Tapetum Lucidum: Why Eyes Glow at Night

Species preset

Eye optics

Tapetum reflectivity R 0.65

Absorption per pass a 0.22

Ambient light level 30

Pupil dilation 0.6

Light incidence angle 0°

Drag the yellow flashlight handle on the arc (left side of the diagram) to sweep the angle — eyeshine only fires back near the source when the ray is close to the eye's own optical axis.

Live readouts

1st-pass absorbed

22%

2nd-pass absorbed

17%

Total captured

39%

Sensitivity gain

×1.8

Escaping light

31%

Eyeshine visible

100%

How it works

Behind the photoreceptor layer of many nocturnal animals sits the tapetum lucidum, a mirror-like tissue that reflects unabsorbed light back through the retina for a second chance at absorption:

P_total = P1 + (1 - P1)·R·P2

P1, P2 = fraction absorbed per pass (per-pass absorption, ~equal)
R      = tapetum reflectivity

This double pass genuinely boosts scotopic sensitivity — but the same reflected light that was not recaptured also has to exit the eye somewhere. Because the tapetum is a curved mirror, it reflects a ray back close to its own incoming direction only when that ray entered nearly along the eye's optical axis — real retroreflection. That is why eyeshine "switches on" only when a flashlight (or car headlight) sits close to the observer's own line of sight, and vanishes the moment the light is held off to the side, even though the underlying sensitivity boost never changes.

Cross-section ray diagram — drag flashlight, drag canvas to pan

scroll to zoom