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Visible spectrum

From Wikipedia, the free encyclopedia
"Color spectrum" redirects here. For the music album, seeThe Color Spectrum.
Whitelightisdispersedby aprisminto the colors of the visible spectrum.

Thevisible spectrumis thebandof theelectromagnetic spectrumthat isvisibleto thehuman eye.Electromagnetic radiationin this range ofwavelengthsis calledvisible light(or simply light). Theoptical spectrumis sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include theultravioletandinfraredparts of the electromagnetic spectrum as well, known collectively asoptical radiation.[1]

A typicalhuman eyewill respond to wavelengths from about 380 to about 750nanometers.[2]In terms of frequency, this corresponds to a band in the vicinity of 400–790terahertz.These boundaries are not sharply defined and may vary per individual.[3]Under optimal conditions these limits of human perception can extend to 310 nm (ultraviolet) and 1100 nm (near infrared).[4][5][6]

The spectrum does not contain all thecolorsthat the humanvisual systemcan distinguish.Unsaturated colorssuch aspink,orpurplevariations likemagenta,for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are also calledpure colorsorspectral colors.[7]

Visible wavelengths pass largely unattenuated through theEarth's atmospherevia the "optical window"region of the electromagnetic spectrum. An example of this phenomenon is when clean airscattersblue light more than red light, and so the midday sky appears blue (apart from the area around the Sun which appears white because the light is not scattered as much). The optical window is also referred to as the "visible window" because it overlaps the human visible response spectrum. Thenear infrared(NIR) window lies just out of the human vision, as well as the medium wavelength infrared (MWIR) window, and the long-wavelength or far-infrared (LWIR or FIR) window, although other animals may perceive them.

Spectral colors

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sRGB rendering of the spectrum of visible light
sRGB rendering of the spectrum of visible light
Color Wavelength
(nm) 		Frequency
(THz) 		Photon energy
(eV)
violet
 380–450 670–790 2.75–3.26
blue
 450–485 620–670 2.56–2.75
cyan
 485–500 600–620 2.48–2.56
green
 500–565 530–600 2.19–2.48
yellow
 565–590 510–530 2.10–2.19
orange
 590–625 480–510 1.98–2.10
red
 625–750 400–480 1.65–1.98
Main article:Spectral color

Colors that can be produced by visible light of a narrow band of wavelengths (monochromatic light) are calledpure spectral colors.The various color ranges indicated in the illustration are an approximation: The spectrum is continuous, with no clear boundaries between one color and the next.[8]

History

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Newton's color circle, fromOpticksof 1704, showing the colors he associated withmusical notes.The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a fulloctave,from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectralpurplecolors are observed when red and violet light are mixed.

In the 13th century,Roger Bacontheorized thatrainbowswere produced by a similar process to the passage of light through glass or crystal.[9]

In the 17th century,Isaac Newtondiscovered that prisms could disassemble and reassemble white light, and described the phenomenon in his bookOpticks.He was the first to use the wordspectrum(Latinfor "appearance" or "apparition" ) in this sense in print in 1671 in describing hisexperimentsinoptics.Newton observed that, when a narrow beam ofsunlightstrikes the face of a glassprismat an angle, some isreflectedand 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.

Newton's observation of prismatic colors (David Brewster1855)

Newton originally divided the spectrum into six named colors:red,orange,yellow,green,blue,andviolet.He later addedindigoas the seventh color since he believed that seven was a perfect number as derived from theancient Greeksophists,of there being a connection between the colors, the musical notes, the known objects in theSolar System,and the days of the week.[10]The human eye is relatively insensitive to indigo's frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some later commentators, includingIsaac Asimov,[11]have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet. Evidence indicates that what Newton meant by "indigo" and "blue" does not correspond to the modern meanings of those color words. Comparing Newton's observation of prismatic colors with a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas his "blue" corresponds tocyan.[12][13][14]

In the 18th century,Johann Wolfgang von Goethewrote about optical spectra in hisTheory of Colours.Goethe used the wordspectrum(Spektrum) to designate a ghostly opticalafterimage,as didSchopenhauerinOn Vision and Colors.Goethe argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges withwhitebetween them. The spectrum appears only when these edges are close enough to overlap.

In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized byWilliam Herschel(infrared) andJohann 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 andcolor visionwas explored by Thomas Young andHermann von Helmholtzin the early 19th century. Theirtheory of color visioncorrectly proposed that the eye uses three distinct receptors to perceive color.

Limits to visible range

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See also:Color vision § Physiology of color perception
Photopic(black) andscotopic(green) luminous efficiency functions. The horizontal axis is wavelength innm.Seeluminous efficiency functionfor more info.

The visible spectrum is limited to wavelengths that can both reach the retina and triggervisual phototransduction(excite avisual opsin). Insensitivity toUV lightis generally limited by transmission through thelens.Insensitivity toIR lightis limited by thespectral sensitivityfunctions of the visual opsins. The range is definedpsychometricallyby theluminous efficiency function,which accounts for all of these factors. In humans, there is a separate function for each of two visual systems, one forphotopic vision,used in daylight, which is mediated bycone cells,and one forscotopic vision,used in dim light, which is mediated byrod cells.Each of these functions have different visible ranges. However, discussion on the visible range generally assumes photopic vision.

