Gibson [J. Exp. Psychol. 16, 1 (1993)] observed that during prolonged viewing, a line perceptually rotates toward the nearest vertical or horizontal meridian (the normalization effect), and moreover, the perceived orientation of a subsequently presented line depends on the orientation of the adapting one (the tilt after-effect). The mechanisms of both phenomena remain poorly understood. According to our experimental results, the adapting line perceptually rotates to the nearest of three orientations: vertical, horizontal, and diagonal. We propose a simple neuronal model of orientation detectors whose responses are determined by the cardinal detectors. It is shown that both normalization and tilt after-effect may be explained by adaptation of these cardinal detectors.

Colour-perception differences under illumination by two quadrichromatic solid-state sources of light have been studied with respect to a tungsten radiator with the same correlated colour temperature (2600 K). A virtual RYgCB source (illuminant), which contains red, yellow–green, cyan and blue components with the line width typical of AlGaInP and AlInGaN light-emitting diodes (LEDs), was fully optimized for the highest value of the general colour-rendering index (CRI) (Ra = 98.3). An implemented RAGB source (lamp) contained commercially available red, amber, green and blue LEDs (Ra = 79.4). Colorimetric calculations in the Commission Internationale de l'Eclairage 1976 (u′,v′) colour plane for 40 Munsell colour samples (value 6, chroma/6, hue increment 2.5) revealed the differences in hue discrimination and distortion for both sources in the yellow–green and blue–cyan ranges. These differences were not revealed by the standard analysis of the special CRIs and were lower for the RYgCB illuminant, which contained primary LEDs in the sensitive ranges. A psychophysical experiment on seven subjects was performed using the RAGB lamp stabilized against thermal and ageing drifts. Despite different colour-perception abilities of the subjects under investigation, the experiment confirmed the calculation results. Methods of obtaining composite white light with high subjective ratings are discussed, based on the obtained data.

Solid-state white light sources based on multiple light emitting diodes (LEDs) have three features that make them radically different from other sources of electrically generated light. These are the narrowband spectra of the individual LEDs, the possibility of controlling the light output of the individual LEDs and the limited stability of the LED’s colour. These features make it possible to achieve a set of solid-state white light sources that are metameric to a selected reference light source. A computational method to optimise the intensities of narrowband LEDs to obtain these metameric light sources is proposed. The appearance of object colours under these metameric light sources differs and can be used to satisfy users’ requests according to the various aspects of colour quality required, without altering the visual appearance of the light source.

We report on a blue-amber (Bfirelight[) cluster of light-emitting diodes (LEDs) with extra-low correlated color temperature ($1860 K) optimized for outdoor lighting under mesopic conditions. When compared with common white LEDs, the firelight LED cluster shows considerably reduced indexes of melatonin suppression and skyglow, increased retinal illuminance for elderly people, but a reduced performance of perceiving colors, which, however, can be tolerated at mesopic luminance. In comparison with an almost metameric high-pressure sodium lamp, the cluster exhibits a potentially higher luminous efficacy, similar reaction time and detection threshold of luminance contrasts for achromatic targets, and noticeably improved color discrimination characteristics.

The scoring artefact in the Farnsworth-Munsell 100-Hue test, arising from the grouping of the caps into four boxes, was investigated. The traditional method of scoring performed with the numbers of the anchor caps disregarded and the alternative scoring performed with the numbers of the anchor caps employed, were compared. For the traditional method of scoring, we revealed an increase of the error score of the outside (end-box) caps when the total error score was above 240. On the contrary for scoring performed with the numbers of the anchor caps employed, the difference between the error score of the outside caps and the average error per cap is not significant. To mitigate the end-box artefact and to improve the reliability of the Farnsworth-Munsell 100-Hue test, corrections to the traditional method of scoring are proposed.