Visibility curves by direct comparison in a 10 degree field at 1000 Td

Twenty-four observers matched monochromatic lights to a white reference light in a 10 degree field. The retinal illuminance was 1000 Td over most of the spectrum. Twelve persons' visibility curves were more or less additive and resembled the y10 color-matching function of the CIE. Eight others had a double-peaked function, which failed badly to obey Abney's law. The remaining four were more additive, although their curves were broader than y10.