Color measurements: a computation tutorial, Rs d r d – Ocean Optics OOIColor User Manual

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Color Measurements: A Computation Tutorial

The fundamental computation involved in the measurement of color is to determine the reflection
spectrum of an object and apply it to a standard illuminant (a fictitious light source). The amount

of light from this standard illuminant that the object reflects (in terms of energy, not percent

reflection) is manipulated and reduced to the tristimulus values X,Y and Z. These values
correspond to the physiological response of the three types of color receptors that the typical

human has in his/her eye. These X,Y and Z values are then combined into uniform color spaces

(L*,a* and b*, for example).

The process begins by the acquisition of a reflection spectrum (R

) at 5nm intervals over the

wavelength range of 380-780nm (Note: All sums shown here are over this same interval).

−

…where R

is the reflectivity at wavelength

, S

is the sample spectrum intensity at wavelength

, R

is the reference spectrum intensity at wavelength

and D

is the dark spectrum intensity at

wavelength

The first step in the calculation is to compute the color matching values for the sample and the
reference:

I x

I y

I z

R I x

R I y

R I z

ref

sam

∑

λ λ

λ λ λ

…where x

, y

and z

are the color matching coordinates at a specific wavelength, and I

is the

intensity of the illuminant at a specific wavelength.