – individually tested
          Plastic lenses – batch tested; ruin lens

2-1    steel ball on left is used for industrial and the one on the right is used for dress lenses.

2/3    Plastic lenses are batch tested. Non-Rx sunglasses and OTC (over the counter) reading
       glasses are batch tested. Sunglasses are a hidden source of liability b/c often worn in
       hazardous situations. Where do people wear them? Golfing, fishing, baseball, active
       sports where there is plenty of opportunity for impact accidents. So, sports lenses should
       ALWAYS be poly.

3-1    #2 FDA requires you keep records (where bought and who made) of all glasses
       dispensed for 3 years.

3-2    FDA has NO MINIMUM thickness requirement. What actually determines thickness?
       Practical impact resistant minimum of 1.0mm and it has to be thick enough for the lab to
       surface. If you waive impact resistance testing or treatment, you MUST notify
       patient in WRITING. Not recommended to do this.

3-3    Why might we waive requirements?
          Cosmetics – FDA says it is not an acceptable reason
          Flint glass – used for x-ray protection; this material can’t be made impact
OSHA Requirements

4-1   Requires that spectacles (occupational and educational) meet ALL of the requirements of
      the ANSI Z87.1 standard. TWO Level: basic impact and high impact.

4-2   Basic Impact
       1 inch steel ball dropped from 50 inches
       3.0 mm minimum thickness
       labeled by manufacturer’s logo
       anything meets this standard if thick enough and tempered
      High Impact
       ¼ inch steel ball at velocity of 150ft/sec
       2.0mm minimum thickness; causes a problem with poly b/c it flexes and could pop
         out of frame
       labeled with manufacturer’s logo and ―+‖ sign

4/3   What materials can meet the high impact requirements? (1) poly and (2) Trivex

5-1   Top two pics are dress eyewear
      Side note: non-Rx glasses are tested straight-ahead and at 30, 60 and 90 degrees

5-2   Photochromics – etched with ―V‖ for variable tint lens. Most common b/c these lenses
      are paid for by the company and it’s kind of a two for one deal. Usually contraindicated
      for forklift driver; why? Takes a while for lenses to lighten when going back indoors.
      Protective tints – identified by their shade number; tints for people doing welding;
      etched on lens

5-3   Protective tints have VERY SPECIFIC UV, visible and IR transmittance requirements.
      Welding arcs give off all wavelengths. Standard ophthalmic tints DO NOT meet the
      requirements. Lenses with protective tints have their shade number marked on the lens.

6-1   How to get the shade number. Crazy formula

6-2   Generally if you inc shade # you inc darkness of lens and dec transmittance

6-3   Frames must meet testing requirements. When tested the frames have lenses in them. Rx
      frames are labeled with Z87.2 and non-Rx are labeled with Z87
      Why Z87.2? Means the frames were tested to the new standard.

7-1   Why are we so concerned about labeling?
      1. tells you that the frames/lenses are impact resistant; that they meet the standards
      2. people (employers) at the factory/institution/school/company need to make sure that
         their employees are wearing the correct eyewear.

End of Exam 2

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