What is Meant by the Term "Observer Angle"?

In addition to defining standard illuminants, the CIE conducted experiments to quantify the standard observer. The development of the standard observer is the basis for all instrumental color measurement. 

1931 Standard Observer (2-degree observer).

In 1927, a test fixture was designed to allow an observer to "dial" in the correct amount of red, green and blue light to match a given color. Imagine an observer seated in front of this fixture. The observer would look at a white screen through an aperture having a 2 degree field of view (much like looking at one's thumbnail at arm's length). The observer then would be asked to match a test light visually on one side of the screen by adjusting the intensity of red, green and blue lights. The quantities of the three primary light sources required to match the test source were named tristimulus values. The test was continued until the observers matched colors representing the entire visual spectrum.

A 2 degree field of view was used in the experiments. This meant the observer would be able to use only the region of the retina, known as the fovea, that is most sensitive to color.

Two separate experiments were conducted by physicists John Guild and David Wright. Guild used 7 observers and Wright used 10. The data from the two independent observer groups were combined mathematically.

The experimental results proved that not all colors can be matched using the set of primaries. In some cases, light from one of the primaries had to be added to the test color to deliver a match. Adding light to the test color was considered the equivalent of subtracting it from the other two primaries, resulting in the test color being described by a combination of positive and negative tristimulus values.

In order for the data to be easily used in ongoing standardization work, the CIE believed it would be necessary to eliminate all negative numbers. To accomplish this, they mathematically transformed the standard tristimulus curves so that all red, green and blue responses were positive. This change meant that the new primary set of red, green and blue output could not be produced by an existing lamp. Despite this, the 1931 CIE standard observer curves were adopted as "standard" response curves for the average observer.

The three standard observer curves relate directly back to how the human eye functions. They represent the three responses of the human eye. We know these as our color matching function. The three color matching functions are given symbols x-bar, y-bar, and z-bar. These functions correspond to the spectral sensitivity of the human eye with the highest sensitivity in the green range (electromagnetic spectral energy at 550 nm). This standard observer function and the related mathematical data remain the worldwide basis for color measurement and computation.

1964 Observer (10 Degree Observer)

It was later determined that color values calculated using the 2-degree observer do not always correlate well with visual assessment, since most visual assessments are done with a field of view greater than 2 degrees. Subtle differences exist when a wider area of view is used, particularly in the blue-green region of the spectrum.

In 1964, the CIE defined a supplemental observer to provide better correlation with commercial color matching. The supplemental observer is based on color matching experiments which were conducted using a 10-degree field of view.

Repeatability of the standard observer was found to be more accurate using a larger field of view. Today, the 10-degree observer is most widely used in color formulation and color quality control.

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