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The threshold illumination i at the eye from a steady source of light of 1′ angular diameter in a field of brightness b was measured for b ranging from zero to about 1500 candles per square foot. The data were obtained by five young experienced observers using both eyes unaided and with natural pupil. A bend in the i, b curve at about b = 1000 mμL occurred at the transition from foveal to extra-foveal vision. The relation i=10−10(1+b)12, where i is in footcandles and b is in millimicrolamberts, expressed the experimental data within a factor of 3 over the entire range.
Knoll et al. (Thu,) studied this question.