Lights, shadows, and filters: A parametric study of neon brightness spreading

Abstract

We investigated in three separate experiments the perceived achromatic colour, transparency, and phenomenal attributes of concentric-circle neon-brightness-spreading displays. Stimuli were obtained by varying the luminances of the outer arc (OA) and inner arc (IA) of circle, while keeping the background luminance constant. In the first experiment, observers modified the luminance of an adjustable region to match the achromatic colour of the central area of the displays; additional adjustments were performed on sets of control figures without IA (subjective figures) and without OA. In the second experiment, observers judged the transparency of the central area of the displays using either numerical ratings or dichotomous judgments. In the third experiment, they used four possible categories (light, shadow, filter, opaque) to describe the appearance of the displays. Only displays with intermediate background luminance were perceived as non-transparent. Displays with intermediate IA luminance were seen as transparent filters, whereas those with intermediate OA luminance were categorised as either lights or shadows, depending on luminance polarity. Adjustment data show that a simple contrast rule accounts well for induction effects in OA-only and IA-only figures. Data concerning complete figures show an interaction between OA and IA luminances in determining the percept; in particular, the contrast effect of OA luminance appears modulated by IA luminance in a nonmonotonic fashion. We discuss the implications of these findings on our current understanding of the mechanisms of neon brightness and colour spreading.