The method of magnitude estimation was used to investigate how the level of adaptation affects the power function relating brightness to luminance. With the left eye dark adapted and the right eye light adapted, a test field was presented briefly to one eye. The observers' estimates generated a pair of brightness functions, one for each eye. The validity of these functions was checked by interocular brightness matching. The results are described by the equation ψ= k(L-L₀)^β, when ψ is brightness, L luminance, and L₀ the absolute threshold. All the parameters—k, L₀, and β—change systematically with light adaptation. The exponent β increases from 0.33 for the dark-adapted eye to 0.44 for the eye adapted to 1 lambert.

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