Atmospheric transmission

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The visible range of most animals evolved to match theoptical 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

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Cumulativetransmission spectraof light as it passes through the ocular media, namely after thecornea(blue), before thelens(red), after the lens (gray) and before theretina(orange). The solid lines are for a 4.5 year old eye. The dashed orange line is for a 53 year old eye, and dotted for a 75 year old eye, indicating the effect of lens yellowing.)

Before reaching theretina,light must first transmit through thecorneaandlens.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 causexanthopsiaas well as a slight truncation of the short-wave (blue) limit of the visible spectrum. Subjects withaphakiaare missing a lens, so UVA light can reach the retina and excite the visual opsins; this expands the visible range and may also lead tocyanopsia.

Opsin absorption

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Each opsin has aspectral sensitivityfunction, which defines how likely it is to absorb a photon of each wavelength. The luminous efficiency function is approximately the superposition of the contributingvisual opsins.Variance in the position of the individual opsin spectral sensitivity functions therefore affects the luminous efficiency function and the visible range. For example, the long-wave (red) limit changes proportionally to the position of the L-opsin. The positions are defined by the peak wavelength (wavelength of highest sensitivity), so as the L-opsin peak wavelength blue shifts by 10 nm, the long-wave limit of the visible spectrum also shifts 10 nm. Large deviations of the L-opsin peak wavelength lead to a form ofcolor blindnesscalledprotanomalyand a missing L-opsin (protanopia) shortens the visible spectrum by about 30 nm at the long-wave limit. Forms of color blindness affecting the M-opsin and S-opsin do not significantly affect the luminous efficiency function nor the limits of the visible spectrum.

Different definitions

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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,[19][20]while others may be concerned withpsychometricsand achieving the broadest spectrum would liberally report 380–750, or even 380–800 nm.[21][22]The luminous efficiency function in theNIRdoes not have a hard cutoff, but rather an exponential decay, such that the function's value (or vision sensitivity) at 1,050 nm is about 109times weaker than at 700 nm; much higher intensity is therefore required to perceive 1,050 nm light than 700 nm light.[23]

Vision outside the visible spectrum

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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 suggeststwo-photon absorptionmay 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 tofluorescenceof 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

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In addition to the photopic and scotopic systems, humans have other systems for detecting light that do not contribute to the primaryvisual system.For example,melanopsinhas an absorption range of 420–540 nm and regulatescircadian rhythmand other reflexive processes.[28]Since the melanopsin system does not form images, it is not strictly consideredvisionand does not contribute to the visible range.

In non-humans

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See also:Evolution of color vision

The visible spectrum is defined as that visible to humans, but the variance between species is large. Not only cancone opsinsbe spectrally shifted to alter the visible range, butvertebrateswith 4 cones (tetrachromatic) or 2 cones (dichromatic) relative to humans' 3 (trichromatic) will also tend to have a wider or narrower visible spectrum than humans, respectively.

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

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Most mammals have retained only two opsin classes (LWS and VS), due likely to thenocturnal 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

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See also:Bird_vision § Ultraviolet_sensitivity

Most other vertebrates (birds, lizards, fish, etc.) have retained theirtetrachromacy,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 avianoil 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 theirplumagethat are visible only in the ultraviolet range.[33][34]

Fish

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See also:Vision_in_fish § Ultraviolet

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 alternativechromophorescan 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

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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] Beesand many other insects can detect ultraviolet light, which helps them findnectarin 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 shrimpexhibit up to 14 opsins, enabling a visible range of less than 300 nm to above 700 nm.[17]

Thermal vision

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Main article:Infrared sensing in snakes

Some snakes can "see"[40]radiant heat atwavelengthsbetween 5 and 30μmto a degree of accuracy such that a blindrattlesnakecan 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 inthermoregulationandpredatordetection.[43][44]

Spectroscopy

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Earth's atmospherepartially or totallyblockssome wavelengths of electromagnetic radiation, but in visible light it is mostly transparent
Main article:Spectroscopy

Spectroscopyis the study of objects based on the spectrum of color they emit, absorb or reflect. Visible-light spectroscopy is an important tool inastronomy(as isspectroscopyat other wavelengths), where scientists use it to analyze the properties of distant objects.Chemical elementsand smallmoleculescan be detected in astronomical objects by observingemission linesandabsorption lines.For example,heliumwas first detected by analysis of the spectrum of theSun.The shift in frequency of spectral lines is used to measure theDoppler shift(redshiftorblueshift) of distant objects to determine their velocities towards or away from the observer.Astronomical spectroscopyuses high-dispersiondiffraction gratingsto observe spectra at very high spectral resolutions.

See also

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Wikisourcehas original text related to this article:
Wikimedia Commons has media related toVisible spectrum.

References

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