The Threshold of Sensitivity
Rods are most sensitive near 500 nm
Rods are most abundant at 20° (from visual axis)
Threshold: 90 photons/s entering the eye in 0.1 sec! (seen 60% of the time) E= 90 hf = 100 hc/l = 4x10-17 watts (10% efficiency)
Efficiency: reflection (n-1)2/(n+1)2 = 94% absorption (50%) rods (20%)