Reliable results cannot be expected from this test unless standard illumination is used. The illumination should approximate 6,740 degrees Kelvin (Standard Illuminant C or average daylight) preferably at 25 foot candles or more. Equivalent results can be achieved with using a tungsten halogen daylight source at 6,500 degrees K, such as the one found in the SpectraLight II and SolSource manufactured by Macbeth.
If natural daylight is used, the test should be given near a window illuminated chiefly from the north sky, lightly to moderately overcast. Daylight Fluorescent (6500 degrees) may be used if necessary such as the one found in the Judge II from Macbeth. Since natural daylight is variable, the results cannot be expected to be as stable as when the test is given under standard illumination. Ordinary incandescent room lights should be switched off or shielded from the test area.
The most convenient position for the administrator of the test is across the table from the subject. The light should be from above so that the angle of illumination is about 90 degrees and the angle of viewing about 60 degrees.