Color vision and colorimetry In optics, the
perception of green is evoked by light having a spectrum dominated by energy with a
wavelength of roughly 495–570
nm. The sensitivity of the dark-adapted human eye is greatest at about 507 nm, a blue-green color, while the light-adapted eye is most sensitive about 555 nm, a yellow-green; these are the peak locations of the rod and cone (scotopic and photopic, respectively)
luminosity functions. The perception of greenness (in opposition to redness forming one of the
opponent mechanisms in human
color vision) is evoked by light which triggers the medium-wavelength
M cone cells in the eye more than the long-wavelength
L cones. Light which triggers this greenness response more than the yellowness or blueness of the other color opponent mechanism is called green. A green light source typically has a spectral power distribution dominated by energy with a wavelength of roughly 487–570 nm. . All the colors seen are made by mixing them in different intensities. Human eyes have color receptors known as cone cells, of which there are three types. In some cases, one is missing or faulty, which can cause
color blindness, including the common inability to distinguish red and yellow from green, known as
deuteranopia or red-green color blindness. Green is restful to the eye. Studies show that a green environment can reduce fatigue. In the
subtractive color system, used in painting and color printing, green is created by a combination of yellow and blue, or yellow and
cyan; in the
RGB color model, used on television and computer screens, it is one of the
additive primary colors, along with red and blue, which are mixed in different combinations to create all other colors. On the
HSV color wheel, also known as the
RGB color wheel, the
complement of green is
magenta; that is, a color corresponding to an equal mixture of
red and
blue light (one of the
purples). On a traditional color wheel, based on subtractive color, the complementary color to green is considered to be red. In additive color devices such as computer displays and televisions, one of the
primary light sources is typically a narrow-spectrum yellowish-green of dominant wavelength ≈550
nm; this "green" primary is combined with an orangish-red "red" primary and a purplish-blue "blue" primary to produce any color in between – the RGB color model. A
unique green (green appearing neither yellowish nor bluish) is produced on such a device by mixing light from the green primary with some light from the blue primary.
Lasers Lasers emitting in the green part of the spectrum are widely available to the general public in a wide range of output powers. Green laser pointers outputting at 532
nm (563.5
THz) are relatively inexpensive compared to other wavelengths of the same power, and are very popular due to their good beam quality and very high apparent brightness. The most common green lasers use diode pumped solid state (
DPSS) technology to create the green light. An infrared
laser diode at 808 nm is used to pump a crystal of neodymium-
doped yttrium vanadium oxide (Nd:YVO4) or neodymium-doped yttrium aluminium garnet (Nd:YAG) and induces it to emit 281.76 THz (1064 nm). This deeper infrared light is then passed through another crystal containing potassium, titanium and phosphorus (KTP), whose non-linear properties generate light at a frequency that is twice that of the incident beam (563.5 THz); in this case corresponding to the wavelength of 532 nm ("green"). Other green wavelengths are also available using DPSS technology ranging from 501 nm to 543 nm. Green wavelengths are also available from
gas lasers, including the
helium–neon laser (543 nm), the Argon-
ion laser (514 nm) and the Krypton-ion laser (521 nm and 531 nm), as well as liquid
dye lasers. Green lasers have a wide variety of applications, including pointing, illumination, surgery,
laser light shows,
spectroscopy,
interferometry,
fluorescence,
holography,
machine vision,
non-lethal weapons, and
bird control. As of mid-2011, direct green laser diodes at 510 nm and 500 nm have become generally available, although the price remains relatively prohibitive for widespread public use. The efficiency of these lasers (peak 3%) compared to that of DPSS green lasers (peak 35%) may also be limiting adoption of the diodes to niche uses.
Pigments, food coloring and fireworks is dyed green every year to mark
St. Patrick's Day Many minerals provide
pigments which have been used in green paints and dyes over the centuries. Pigments, in this case, are minerals which reflect the color green, rather that emitting it through
luminescent or
phosphorescent qualities. The large number of green pigments makes it impossible to mention them all. Among the more notable green minerals, however is the
emerald, which is colored green by trace amounts of
chromium and sometimes
vanadium. Chromium(III) oxide (Cr2O3), is called
chrome green, also called
viridian or institutional green when used as a pigment. Copper is the source of the green color in
malachite pigments, chemically known as basic
copper(II) carbonate.
