ErgoSense
Issue 18
Eye Strain
What is the Number One Complaint from Computer Users Today? Is it carpal-tunnel syndrome, a painful and sometimes debilitating problem of wrists and fingers due to improper postures in using a keyboard device? Or is it back problems…persistent lower-back pain that develops from poor seated postures, poor support from chair to few breaks? As important as these injuries are, they ignore the most widespread, yet all too often overlooked health complaint among computer users: eyestrain. According to the National Institute of Occupational Safety and Health (NIOSH), the number one contributor to high levels of fatigue, errors at work, and lost days of production is eyestrain. NIOSH says that of the 66 million people in U.S. who work at a computer 3 or more hours a day, a whopping 88 percent suffer from eyestrain. Unless corrective steps are taken, eyestrain can cause blurred vision, headaches, gritty eye sensation, eye fatigue, neck and shoulder pain, color perception change, decreased visual efficiency, more frequent errors, and reduced overall efficiency. For computer users, that's unfortunate and probably dangerous. For employers, it's counterproductive, and an invitation for legal proceedings. What Causes Computer Eyestrain? Our eyes have what is called a "dark focus." This is the point in space where the focus naturally rests if the eyes are totally relaxed. Imagine being in a pitch-black room. In such an environment, where the eyes can't see anything, the eyes are actually focusing on a fixed point in space, the "dark focus" point. This point, also known as the resting point of accommodation (RPA) is unique to every individual. It generally rests three or more feet away from the face, and will not vary through the course of the day or with age. When looking at an object, the eyes exert effort in moving their focus away from the RPA and towards the object. The difference between the RPA and the focused object is known as the lag of accommodation.
Lag of Accommodation
RPA
Now, imagine tug-of-war that takes place as focus moves from the RPA toward say the computer monitor. The monitor screen is composed of pixels. The pixels are known as gaussian images. That is, they are brightest, and have the clearest definition at their centers, while their edges fade and are less distinct. This kind of image, which makes up everything seen on the computer screen, actually forces the eyes to want to move toward their dark focus point. The fact that the focus is pushed to the RPA isn't a problem in and of itself. Rather, it’s the brain's attempt at pulling the focus back to the plane of the computer screen, the tug-of-war, that is the cause of eyestrain. For a person who uses a computer up to eight hours a day, the eyes will do this re-focusing as much as 25,000 times. Hence, the occurrence of headaches, fatigue, and other symptoms related to computer eyestrain. To complicate things even further, there is the added problem of convergence. Convergence is when the eyes turn inward toward the nose when viewing close objects. Convergence allows the image of the objects to be projected to the same relative place on each retina. Without accurate convergence, we see double images. The closer the objects, the greater the strain on the muscles that converge the eyes. The visual system also has a resting point of vergence (RPV). It is similar to the resting point of accommodation, but it's the point in the distance where the eyes will naturally converge when there is no object to focus on. It's also known as "dark vergence." The PRV averages about 45 inches when looking straight ahead and will decrease to approximately 35 inches with a 30-degree downward gaze angle. Recent
studies indicate that the stress of convergence contributes more to visual discomfort than the stress of accommodation. How is Eyestrain Avoided? In computer workstations, the principal factors affecting the ability to see well are:
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Glare – direct and reflected The luminance (brightness) difference between what is being looked at and its immediate environment The amount of light The distance between the eyes and the screen and document The readability of the screen and document The worker's vision and his or her corrective lenses
Watch out for direct glare. Direct glare involves a light source shining directly into the eyes: ceiling lights, task lights or bright windows. To determine the degree of direct glare, temporarily shield the eyes with a hand. If glare is a problem the eyes should feel immediate relief. Reflected glare, light bouncing off computer screens, can cause eyestrain. However, the more harmful effect of reflected glare can be how it causes a person to adopt awkward postures to avoid the glare. The most overlooked cause of eyestrain in offices is contrast: usually, a dark screen surrounded by a bright background such as a window or a brightly-lit wall. The best solution is to find a way to darken the area around the screen. This problem occurs mainly on screens with light letters on a black background. Ergonomists have long recommended users set their computer screens options to display a light background with dark print. This format will better blend with the workspace background and provide the best on-screen contrast. How much light is right? The right amount of light depends on age, the quality of the print being read, and the task being performed. There should be plenty of light for reading paper documents. However, light levels for computer usage should be more subdued and as indicated above should never shine directly into the computer screen.
