Abstract

We first measured red/green equilibria of a 1^o test field superimposed on either a green (540 nm) or red (660 nm) 5°, 32-troland background. The background was then modified by (1) reducing its size to 3° or (2) introducing a dark, thin concentric ring with an outer diameter of 3°. The ring was 12′ wide so, effectively, it changed the 5° background to a 2.6° background surrounded by, but not contiguous with, a 35° (inner-outer diameter) ring. The test was an admixture of 549-and 660-nm light, and was varied from 6 to 300 trolands. The observer adjusted the 549-nm component of the test until the test appeared neither reddish nor greenish. Two color-normal observers participated in the study. Reducing the size of the background from 5° to 3° caused a shift in color appearance at all illuminances of the test. The shift was toward greenness with a 540-nm background and toward redness with a 660-nm background and can be explained by a smaller change in receptoral sensitivity with the smaller background. Introducing a 12′ dark gap in the 5°, 540-nm background had a larger effect than reducing the size from 5° to 3°, implying the influence of the gap is not due simply to less background light. The same gap on a 5°, 660-nm background had an effect opposite the change in color appearance caused by reducing the size of the background: reducing the size from 5° to 3° caused the test to appear more reddish, but adding the dark gap to the 5° background caused the test to appear more greenish. Similar results were found with 320-troland backgrounds. These observations reveal that contours in a background field strongly affect color appearance, and do so in ways qualitatively different from varying the retinal area stimulated by background light. The results point to higher level processes that use edge contrast to differentiate remote from contiguous adapting light.

© 1989 Optical Society of America