| luminance of a comparison field in which radiation of frequency 540 × 1012 Hz has the same brightness as the field considered under the specified photometric conditions of measurement, where the comparison field must have a specified size and shape which can be different from that of the field considered Note 1 to entry: Radiation at a frequency of 540 × 1012 Hz has a wavelength in standard air of approximately 555,017 nm. Note 2 to entry: A comparison field can also be used in which the radiation has any relative spectral distribution, if the equivalent luminance of this field is known under the same conditions of measurement. Note 3 to entry: The equivalent luminance is expressed in candela per square metre (cd·m−2). Note 4 to entry: This entry was numbered 845-01-58 in IEC 60050-845:1987.
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