An empirical formula can express the equivalent luminance of any light as a function of the scotopic and photopic luminances. Tests show the suitability of this standard observer for large-field photometry at any level. A restriction on the use of the term equivalent luminance is suggested.
D. A. PALMER, "Standard Observer for Large-Field Photometry at any Level," J. Opt. Soc. Am. 58, 1296-1298 (1968)