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Harry Zwick, Peter Edsall, Louis Hare, James Ness; Utilization of the Crawford transformation in evaluation of spectral background efficiency of solid state light sources. Journal of Vision 2006;6(6):227. doi: 10.1167/6.6.227.
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Visual sensitivity is governed by target and background lighting conditions. The Crawford transformation provides a link between the test target threshold function of dark adaptation and the background increment threshold intensity function obtained by incrementing background energy and redetermining threshold. The equivalent background is assessed as a function of time in the dark, providing assessment of increment background intensity, as the visual system returns to a dark adapted state (Blakemore and Rushton, 1965). Spectral dark adaptation functions obtained from four LED visible light sources were merged with four equivalent background threshold functions for measurement of equivalent background energy as functions of time in the dark for two LED background sources; the first had a single peak transmission at 630 nm; the second had a bimodal transmission at 440 and at 540 nm. Equivalent background measurements for the first LED background source showed minimal variation in dark adaptation equivalent background functions with test LED sources 441, 501, and 562 nm. The 525 nm test LED remained a log unit higher in threshold over test period. The second LED background source with peaks at 440 and 540 nm showed maximum early sensitivity for the 525nm LED source. After 7 min of dark adaptation, both the 525 and 501 LED sensitivities were equal. Equivalent background thresholds remained higher at 562 and 441 nm. Utilization of the Crawford transformation to evaluate solid state spectral light sources with varying spectral content supports its utility for evaluation of cone sensitivity associated with emerging LED light sources.
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