scotopic and photopic vision (original) (raw)

Author: the photonics expert (RP)

Definition: vision of the human eye at lower and higher light intensity levels, respectively

Category: vision, displays and imaging

• color vision
  • scotopic and photopic vision
  • color spaces
  • color rendering index

Related: color visionluminosity functionsspectral response of a photodetectorluminous efficacy and efficiencynight vision devices

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DOI: 10.61835/0jh Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

The vision of the human eye is based on four different kinds of photoreceptors: rods and three types of cones (L, M and S cones). Depending on the available light levels (more precisely, the incoming luminance), the brain uses different photoreceptor signals for determining the brightness of seen objects.

Scotopic Vision

The rods, containing rhodopsin photoreceptors, are substantially more light-sensitive than the cones and are therefore used for vision at low light levels, e.g. at night with a minimum amount of illumination, where the cones hardly deliver any usable signals. Because the eye has only one type of those rods, it cannot obtain any spectral information: it cannot “know” whether a certain brightness results from green light, where the rods are most sensitive, or from light in the red or blue spectral region, for example, where the rod sensitivity is comparatively low, but the light intensity may be correspondingly higher. Therefore, in that mode of vision, called scotopic vision, all objects are perceived as gray, a kind of neutral color impression.

Figure 1: The scotopic response function according to CIE (1951). Data source: Colour & Vision Research Laboratory of the University College London, page on luminous efficiency functions.

Purely scotopic vision occurs if the input luminance stays below 10−3 cd/m2. After seeing brighter light, the eye requires some time to adapt to the new light conditions — full adaptation can take around 30 minutes. Therefore, only after that time one begins seeing something in the dark.

Photopic Vision

At higher light levels, which are sufficient for substantially exciting the cone receptors, their outputs are used for color vision and also for the determination of apparent brightness. That mode of operation is called photopic vision.

Purely photopic vision is possible at luminance values of at least a couple of cd/m2. There is also an intermediate regime, called mesopic vision, which is a kind of combination of scotopic and photopic vision.

Figure 2: The photopic response function according to CIE. Data source: Colour & Vision Research Laboratory of the University College London, page on luminous efficiency functions.

Different Vision of Animals

The eyes of various animals differ substantially from those of human beings. For example, the retina of a cat's eye contains a larger proportion of rod receptors, leading to an improved sensitivity at low lighting conditions — at the expense of substantially reduced color vision, which is dichromatic instead of trichromatic.

Luminosity Functions

For both scotopic and photopic vision, there are luminosity functions describing the spectral sensitivity of the eye. While the scotopic response peaks at a wavelength of approximately 500 nm, the maximum photopic response is achieved around 555 nm. Both maxima are in the green spectral region.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is scotopic vision?

Scotopic vision is the vision of the human eye at very low light levels, for example at night. It relies on the highly sensitive rod photoreceptors and does not provide color information, causing objects to be perceived in shades of gray.

What is photopic vision?

Photopic vision is the mode of vision used at higher light levels, sufficient to excite the cone photoreceptors. This mode of vision allows for the perception of colors and apparent brightness.

Why is vision at night colorless?

Night vision (scotopic vision) is colorless because it relies on only a single type of photoreceptor: the rods. With only one spectral sensitivity profile, the brain cannot distinguish different colors and perceives only shades of gray.

How does the eye's spectral sensitivity change with light levels?

The eye's spectral sensitivity is described by different luminosity functions. In scotopic (low-light) vision, the peak sensitivity is at a wavelength of about 500 nm. In photopic (bright-light) vision, this peak shifts to around 555 nm.

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