What is the correct viewing distance? Eyes are strained more by close viewing than by distant viewing. The "right" distance for computer monitors and documents depends entirely on how clearly they can be read at a given distance. Until recently it was believed that the proper distance between user and monitor should be 18-24 inches. Today most ergonomic professionals feel the best distance is "as far away as possible while still being able to read it clearly." Longer distances relax the eyes. Many cite 20-30 inches or arm length to be closer to optimum. The general rule is to keep viewed material as far away as possible, provided it can be read easily!!! Focusing one the same object for too long will cause the eyes to tire. Eyes need to focus at different distances from time to time. It's a good idea to follow the "20/20 rule" – every twenty minutes, look twenty feet away for twenty seconds. If two objects are only a couple of inches different in their distance from the eyes, the eyes actually do NOT have to refocus to look from one to another. Greater distance differences, however, can overwork the eyes if it is necessary to look from one object to another often – as when typing from printed copy and looking at the screen. In general, it is best to keep viewed objects at about the same distance if shifting focus frequently. What is the proper monitor Height? In the past proper monitor height was believed to be achieved by placing the top of the screen at about at eye height. While this appropriate for some, it is incorrect for still others. Having the top of the monitor level with the height of the eyes is considered the highest a monitor should be. It is not generally the best height. Many people find a lower monitor height to be more comfortable for the eye and neck. Note too, that it is rarely advisable to place the monitor on top of the CPU. This practice generally raises the screen height above the optimal position for most users. Can Computer Work Cause Nearsightedness? Rarely, according to optometrists. It's more likely that computer work makes an individual realize that glasses are needed.
Sometimes eyestrain is just a case of dry eyes. Lowering the monitor can help. Looking downward means more of the eye surface is covered by the eyelid, and two other things happen: the eyes unconsciously blink more, and they produce more lubrication. What About People Who Wear Bifocals? People who need bifocals should consider other possible options to insure improve their interaction with computers. Such options may be:
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"Computer Glasses" that focus at the right distance for the computer screen. Wearing contact lenses – corrected for computer or reading distance in one eye, and for far distance (if needed) in the other eye. Lower the computer screen as much as possible. This will reduce the number of sore necks and shoulders due to frequent tipping of the head back to see the screen through the lower portion of the glasses. If possible, remove the monitor's tilt/swivel base (consult a computer hardware person first) to gain a couple additional inches or at least make certain it is not sitting on top the CPU. If possible, lower the work surface that the monitor sits on.
Some optometrists have special training in computer-related vision. Many make use of the PRIO system. The PRIO system reformulates the eye exam using a device which simulates an actual computer screen. Instead of using the eye charts with printed letters and numbers, which have hard edges and are not gaussian images, the doctor creates a prescription meant specifically to eliminate the lag of accommodation when looking at a computer monitor. The two focusing planes come together, eliminating the tug-of-war, thereby reducing some of the major effects of computer eyestrain. For more information regarding the PRIO System, visit: www.prio.com. Conclusion We all know that computers are machines. Often, we treat ourselves as machines as well. As we hammer away on our maxed-out Pentiums and spend hours at a time typing on keyboards, it would seem natural that some sort of greater harmony should occur between people and computers. Unfortunately, to date we have been more successful adapting our bodies to our mechanical counterparts than the reverse. If “where the eyes go, the body follows” it makes sense to let concerns for eyesight lead us to better all around office health.
References Seymoure, Jim, "Your Eyes Come First", PC Magazine 14 (Oct. 24, 1995): 93-94 PRIO-The Solution to Computer Eyestrain www.prio.com?The _Solution.html Eye Care Methodist Health Care System www.methodisthealth.com/eye/vdts.thm Atencio, Rosemare, Eyestrain: The Number One Complaint of Computer Users / Computers Libraries, Nov. 8, 1996 Moon, Ken, OUCH! The Physical Toll of Computer Use IS/OOP Group's Online Magazine http://www.online-magazine.com/ouch.htm#eyestrain F-One Ergonomics, Ann Arbor, Michigan http://ww.office-ergo.com/12things1.htm American Academy of Ophthalmology, Computer and Eye Strain http://www.eyenet.org/public/faqs/computers_faq.htm. Tahran, Rod, O.D. Rx for VDT Eyestrain: First, Pop the Question, Then, Prescribe the Right Lenses http://www.prio.com?RX_VDT_Eyestrain.html
This is paper was written for The Ergonomics Consortium by Kay ArDilonge. Ms. ArDilonge, a certified OSHA management consultant specializing in office ergonomics, is a market manager for Details Inc., which serves Steelcase and Details dealers throughout North and South Carolina. She is a member of the Human Factors and Ergonomics Society. Kay ArDilonge is also a member of The Ergonomics Consortium.