Biomedical subjects

M P Lucassen

Publications and source records attributed to M P Lucassen.

3 recordsLinked to original sources

PubMed1996

Color constancy under natural and artificial illumination.

Color constancy was studied under conditions simulating either natural or extremely artificial illumination. Four test illuminants were used: two broadband phases of daylight (correlated color temperatures 4000 and 25,000 K) and two spectrally impoverished metamers of these lights, each consisting of only two wavelengths. A computer controlled color monitor was used for reproducing the chromaticities and luminances of an array of Munsell color samples rendered under these illuminants. An asymmetric haploscopic matching paradigm was used in which the same stimulus pattern, either illuminated by one of the test illuminants, or by a standard broadband daylight (D65), was alternately presented to the left and right eye. Subjects adjusted the RGB settings of the samples seen under D65 (match condition), to match the appearance of the color samples seen under the test illuminant. The results show the expected failure of color constancy under two-wavelengths illumination, and approximate color constancy under natural illumination. Quantitative predictions of the results were made on the basis of two different models, a computational model for recovering surface reflectance, and a model that assumes the color response to be determined by cone-specific contrast and absolute level of stimulation (Lucassen & Walraven, 1993). The latter model was found to provide somewhat more accurate predictions, under all illuminant conditions.

Color Perception↗

PubMed1993

An adaptation-induced pop-out in visual search.

The present study demonstrates that an object embedded in an array of identical objects can pop-out. Dependent on the stimuli preceding the search display, local (chromatic) adaptation causes an identical object to pop-out because it appears to have a colour (Expt 1) or brightness (Expt 2) that is slightly different from the colour and brightness of the other objects in the display. Experiment 3 shows that this pop-out even occurs when the stimulus preceding the search display is presented for only 100 msec.

Adaptation, Ocular↗

PubMedDate not supplied

Quantifying color constancy: evidence for nonlinear processing of cone-specific contrast.

Color constancy was studied by the method of comparing color samples under two different illuminants using a CRT color monitor. In addition to the classical approach in which one of the illuminants is a (standard) white, we performed experiments in which the range of differential illumination was extended by using pairs of lights that were both colored. The stimulus pattern consisted of an array of thirty-five color samples (including five neutral samples) on a white background. A trichromatic illuminant-object interaction was simulated analogous to that resulting from illumination by three monochromatic lights. The test samples, as seen under "test" and "match" illumination, were successively presented to the left and right eye (haploscopic matching). The data show systematic deviations from predictions on the basis of cone-specific normalization procedures like those incorporated in the Retinex algorithm and the von Kries transformation. The results can be described by a nonlinear response transformation that depends on two factors, receptor-specific sample/background contrast and the extent to which the illuminant stimulates the receptor system in question. The latter factor explains the deviations. These are mainly caused by the short-wave-sensitive system, as a consequence of the fact that this system can be more selectively stimulated than the other, spectrally less separated, cone systems.

Color Perception↗