An understanding of the material in this section is not necessary for the practical use of this test. However, the information is important if the test is used in connection with research in color vision.
Perceptible Color Differences
Imagine a chart made up of all possible discriminable hues - reds, greens, blues, etc., in all strengths from neutral to high purity, but all of the same brightness. If these colors were organized systematically, they would form a color array similar to that shown below. Equal distances on this diagram represent equal differences of color to the normal eye.
Suppose that each dot represents a color which is just easily perceptibly different from each adjacent color. We now have a unit of measurement which can be applied to color discrimination. It may be that in some regions of the chart, certain adjacent colors cannot be distinguished by a particular person or by a normal under some particular conditions of viewing, and it may be necessary to skip three or five or ten color units before coming to a color which is just easily perceptibly different from the first. If, for each particular purpose, or individual, it were possible to test each of the thousands of combinations on the diagram, a full description of color discrimination could be obtained for that condition or individual.
Selection of Colors in the 100 Hue Test
The principle or method of the Farnsworth-Munsell Hue Test is to sample the color diagram in all directions and thereby indicate the degree and orientation of discrimination throughout the color field. Because changes in discrimination are systematic throughout the chromaticity plane, generalized deductions can be made from the sample data furnished by the test.
By elimination and replacement in a series which originally consisted of one hundred Munsell colored papers, a circuit of eighty-five papers was constructed, in which the hue differences were just easily noticeable by normals when the papers were suitably mounted. This mounting consists of plastic caps with black rims which separate the exposed parts of the color discs by about their own diameter. It is also necessary that there be slight differences from disc to disc in value and chroma. The latter factor is the device which requires some expression of aptitude in normals, which detects color defects by forcing them to resort to criteria of other than hue difference and, with the factor of black-rim separation, makes the test short enough for practical purposes. The positions of the 85 test colors are shown in the figure below, plotted on the Farnsworth Uniform Chromaticity Scale Diagram #44. Their relation to the schematic is suggested by the white circle in the previous diagram.
