ASD FieldSpec 3, User Manual

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ASD Document 600540 Rev. J 69 FieldSpec® 3 User Manual
www.asdi.com Chapter 7 Field Measurements
7.11 White Reference
A material with 95-99% reflectance across the entire spectrum is called a
white
reference panel or white reference standard .
The raw measurement made by the spectroradiometer is influenced by both
the sample and the light source. An independent measure of the light source
illumination on a reference of known reflectance is required to calculate the
reflectance of the sample. The use of a white reference standard with near
100% reflectance simplifies this calculation.
The ASD application software, such as RS
3
and Indico, can calculate the
ratios for reflectance or transmittance of the material being sampled by the
FieldSpec spectroradiometer using the white reference as the standard.
Spectralon from Labsphere is the white reference standard that is very
suitable for the VNIR and SWIR spectral ranges of ASD instruments.
Spectralon is made of polytetraflouroethylene (PTFE) and cintered halon. It
has the characteristic of being nearly 100% reflective within the wavelength
range of 350 nm to 2500 nm. A Spectralon white reference scatters light
uniformly in all directions within that wavelength range.
7.11.1 Spectralon Reflectance Data
Table 7-2 shows the published reflectance data for an uncalibrated SRM-990.
When SRM-990 is used as the white reference for a reflectance measurement
with the FieldSpec spectroradiometer, an even closer reflectance value for the
sample can be calculated.
Table 7-2Labsphere's published reflectance data for uncalibrated Spectralon (+/- 0.5%)
Wavelength (nm)
SRM-990 “Uncalibrated”
Spectralon Reflectance
250 0.950
300 0.985
400 0.990
500 0.991
600 0.992
700 0.992
800 0.991
900 0.991
1000 0.993
1100 0.993