All ASD spectrometers are “post-dispersive. In a pre-dispersive spectrometer, the sample is illuminated with monochromatic light. Light scattered off or transmitted through the sample is then collected and delivered to the instrument’s detector. Ambient light that strays into the sample being measured is also collected. Thus, both the monochromatic illumination from the instrument and all wavelengths of the ambient stray light are delivered to the detector. Because the stray ambient light signal can represent a large fraction of the total light signal measured by the detector, it is a major source of error. While this source or error can be minimized by completely shielding the sample form all sources of ambient illumination, this often precludes the use of most reflectance and transmittance fiber optic probes.

In a post-dispersive spectrometer, the sample is illuminated with white light. Light scattered off or transmitted through the sample is then dispersed and delivered to the instrument’s detector. As with the pre-dispersive spectrometer, ambient light that strays into the sample being measured is also collected. The difference is that in the post-dispersive instrument, only ambient stray light of the same wavelength as that being measured by the detector is added to the signal resulting from the instrument’s illumination of the sample. Thus, the stray ambient light signal represents a small fraction of the total light signal measured by the detector.