Visible spectrum
The visible spectrum is the
A typical
The spectrum does not contain all the
Visible wavelengths pass largely unattenuated through the
Spectral colors
Color | nm )
|
Frequency (THz) |
Photon energy (eV) |
---|---|---|---|
380–450 | 670–790 | 2.75–3.26 | |
450–485 | 620–670 | 2.56–2.75 | |
485–500 | 600–620 | 2.48–2.56 | |
500–565 | 530–600 | 2.19–2.48 | |
565–590 | 510–530 | 2.10–2.19 | |
590–625 | 480–510 | 1.98–2.10 | |
625–750 | 400–480 | 1.65–1.98 |
Colors that can be produced by visible light of a narrow band of wavelengths (
History
In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal.[9]
In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. He was the first to use the word spectrum (Latin for "appearance" or "apparition") in this sense in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of "corpuscles" (particles) of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass. The result is that red light is bent (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors.
In the 18th century,
In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel (infrared) and Johann Wilhelm Ritter (ultraviolet), Thomas Young, Thomas Johann Seebeck, and others.[15] Young was the first to measure the wavelengths of different colors of light, in 1802.[16]
The connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century. Their theory of color vision correctly proposed that the eye uses three distinct receptors to perceive color.
Limits to visible range
The visible spectrum is limited to wavelengths that can both reach the retina and trigger
Atmospheric transmission
The visible range of most animals evolved to match the optical window, which is the range of light that can pass through the atmosphere. The ozone layer absorbs almost all UV light (below 315 nm).[17] However, this only affects cosmic light (e.g. sunlight), not terrestrial light (e.g. Bioluminescence).
Ocular transmission
Before reaching the retina, light must first transmit through the cornea and lens. UVB light (< 315 nm) is filtered mostly by the cornea, and UVA light (315–400 nm) is filtered mostly by the lens.[18] The lens also yellows with age, attenuating transmission most strongly at the blue part of the spectrum.[18] This can cause xanthopsia as well as a slight truncation of the short-wave (blue) limit of the visible spectrum. Subjects with aphakia are missing a lens, so UVA light can reach the retina and excite the visual opsins; this expands the visible range and may also lead to cyanopsia.
Opsin absorption
Each opsin has a
Different definitions
Regardless of actual physical and biological variance, the definition of the limits is not standard and will change depending on the industry. For example, some industries may be concerned with practical limits, so would conservatively report 420–680 nm,
Vision outside the visible spectrum
Under ideal laboratory conditions, subjects may perceive infrared light up to at least 1,064 nm.[23] While 1,050 nm NIR light can evoke red, suggesting direct absorption by the L-opsin, there are also reports that pulsed NIR lasers can evoke green, which suggests two-photon absorption may be enabling extended NIR sensitivity.[23]
Similarly, young subjects may perceive ultraviolet wavelengths down to about 310–313 nm,[24][25][26] but detection of light below 380 nm may be due to fluorescence of the ocular media, rather than direct absorption of UV light by the opsins. As UVA light is absorbed by the ocular media (lens and cornea), it may fluoresce and be released at a lower energy (longer wavelength) that can then be absorbed by the opsins. For example, when the lens absorbs 350 nm light, the fluorescence emission spectrum is centered on 440 nm.[27]
Non-visual light detection
In addition to the photopic and scotopic systems, humans have other systems for detecting light that do not contribute to the primary visual system. For example, melanopsin has an absorption range of 420–540 nm and regulates circadian rhythm and other reflexive processes.[28] Since the melanopsin system does not form images, it is not strictly considered vision and does not contribute to the visible range.
In non-humans
The visible spectrum is defined as that visible to humans, but the variance between species is large. Not only can
Vertebrates tend to have 1-4 different opsin classes:[17]
- longwave sensitive (LWS) with peak sensitivity between 500–570 nm,
- middlewave sensitive (MWS) with peak sensitivity between 480–520 nm,
- shortwave sensitive (SWS) with peak sensitivity between 415–470 nm, and
- violet/ultraviolet sensitive (VS/UVS) with peak sensitivity between 355–435 nm.
