CIE Standard Observers
Date：2017-02-17
In
the visual observing situation, the observer is the human eye that
receives the light reflected from or transmitted through an object, and
the brain which perceives the vision.
The visual observing situation
Since
different humans perceive color and appearance in different ways,
subjectively, attempts have been made to “standardize” the human
observer as a numerical representation of what the “average person”
sees. This standard observer could then be used in lieu of a human
observer when assessments are made instrumentally.
Wright
and Guild performed experiments using human volunteers to assess their
color vision and develop an average, or standard, observer. In 1931
they published the 2° CIE Standard Observer function based on their
research. The function is called 2° because their experiments involved
having the subjects judge colors while looking through a hole that
allowed them a 2° field of view. In 1931, it was believed that all the
color-sensing cones of the eye were located within a 2° arc of the
fovea. Thus the 2° field of view was chosen and used in establishing
the standard observer.
By
the 1960s, it was realized that cones were present in a larger area of
the eye than previously believed, and so in 1964, the 10° Standard
Observer was developed. The 10° Standard Observer is currently believed
to best represent the average spectral response of human observers,
although the 2° Standard Observer still has its place for measurement of
objects that will be viewed at a distance, such as road signs. The
relative sizes of the two fields of view are shown below.
The
standard observers, in the form of mathematical functions of the human
response to each wavelength of light, are used in color calculations.
The observers can be graphed as shown below.
The CIE Tristimulus XYZ color scale, for instance, is calculated as follows:
where
R = % reflectance
T = % transmittance
Sums are across the spectral range for which the instrument reads.
Note
that X, Y, and Z include factors for the mathematical standard observer
in their formulas. All other tristimulus color scales (such as Hunter
L, a, b and CIEL*a*b*) may then be calculated from XYZ.