Verdigris is made by placing a plate or blade of copper, brass or bronze, slightly warmed, into a vat of fermenting wine, leaving it there for several weeks, and then scraping off and drying the green powder that forms on the metal. The process of making verdigris was described in ancient times by
Pliny. It was used by the Romans in the murals of Pompeii, and in Celtic medieval manuscripts as early as the 5th century AD. It produced a blue-green which no other pigment could imitate, but it had drawbacks: it was unstable, it could not resist dampness, it did not mix well with other colors, it could ruin other colors with which it came into contact, and it was
toxic.
Leonardo da Vinci, in his treatise on painting, warned artists not to use it. It was widely used in miniature paintings in Europe and Persia in the 16th and 17th centuries. Its use largely ended in the late 19th century, when it was replaced by the safer and more stable
chrome green. Viridian, as described above, was
patented in 1859. It became popular with painters, since, unlike other synthetic greens, it was stable and not toxic.
Vincent van Gogh used it, along with
Prussian blue, to create a dark blue sky with a greenish tint in his painting
Café Terrace at Night.
Cobalt green, sometimes known as
Rinman's green or zinc green, is a translucent green pigment made by heating a mixture of cobalt (II) oxide and zinc oxide.
Sven Rinman, a Swedish chemist, discovered this compound in 1780. Green chrome oxide was a new synthetic green created by a chemist named Pannetier in Paris in about 1835. Emerald green was a synthetic deep green made in the 19th century by hydrating chrome oxide. It was also known as Guignet green. typically use
barium salts to create green sparks There is no natural source for green
food colorings which has been approved by the US
Food and Drug Administration. Chlorophyll, the
E numbers E140 and E141, is the most common green chemical found in nature, and only allowed in certain medicines and cosmetic materials.
Quinoline Yellow (E104) is a commonly used coloring in the United Kingdom but is banned in Australia, Japan, Norway and the United States.
Green S (E142) is prohibited in many countries, for it is known to cause
hyperactivity,
asthma,
urticaria, and
insomnia. To create green sparks,
fireworks use
barium salts, such as
barium chlorate,
barium nitrate crystals, or
barium chloride, also used for green fireplace logs.
Biology File:Plagiomnium affine laminazellen.jpeg|The chloroplasts of plant cells contain a high concentration of
chlorophyll, making them appear green. File:Caerulea3 crop.jpg|
Frogs often appear green because
dermal iridophores reflect blue light through a yellow upperlayer, filtering the light to be primarily green. File:Yellow-naped Amazon.jpg|A yellow-naped Amazon
parrot, colored green for camouflage in the jungle File:Anolis callainus 247244961.jpg|An
Anole combines blue
structural color of
iridiophores with a layered deposit of yellow
carotenoid pigments obtained from its diet to create the green color it uses for
camouflage. File:Micrommata virescens (Arcugnano).jpg|The
green huntsman spider is green due to the presence of
bilin pigments in the spider's
hemolymph and
tissue fluids Green is common in nature, as many plants are green because of a complex chemical known as chlorophyll, which is involved in
photosynthesis. Chlorophyll absorbs the long wavelengths of light (red) and short wavelengths of light (blue) much more efficiently than the wavelengths that appear green to the human eye, so light reflected by plants is enriched in green. Chlorophyll absorbs green light poorly because it first arose in organisms living in oceans where purple
halobacteria were already exploiting photosynthesis. Their purple color arose because they extracted energy in the green portion of the spectrum using
bacteriorhodopsin. The new organisms that then later came to dominate the extraction of light were selected to exploit those portions of the spectrum not used by the halobacteria. at the
Hellabrunn Zoo Animals typically use the color green as
camouflage, blending in with the chlorophyll green of the surrounding environment. It hunts insects in green vegetation, where it is well camouflaged.
Green eyes There is no green pigment in green eyes; like the color of blue eyes, it is an optical illusion; its appearance is caused by the combination of an amber or light brown pigmentation of the
stroma, given by a low or moderate concentration of
melanin, with the blue tone imparted by the
Rayleigh scattering of the reflected light. Nobody is brought into the world with green eyes. An infant has one of two eye hues: dark or blue. Following birth, cells called melanocytes start to discharge melanin, the earthy colored shade, in the child's irises. This begins happening since melanocytes respond to light in time. Green eyes are most common in
Northern and
Central Europe. They can also be found in
Southern Europe,
West Asia,
Central Asia, and
South Asia. In
Iceland, 89% of women and 87% of men have either blue or green eye color. A study of Icelandic and Dutch adults found green eyes to be much more prevalent in women than in men. == In history and art ==