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H. Zwick, P. Edsall, L. Hare; Utilization of the Crawford Transformation Evaluate the Spectral Background Efficiency of Emerging Solid State Light Sources . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3706.
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Visual sensitivity is governed by both target and background lighting conditions. The Crawford transformation provides an important analytical link between test target threshold function obtained during the process of dark adaptation and the background increment threshold intensity function obtained by incrementing a surround background and redeterming threshold for a test target located within the background. From these functions, the equivalent background can be assessed as a function of time in the dark, thereby, providing assessment of changes in increment background intensity as the visual system returns to a fully dark adapted state (Blakemore and Rushton, 1965). The function for equivalent background is employed to evaluate the efficiency of spectral solid state light sources varying in spectral content which may affect visual sensitivity during the early phase of dark adaptation.
Spectral dark adaptation functions obtained from four LED visible light sources were merged with four equivalent background threshold functions to provide the capability for the measurement of equivalent background energy as a function of time in the dark for two LED background sources. The first LED background source had a single peak transmission at 630 nm. The second LED background source 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 at 441, 501, and 562 nm. The test LED source at 525 nm,however, remained about a log unit higher in threshold sensitivity relative to three other test diodes over the entire ten minute test period. The second LED background source with peak emissions at 440 and 540 nm showed maximum early sensitivity for the 525nm LED equivalent background measurement. However, after 7 min of dark adaptation, both the 525 and 501 sensitivities were the same. Equivalent log background threshold measurements were successively higher at 562 and 441 nm, respectively.
The utilization of the Crawford transformation to evaluate solid state spectral light sources with varying spectral content reflects the utility of this technique for evaluation of cone sensitivity variations associated with emerging LED light sources.
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