Eigengrau (

German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...

for "intrinsic gray"; ), also called Eigenlicht ( Dutch and German for "intrinsic light"), dark light, or brain gray, is the uniform dark

gray Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...

background color that many people report seeing in the absence of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

. The term ''Eigenlicht'' dates back to the nineteenth century, but has rarely been used in recent scientific publications. Common scientific terms for the phenomenon include "visual noise" or "background adaptation". These terms arise due to the perception of an ever-changing field of tiny black and white dots seen in the phenomenon. Eigengrau is perceived as lighter than a

black Black is a color which results from the absence or complete absorption of visible light. It is an achromatic color, without hue, like white and grey. It is often used symbolically or figuratively to represent darkness. Black and white have o ...

object in normal lighting conditions, because contrast is more important to the visual system than absolute brightness. For example, the night sky looks darker than Eigengrau because of the contrast provided by the stars.

Cause

Researchers noticed early on that the shape of intensity-sensitivity curves could be explained by assuming that an intrinsic source of noise in the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

produces random events indistinguishable from those triggered by real

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

s. Later experiments on

rod cell Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in per ...

s of cane toads (''Rhinella marina'') showed that the frequency of these spontaneous events is strongly temperature-dependent, which implies that they are caused by the thermal isomerization of

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...

. In human rod cells, these events occur about once every 100 seconds on average, which, taking into account the number of rhodopsin molecules in a rod cell, implies that the

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of a rhodopsin molecule is about 420 years. The indistinguishability of dark events from photon responses supports this explanation, because rhodopsin is at the input of the transduction chain. On the other hand, processes such as the spontaneous release of

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...

s cannot be completely ruled out.

References

{{Authority control Color Darkness Shades of gray Vision

Cause

See also

References