[8.4.1.1] A device or instrument with which luminance is measured by visually comparing the appearance of two fields.
Note: The two fields are usually viewed simultaneously side by side, and a direct brightness match is made. This is satisfactory when the color difference between the test field and the comparison field is small. However, when there is a significant color difference, direct brightness matching does not produce equal luminances of the two fields, and either a flicker photometer or the minimally distinct border (MDB) method of photometry provides more-valid measurements. In flicker photometry, the two fields are viewed alternately at such a rate that the color sensations either nearly or completely blend, and the flicker, due to brightness difference, is minimized by adjusting the comparison field. In the MDB method, two juxtaposed fields are adjusted until the border between the two is least conspicuous, even though the brightness of the two fields may appear unequal; the MDB setting yields equal luminances between the two fields.
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