Radiometric units represent physical energy (e.g., radiance has units of watts \(sr^{-1}~m^{-2}\))
Colorimetric units adjust radiometric units for visual wavelength sensitivity (e.g. luminance has units of \(\text{cd m}^{-2}\) scotopic units are proportional to rod absorptions; photopic luminance units are proportional to a weighted sum of the L and M cone absorptions
Typical ambient luminance levels (in \(\text{cd m}^{-2}\)): starlight \(10^{-3}\); moonlight \(10^{-1}\); indoor lighting \(10^{2}\); sunlight \(10^{5}\); max intensity of common CRT monitors, \(10^{2}\)
One Troland (Td) of retinal illumination is produced on the retina when the eye looks at a surface of 1 \(\text{cd m}^{-2}\) through a pupil of area \(1~mm^{2}\).
Lens focal length: f (meters); lens power \(=1/f\) (diopters)
Conversion of linear units (X) to decibels: \(Y=20~log~10(X)\); a change of 0.3 log10 units is a factor of 2, or 6 dB
Image Formation
The eyes are 6 cm apart and half-way down the head
Visual angle of the sun or moon \(=0.5\) deg
At arm’s length: thumbnail \(=1.5\) deg; thumb joint \(=2.0\) deg; fist \(=8-10~deg\)
Monocular visual field measured from central fixation: 160 deg (w) x 175 deg (h)
Binocular visual field measured from central fixation: 200 deg (w) x 135 deg (h)
Region of binocular overlap: 120 deg (w) x 135 deg (h)
Range of pupil diameters: 2mm - 8mm.
Refractive indices: air 1.000; glass 1.520; water 1.333; cornea 1.376
Axial chromatic aberration over the visible spectrum: 2 diopters
Retina
Retinal size: \(5~cm\times5\) cm; 0.4 mm thick
One degree of visual angle \(=0.3\) mm on the retina
Number of cones in each retina: \(5\times10^{6}\)
Number of rods in each retina: \(10^{8}\)
Diameter of the fovea: 1.5 mm (5.2 deg); rod-free fovea: 0.5 mm (1.7 deg); foveola (rod-free, capillary-free fovea): 0.3 mm (1 deg); size of the optic nerve head: \(1.5~\text{mm}\times2.1~mm\) (5 deg (w) x 7 deg (h)) location of the optic nerve head: 15 deg nasal
Size of rods near fovea: 1 \(\mu \text{m}\) (diameter) x 60 \(\mu \text{m}\) (length)
S cone spacing (foveal): 10 arc min
L and M cone spacing (foveal): 0.5 arc min
Number of \((L+M)\) cones / Number of \(S~cones=14\) (though the ratio may be higher in the foveola)
\(1.5\times10^{6}\) optic nerve fibers/retina; ratio of receptors to ganglion cell in fovea 1:3; ratio of receptors to ganglion cells for whole retina, 125:1
Cortex
Area of entire cortex: \(1.3\times10^{5}~\text{mm}^{2}\) 1.7 mm thick
Total number of cortical neurons: \(10^{10}\); density: \(10^{5}\) neurons \(/\text{mm}^{3}\)
Number of corpus callosum fibers: \(5\times10^{8}\)
Number of macaque visual areas: 30
Size of each area V1: 3cm by 8 cm
Half of area V1 represents the central 10 deg (2% of the visual field)
Width of a human ocular dominance column 0.5-1.0 mm; width of a macaque ocular dominance column 0.3 mm
Sensitivity
Minimum number of absorptions for: scotopic detection 1-5; detectable electrical excitation of a rod 1; photopic detection 10-15
The number of photoisomerisations per rod (per sec?) required to saturate the retinal rod circuit: 1
Following exposure to a sunny day, dark adaptation to a moonless night involves: 10 minutes (photopic); 40 minutes (scotopic); change in visual sensitivity 6 log10 units \(\mu \text{m}\)
Highest detectable spatial frequency at high ambient light levels, 50-60 cpd; low ambient light levels, 20-30 cpd
The contrast threshold (Delta \(L/L)\) for a static edge at photopic luminances is 1%.
Highest detectable temporal frequency: high ambient large field, 80 Hz; low ambient, large field 40 Hz.
Typical localization threshold: 6 arc sec (0.5 \(\mu \text{m}\) on the retina)
Minimum temporal separation needed to discriminate two small, brief light pulses from a single equal-energy pulse: 15-20 ms