Testing the visual systems of animals behaviorally is difficult, so the visible range of animals is usually estimated by comparing the peak wavelengths of opsins with those of typical humans (S-opsin at 420 nm and L-opsin at 560 nm).
Mammals
Most mammals have retained only two opsin classes (LWS and VS), due likely to the nocturnal bottleneck. However, old world primates (including humans) have since evolved two versions in the LWS class to regain trichromacy.[17] Unlike most mammals, rodents' UVS opsins have remained at shorter wavelengths. Along with their lack of UV filters in the lens, mice have a UVS opsin that can detect down to 340 nm. While allowing UV light to reach the retina can lead to retinal damage, the short lifespan of mice compared with other mammals may minimize this disadvantage relative to the advantage of UV vision.[29] Dogs have two cone opsins at 429 nm and 555 nm, so see almost the entire visible spectrum of humans, despite being dichromatic.[30] Horses have two cone opsins at 428 nm and 539 nm, yielding a slightly more truncated red vision.[31]
Birds
Most other vertebrates (birds, lizards, fish, etc.) have retained their tetrachromacy, including UVS opsins that extend further into the ultraviolet than humans' VS opsin.[17] The sensitivity of avian UVS opsins vary greatly, from 355–425 nm, and LWS opsins from 560–570 nm.[32] This translates to some birds with a visible spectrum on par with humans, and other birds with greatly expanded sensitivity to UV light. The LWS opsin of birds is sometimes reported to have a peak wavelength above 600 nm, but this is an effective peak wavelength that incorporates the filter of avian oil droplets.[32] The peak wavelength of the LWS opsin alone is the better predictor of the long-wave limit. A possible benefit of avian UV vision involves sex-dependent markings on their plumage that are visible only in the ultraviolet range.[33][34]
Fish
Teleosts (bony fish) are generally tetrachromatic. The sensitivity of fish UVS opsins vary from 347-383 nm, and LWS opsins from 500-570 nm.[35] However, some fish that use alternative chromophores can extend their LWS opsin sensitivity to 625 nm.[35] The popular belief that the common goldfish is the only animal that can see both infrared and ultraviolet light[36] is incorrect, because goldfish cannot see infrared light.[37]
Invertebrates
The visual systems of invertebrates deviate greatly from vertebrates, so direct comparisons are difficult. However, UV sensitivity has been reported in most insect species.[38] Bees and many other insects can detect ultraviolet light, which helps them find nectar in flowers. Plant species that depend on insect pollination may owe reproductive success to their appearance in ultraviolet light rather than how colorful they appear to humans. Bees' long-wave limit is at about 590 nm.[39] Mantis shrimp exhibit up to 14 opsins, enabling a visible range of less than 300 nm to above 700 nm.[17]
Thermal vision
Some snakes can "see"[40] radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes,[41] and other snakes with the organ may detect warm bodies from a meter away.[42] It may also be used in thermoregulation and predator detection.[43][44]
Spectroscopy
See also
- High-energy visible light
- Cosmic ray visual phenomena
- Electromagnetic absorption by water
- Two-photon absorption - A method for seeing outside the visible spectrum
References
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- ^ a b Boettner, Edward A.; Wolter, J. Reimer (December 1962). "Transmission of Ocular Media". Investigative Ophthalmology & Visual Science. 1: 776-783.
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The foveal sensitivity to several near-infrared laser wavelengths was measured. It was found that the eye could respond to radiation at wavelengths at least as far as 1,064 nm. A continuous 1,064 nm laser source appeared red, but a 1,060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina.
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Normally the human eye responds to light rays from 390 to 760 nm. This can be extended to a range of 310 to 1,050 nm under artificial conditions.
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