Other Causes for Photometric non-linearity
Appendix C Spectroscopy Background
58
AA003700-01 rev. G. 2.0
A good example is extensive use of the Miracle accessory. It attenuates the beam by
about a factor 3 and thus provides excellent sensitivity with the attenuator removed.
Likewise a diffuse reflectance accessory will usually attenuate the beam
significantly more than a factor 3 and the attenuator should be removed for best
results.
In summary at very high throughput, such as an MB3000 with open beam, there is a
slight tendency to non-linear response of the detector causing some reduction in
photometric linearity.
Other Causes for Photometric non-linearity
Double modulation
Other than detector/electronics non-linearity, Photometric non-linearity can be
caused by “stray light”. In an FTIR there is no stray light when the sample beam is
blocked. However, a small amount of light from one wavelength region can be
found in another wavelength region. This appears as stray light because when the
transmittance at some wavelengths is expected to be zero, there may appear some
residual intensity due to this mixing from other wavelengths.
When the light modulated by the interferometer passes through the sample on its
way to the detector some of the light may be reflected back to the interferometer
where it may be modulated a second time and be reflected back to the sample and
the detector. This second pass modulation actually doubles the frequency of the
original modulation and makes the radiation spectrum appear to occur at twice the
frequency. Thus there is the possibility of some “stray light” due to “double
modulation”.
Figure C- 5
illustrates (by simulation) the principle of double
modulation. The blue trace is the true spectrum. The red trace is a frequency
stretched copy of the blue trace. A small fraction of the intensity of the red trace is
added to the blue trace when double modulation occurs. A totally absorbing band of
