Limits of Human Vision

damitr · Sep 7, 2006

What is the lowest intensity of light that generates any stimulus in the human eye? In what wave length range?

chroot · Sep 7, 2006

The eye is most sensitive to green light, at approximately 515 nm. The actual sensitivity varies enormously from person to person, during the time of day, and with diet. It's difficult to pin down a single, specific number for the eye's sensitivity.

- Warren

NoTime · Sep 7, 2006

damitr said:

What is the lowest intensity of light that generates any stimulus in the human eye? In what wave length range?

IIRC you can actualy see single photon events with the eye rod system.
Can't get any lower than that.
Not as images but simply as light flashes in the dark.
Don't know the frequency range of the rod system.

The eye cone system responds to 3 wavelenghts.
Although I've heard that some people have two different color red receptors, which would make 4.
The cone system requires a lot of light to operate.

Ouabache · Sep 8, 2006

Lowest intensity to obtain a stimulus may be measured in photons.

One online reference describes, the minimum amount of light our brain requires to perceive light, is from 5 to 9 photons reaching the retina in a 100ms period.

.... neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine (photons) arrive within less than 100 ms. If we could consciously see single photons we would experience too much visual "noise" in very low light, so this filter is a necessary adaptation....

Another related concept is the dynamic range of our eye.
see ref2. This is useful when viewing say; a deep space object through a telescope and you are trying to resolve this image. Your eyes need time to adapt to the darkness. The rods of your retina, are more sensitive to dim light than the cones.

At any given instant, the retina can resolve a contrast ratio of around 100:1 (about 6 1/2 stops). As soon as your eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) are mostly complete in thirty minutes. Hence, over time, a contrast ratio of about 1,000,000:1 (about 20 stops) can be resolved. The process is nonlinear and multifaceted, so an interruption by light nearly starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.

Across the whole visible spectrum, what wavelength are we most sensitive to? I found one reference (ref03) which is in agreement with Warren.

A light-adapted eye typically has its maximum sensitivity at around 555 nm, in the green region of the optical spectrum

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Limits of Human Vision

damitr

chroot

NoTime

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