Brightness matching and flicker photometric data obtained over the full mesopic range

Area of Science:

• Vision science
• Photometry
• Color perception

Background:

• Understanding visual sensitivity across different light levels is crucial for vision science.
• The Purkinje shift describes changes in luminosity function with light adaptation.
• Mesopic vision operates between scotopic (low light) and photopic (bright light) conditions.

Purpose of the Study:

• To compare visual sensitivity measurements using heterochromatic flicker photometry and direct comparison brightness matching.
• To investigate the behavior of visual perception in the mesopic range.
• To identify potential discrepancies and phenomena like the Purkinje shift under different photometric conditions.

Main Methods:

• Three normal observers performed visual matches under controlled conditions.
• Measurements were taken at 445, 560, and 630 nm across a luminance range from 0.03 to 100 trolands (td).
• Two distinct psychophysical methods were employed: heterochromatic flicker photometry and direct comparison brightness matching.

Main Results:

• Brightness matching showed smooth changes in relative sensitivity versus illuminance.
• Heterochromatic flicker photometry exhibited a step-like transition around 1 td, indicating distinct response branches.
• A reverse Purkinje shift was observed at 630 nm (high mesopic) and 445 nm (low mesopic), suggesting complex cone-driven mechanisms.

Conclusions:

• The two photometric methods yield different results, particularly in the mesopic range.
• The observed reverse Purkinje shift points to intricate cone-cone interactions or other neural processes.
• Further research is needed to clarify the mechanisms underlying visual matches at low mesopic levels (0.03 td).