Visible spectrum
The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light.
The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well, known collectively as optical radiation.
A typical human eye will respond to wavelengths from about 380 to about 750 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 400–790 terahertz. These boundaries are not sharply defined and may vary per individual. Under optimal conditions, these limits of human perception can extend to 310 nm and 1100 nm.
The spectrum does not contain all the colors that the human visual system can distinguish. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are also called pure colors or spectral colors.
Visible wavelengths pass largely unattenuated through the Earth's atmosphere via the "optical window" region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, and so the midday sky appears blue. The optical window is also referred to as the "visible window" because it overlaps the human visible response spectrum. The near infrared window lies just out of the human vision, as well as the medium wavelength infrared window, and the long-wavelength or far-infrared window, although other animals may perceive them.
Spectral colors
Colors that can be produced by visible light of a narrow band of wavelengths are called 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.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.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 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" 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 less sharply than violet as it passes through the prism, creating a spectrum of colors.
Newton originally divided the spectrum into six named colors: red, orange, yellow, green, blue, and violet. He later added indigo as the seventh color since he believed that seven was a perfect number as derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the Solar System, and the days of the week. 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, including Isaac Asimov, 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 to cyan.
In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On 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 with white between 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 by William Herschel and Johann Wilhelm Ritter, Thomas Young, Thomas Johann Seebeck, and others.
Young was the first to measure the wavelengths of different colors of light, in 1802.
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 visual phototransduction. Insensitivity to UV light is generally limited by transmission through the lens. Insensitivity to IR light is limited by the spectral sensitivity functions of the visual opsins. The range is defined psychometrically by the luminous efficiency function, which accounts for all of these factors. In humans, there is a separate function for each of two visual systems, one for photopic vision, used in daylight, which is mediated by cone cells, and one for scotopic vision, used in dim light, which is mediated by rod cells. Each of these functions have different visible ranges. However, discussion on the visible range generally assumes photopic vision.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. However, this only affects cosmic light, not terrestrial light.Ocular transmission
Before reaching the retina, light must first transmit through the cornea and lens. UVB light is filtered mostly by the cornea, and UVA light is filtered mostly by the lens. The lens also yellows with age, attenuating transmission most strongly at the blue part of the spectrum. This can cause xanthopsia as well as a slight truncation of the short-wave 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 spectral sensitivity function, which defines how likely it is to absorb a photon of each wavelength. The luminous efficiency function is approximately the superposition of the contributing visual 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 limit changes proportionally to the position of the L-opsin. The positions are defined by the peak wavelength, 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 of color blindness called protanomaly and a missing L-opsin 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
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, while others may be concerned with psychometrics and achieving the broadest spectrum would liberally report 380–750, or even 380–800 nm. The luminous efficiency function in the NIR does not have a hard cutoff, but rather an exponential decay, such that the function's value at 1,050 nm is about 109 times weaker than at 700 nm; much higher intensity is therefore required to perceive 1,050 nm light than 700 nm light.Vision outside the visible spectrum
Under ideal laboratory conditions, subjects may perceive infrared light up to at least 1,064 nm. 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.Similarly, young subjects may perceive ultraviolet wavelengths down to about 310–313 nm, 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, it may fluoresce and be released at a lower energy 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.
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. 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 cone opsins be spectrally shifted to alter the visible range, but vertebrates with 4 cones or 2 cones relative to humans' 3 will also tend to have a wider or narrower visible spectrum than humans, respectively.Vertebrates tend to have 1-4 different opsin classes:
- longwave sensitive with peak sensitivity between 500–570 nm,
- middlewave sensitive with peak sensitivity between 480–520 nm,
- shortwave sensitive with peak sensitivity between 415–470 nm, and
- violet/ultraviolet sensitive with peak sensitivity between 355–435 nm.