Some useful numbers in vision science
(from B. Wandell, Stanford University)


Radiometric units represent physical energy (e.g., radiance has units of watts/sr-m)

Colorimetric units adjust radiometric units for visual wavelength sensitivity (e.g. luminance has units of cd/m); 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 cd/m): starlight 10; moonlight 10; indoor lighting 10; sunlight 10; max intensity of common CRT monitors, 10.

One Troland (Td) of retinal illumination is produced on the retina when the eye looks at a surface of 1 cd/mthrough a pupil of area 1 mm.

Lens focal length: f (meters); lens power = 1/f (diopters)

Conversion of linear units (X) to decibels: Y = 20 (X); a change of 0.3 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) 175 deg (h)

Binocular visual field measured from central fixation: 200 deg (w) 135 deg (h) ??

Region of binocular overlap: 120 deg (w) 135 deg (h)

Range of pupil diameters: 2mm -8mm.

Refractive indices: air 1.000; glass 1.520; water 1.333; cornea 1.376

Optical power (diopters): cornea, 43; lens, 20 (relaxed); whole eye, 60

Change in power due to accommodation, 8 diopters

Axial chromatic aberration over the visible spectrum: 2 diopters


Retinal size: 5 cm x 5 cm; 0.4 mm thick

One degree of visual angle = 0.3 mm on the retina

Number of cones in each retina: 510

Number of rods in each retina: 10

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 mm x 2.1 mm (5 deg (w) x 7 deg (h)) location of the optic nerve head: 15 deg nasal

Peak cone density: 1.6 x 10 cones/mm

Foveal cone size: 1-4 (diameter) x 50-80 (length); extrafoveal cone size: 4-10 (diameter) x 40 (length)

Size of rods near fovea: 1 (diameter) x 60 (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 (but the ratio may be higher in the foveola)

1.510 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


Area of entire cortex: 1.3 x 10 mm; 1.7 mm thick

Total number of cortical neurons: 10; density: 10 neurons/mm

Synapses: 5 x 10 synapses/mm 4 x 10 synapses/neuron;

Axons: 3 kilometers/mm

Number of corpus callosum fibers: 5 x 10

Number of macaque visual areas: 30

Size of each area V1: 3 cm 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.

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.

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

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 on the retina)

Minimum temporal separation needed to discriminate two small, brief light pulses from a single equal-energy pulse: 15-20 ms

Stereoscopic depth discrimination: step threshold, 3 arc sec; point threshold, 30 arc sec


Visible spectrum: 370-730 nm

Peak wavelength sensitivity: 507nm (scotopic) and 555 nm (photopic)

Spectral equilibrium hues: 475 nm (blue), 500 nm (green), 575 nm (yellow), no spectral equilibrium red

Number of basic English color names: 11

Incidence of: anomalous trichromacy, 1/100 (male), 1/10,000 (female); protanopia and deuteranopia, 1/100 (male), 1/10,000 (female); tritanopia, 1/10,000; rod monochromacy, 1/10,000; cone monochromacy, 1/100,000