Pre-Dispersive versus Post-Dispersive, what’s the difference?

What is the difference between pre-dispersive and post-dispersive?

ANSWER:

In a pre-dispersive spectrometer, the sample is illuminated with monochromatic light. Light reflected from, or transmitted through, the sample is collected and delivered to the instrument’s detector. Any stray ambient light scattered from the sample is also collected, thus, both the instrument’s monochromatic illumination and all wavelengths of the ambient stray light are delivered to the detector.



In a post-dispersive spectrometer, the sample is illuminated with white light. Light reflected from, or transmitted through, the sample is collected and delivered to the spectrometer where it is first dispersed into it’s component wavelengths and then measured by the instrument’s detector.

Generally two categories of spectrometer performance are affected by Pre versus Post-Dispersive, and in most situations a Post-Dispersive system is advantageous over the Pre-Dispersive system.
 

Stray light interference

In a Pre-dispersive spectrometer the ambient stray light signal can represent a large fraction of the total light signal measured by the detector, thus, it can be a major source of error. While this error can be minimized by completely shielding the sample from all ambient sources of light, this often precludes the use of most reflectance and transmittance fiber optic probes and non-contact measurement heads.

In a Post-Dispersive spectrometer the ambient stray light scattered from the sample is also collected, but with the post-dispersive instrument only ambient stray light of the same wavelength as that being measured by the detector is added to the signal. Thus, the stray light signal represents a much smaller fraction of the total light signal measured by the detector resulting in an increase in instrument baseline stability.
Sampling, or sample presentation flexibility

Post-Dispersive spectrometers provide the greatest amount of sampling, or sample interface flexibility. Post-Dispersive systems allow for non-contact and remote measurements where Pre-Dispersive systems generally have problems with ambient stray light.

ASD spectrometers are post-dispersive.