Brightness of equal-luminance lights

Area of Science:

• Visual perception
• Photometry
• Color science

Background:

• Heterochromatic flicker photometry (HFP) is a standard method for equating light luminance.
• Perceived brightness can differ from luminance, especially for chromatic lights.
• Understanding these discrepancies is crucial for accurate color and light measurement.

Purpose of the Study:

• To quantify the relationship between luminance (defined by HFP) and perceived brightness.
• To investigate how primary light color and colorimetric purity affect brightness perception.
• To test the validity of simple models of brightness generation.

Main Methods:

• Direct brightness matching of chromatic lights against a white standard.
• Stimuli included white light mixed with eleven different primary colors at eight purity levels.
• Participants maintained adaptation to a known white light.
• Brightness-to-flicker ratio (B:F) was calculated.

Main Results:

• B:F ratios varied significantly across different primary colors, with red and blue primaries showing the largest ratios.
• The relationship between B:F ratio and colorimetric purity depended on the primary color.
• Higher purity stimuli consistently yielded larger B:F ratios for a given dominant wavelength.
• The observed variations qualitatively contradicted several basic models of brightness perception.

Conclusions:

• Perceived brightness is not solely determined by luminance as measured by HFP.
• Chromaticity and purity significantly modulate the perceived brightness of light.
• Existing simple models of brightness generation are insufficient to explain the observed data.