Parameters:
(1) light source
(2) standardized presentation
(3) choice of screening instrument/chart
(4) incentive not to be classified as color blind
The light source should be diffused daylight or an approximation (using full spectrum daylight bulbs). Ordinary tungsten lamps should be avoided since a deuteranomalous patient may be able to read the charts.
The charts should be given in a standardized presentation for light and distance from the patient. Variation in gloss for different colored dots may be detectable by a patient with mild color vision defects.
Several screening charts are available for screening. While one basic type can be used routinely, one or more backup methods should be available for handling difficult cases. Some charts may miss certain color vision defects; for example, the Ishihar chart may not detect tritanopia.
A person may want to avoid being classified as being color blind for a variety of reasons. Such a person may be able to falsify results (false negative) if:
(1) the person is familiar with the chart that is used for screening
(2) the person tries to take advantage of light source or other factors
(3) the degree of color blindness is mild
