Spectral
data are used as a kind of fingerprint, helping us to identify
and classify materials. We can measure spectral data such as
UV-VIS-IR reflectance and transmittance, X-ray fluorescence,
NMR, Raman, and so on. We are particularly interested in visible
spectral reflectance; it enables us to calculate color for any
lightning condition, create highly-accurate color profiles,
and in some cases helps to identify specific pigments an artist
might have used.
An imaging spectrophotometer can be made by combining a monochrome
(grayscale) digital camera with a liquid-crystal tunable filter.
An image is a record of the amount of light reflected as a function
of position for a specific range of wavelengths.
Below is a full-color image of a painting and color target.
By adjusting the slider to select the peak wavelength, the corresponding
image is seen. Dark image areas correspond to low reflectance
(significant light absorption); light image areas correspond
to high reflectance (minimal light absorption). In general,
we see a basic principle of color reflection: an object reflects
wavelengths similar in color to its color name and absorbs the
other wavelengths. For example, the green target color is lightest
for the green wavelengths (around 550 nm) and darker for the
others. Yellow is a notable exception: it reflects all green
and red wavelengths and absorbs blue. (In fact, we can produce
a very chromatic yellow without reflecting any yellow wavelengths!
But that's another story...)
