ASD FieldSpec 3 User Manual page 17

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White Reference
Accessories for Light Sources and Probes
ASD.Document 600540 Rev. C
instrument's initialization file, asdcfg.ini. The controlling software
automatically corrects all spectra taken with the shutter closed to account for
this minor complication.
The VNIR spectrometer is fitted with a unique software and hardware
combination feature called Driftlock. Driftlock deals with DC changes over
long periods of time. It automatically updates DC for every measurement by
looking at a series of masked pixels at the front portion of the VNIR array.
The Driftlock feature corrects for the majority of DC variation over time.
A material with approximately 100% reflectance across the entire spectrum is
called a white reference panel or white reference standard.
Because the internal spectrometers only measure the intensity of a light field
through a given point in space, a white reference is one method to correlate
that measurement with the physical properties of the sample.
Reflectance and transmittance are inherent properties of all materials and are
independent of the light source.
Reflectance is the ratio of energy reflected from a sample to the energy
incident on the sample. Spectral Reflectance is the reflectance at a given
wavelength.
Transmittance is the ratio of the radiant energy transmitted through a sample
to the radiant energy incident on the surface of the sample. Spectral
Transmittance is the transmittance at a given wavelength.
Relative reflectance is computed by dividing the energy reflected from the
sample by that reflected off a white reference panel or standard. Absolute
reflectance is computed by multiplying the relative reflectance by the known
reflectance of the white reference panel.
With the reflectance of the reference standard available and known, RS
compute the absolute reflectance or transmittance for the material being
sampled.
ASD offers several accessories for:
• Delivering illumination to the sample.
• Collecting reflected or transmitted radiance and irradiance from the
sample (where solar light is the primary source of illumination).
• Transmitting the collected light to the spectrometer.
Some accessories, such as many of the optional foreoptic probes, rely on other
illumination sources, and even solar light.
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Chapter 1 Introduction
3
FieldSpec® 3 User Manual
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