Abstract

FR.19.03

Mesopic discrimination thresholds for narrow-band colored targets

Harald Babucke, Ines Horsthemke-Eckhardt, Karen Meyer, Anna-Barbara Schönwälder, Kunibert Krause
Fakultät Optik und Mechatronik, Hochschule Aalen, Aalen

Objective
When driving a car at night, the human eye works in the mesopic (twilight) range. The standard CIE photopic V(lambda) and the scotopic V'(lambda) functions are not valid in this region. They are not applicable for discrimination thresholds or for narrow-band targets. Light sources with narrow-band spectra as light emitting diodes are available in many colors and are now used for car backlights, in the cockpit or for traffic lights. What is the spectral sensitivity curve for such sources for the discrimination in the mesopic range?
Methods
The binocular discrimination threshold was determined from eleven subjects (aged 24-45) with narrow-band Landolt targets (2° and 0.6° in size, lambda = 415..685 nm, FWHM below 10 nm) at background luminances of 0.001 to 10 cd/m², presentation times of  0.5 s and retinal eccentricities from 0° to 20°. The sensitivity (1/Le) was calculated from the target radiance Le needed for correct discrimination of the target orientation.
Results
The spectra of the sensitivity show up to five maxima in the mesopic range. The best fit is achieved with a model after Fulton which uses the sum of the non-negative logarithms of three theoretical cone absorption spectra, here applied with the addition of macular absorption. In the photopic (10 cd/m²) region three maxima are found, with the strongest in the yellow region. From 10 cd/m² to the scotopic region (0.001 cd/m²) the sensitivity increases by a factor of 100, in the blue region more than in the red region. The difference increases with decreasing background luminance and leads to a blue-shift of the strongest maximum.
Conclusions
The Fulton model provides a better fit to the data than the other models tested, the fit procedure is more stable and it is easier to communicate. With the accumulated data, it may serve as base for the development of a suitable sensitivity curve. Developers of traffic-relevant lights should use broad-band (400 to 650 nm) and extended (greater than 2°)  sources which emit strongly in the blue region to secure a better target discrimination in the mesopic range.

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