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Spectroscopic Light Sources

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									  Spectroscopic Light Sources


1. Continuum Sources


2. Line Sources


3. Quasi-continuum Sources
Source Types
Source Characteristics
             Continuum Sources


Emit radiation over a broad spectral range.


Continuum in Wavelength, not necessarily in
time.


Most of these are “black body emitters. The
spectral range depends upon the temperature of
the black body.
                                  Spectral Radiance of a Black Body

                           2          11000
                                         10000
                                             9000
                                                 8000
                           1                            7000
                                                               6000

                                                                      5000
Log B (W cm-2 sr-1 nm-1)




                           0
                                                                              4000



                           -1                                                              3000



                                                                                                        2000
                           -2




                           -3
                                                                                                                                 1000


                           -4
                                2.3     2.4    2.5       2.6      2.7        2.8     2.9          3   3.1      3.2   3.3   3.4      3.5   3.6   3.7
                                                                                   Log Wavelength (nm)
Continuum Sources
Continuum Sources
Tungsten Lamps
1. Absorption Filters

Optical material containing an
absorber that permits transmission
only at certain wavelengths
Reflection Grating:
 Linear Dispersion
         Dl

 Dl = f × Da = dx/dλ

      where:

   f = focal length
(of monochromator)

    x = distance
(along focal plane)
Reciprocal Linear   Spectral Bandpass
Dispersion (RLD)        (s or Δλ)

   RLD = Dl-1          s = RLD × w

     =dλ/dx               where:

    nm/mm           w = slit width (mm)

                         s in nm
    3. Si Photodiode
                                  Si: 3s23p2
                                  Covalent Bonds in Solid

                                  Therefore 1/2–filled
                                  sp3

                                  ΔE ≤ 2.5 eV
                                  (semiconductor)




4 electrons fill a valence band at 0K
At higher T an electron can move to conduction band
Leaving a positive hole behind (both are mobile)
3. Si Photodiode
                   Doping Si with a group 5
                   element (As or Sb) results
                   in extra electrons (n-type).

                   Doping with a group 3
                   element (In, Ga) results
                   in extra holes (p-type)
3. Si Photodiode
    3. Si Photodiode




Forward bias (not very useful for spectroscopy)
3. Si Photodiode

                   Reversed Bias:

                   Depletion zone at the
                   junction.

                   Photons may eject
                   electrons and form
                   holes

                   Current proportional
                   to number of photons
3. Si Photodiode



                   200 – 1000 nm

                   1-10 ns response time

                   0.05 A/W
4. Linear Photodiode Array




        200 – 1000 nm

        1-10 ns response time

        0.05 A/W
4. Linear Photodiode Array

								
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