Month: December 2007

Red
Clockwise, from top left: fresh strawberries; Northern cardinal; Magdalena Frąckowiak wearing a red dress at Paris Fashion Week; Honor guard of Chinese People's Liberation Army holding red flags; Cardinal Théodore-Adrien Sarr, Archbishop of Dakar.
Spectral coordinates
Wavelength	Approx. 625–740[1] nm
Frequency	~480–400 THz
Color coordinates
Hex triplet	#FF0000
sRGBB (r, g, b)	(255, 0, 0)
HSV (h, s, v)	(0°, 100%, 100%)
CIELChuv (L, C, h)	(53, 179, 12°)
Source	X11
B: Normalized to [0–255] (byte) H: Normalized to [0–100] (hundred)

Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres.^[1] It is a primary color in the RGB color model and a secondary color (made from magenta and yellow) in the CMYK color model, and is the complementary color of cyan. Reds range from the brilliant yellow-tinged scarlet and vermillion to bluish-red crimson, and vary in shade from the pale red pink to the dark red burgundy.^[2]

Red pigment made from ochre was one of the first colors used in prehistoric art. The Ancient Egyptians and Mayans colored their faces red in ceremonies; Roman generals had their bodies colored red to celebrate victories. It was also an important color in China, where it was used to color early pottery and later the gates and walls of palaces.^{[3]: 60–61} In the Renaissance, the brilliant red costumes for the nobility and wealthy were dyed with kermes and cochineal. The 19th century brought the introduction of the first synthetic red dyes, which replaced the traditional dyes. Red became a symbolic color of communism and socialism; Soviet Russia adopted a red flag following the Bolshevik Revolution in 1917. The Soviet red banner would subsequently be used throughout the entire history of the Soviet Union, starting from 1922 and ending with its 1991 dissolution. China adopted its own red flag following the Chinese Communist Revolution. A red flag was also adopted by North Vietnam in 1954, and by all of Vietnam in 1975.

Since red is the color of blood, it has historically been associated with sacrifice, danger, and courage. Modern surveys in Europe and the United States show red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love, and joy. In China, India, and many other Asian countries it is the color symbolizing happiness and good fortune.^{[4]: 39–63}

Shades and variations[edit]

Varieties of the color red may differ in hue, chroma (also called saturation, intensity, or colorfulness), or lightness (or value, tone, or brightness), or in two or three of these qualities. Variations in value are also called tints and shades, a tint being a red or other hue mixed with white, a shade being mixed with black. Four examples are shown below.

The cardinal takes its name from the color worn by Catholic cardinals.

Pink is a pale shade of red. Cherry blossoms in the Tsutsujigaoka Park, Sendai, Miyagi, Japan.

Vermilion is similar to scarlet, but slightly more orange. This is sindoor, a red cosmetic powder used in India; some Hindu women put a stripe of sindoor along their hair parting to show they are married.^[5][6]

Ruby is the color of a cut and polished ruby gemstone.

In science and nature[edit]

Seeing red[edit]

The human eye sees red when it looks at light with a wavelength between approximately 625 and 740 nanometers.^[1] It is a primary color in the RGB color model and the light just past this range is called infrared, or below red, and cannot be seen by human eyes, although it can be sensed as heat.^[7] In the language of optics, red is the color evoked by light that stimulates neither the S or the M (short and medium wavelength) cone cells of the retina, combined with a fading stimulation of the L (long-wavelength) cone cells.^[8]

Primates can distinguish the full range of the colors of the spectrum visible to humans, but many kinds of mammals, such as dogs and cattle, have dichromacy, which means they can see blues and yellows, but cannot distinguish red and green (both are seen as gray). Bulls, for instance, cannot see the red color of the cape of a bullfighter, but they are agitated by its movement.^[9] (See color vision).

One theory for why primates developed sensitivity to red is that it allowed ripe fruit to be distinguished from unripe fruit and inedible vegetation.^[10] This may have driven further adaptations by species taking advantage of this new ability, such as the emergence of red faces.^[11]

Red light is used to help adapt night vision in low-light or night time, as the rod cells in the human eye are not sensitive to red.^[12][13]

In color theory and on a computer screen[edit]

In the RYB color model, which is the basis of traditional color theory, red is one of the three primary colors, along with blue and yellow. Painters in the Renaissance mixed red and blue to make violet: Cennino Cennini, in his 15th-century manual on painting, wrote, "If you want to make a lovely violet colour, take fine lac (red lake), ultramarine blue (the same amount of the one as of the other) with a binder"; he noted that it could also be made by mixing blue indigo and red hematite.^[14]

In the CMY and CMYK color models, red is a secondary color subtractively mixed from magenta and yellow.^{[citation needed]}

In the RGB color model, red, green and blue are additive primary colors. Red, green and blue light combined makes white light, and these three colors, combined in different mixtures, can produce nearly any other color. This principle is used to generate colors on such as computer monitors and televisions. For example, magenta on a computer screen is made by a similar formula to that used by Cennino Cennini in the Renaissance to make violet, but using additive colors and light instead of pigment: it is created by combining red and blue light at equal intensity on a black screen. Violet is made on a computer screen in a similar way, but with a greater amount of blue light and less red light.^[15]

• 
  In a traditional color wheel from 1708, red, yellow and blue are primary colors. Red and yellow make orange; red and blue make violet.
• 
  In modern color theory, red, green and blue are the additive primary colors, and together they make white. A combination of red, green and blue light in varying proportions makes all the colors on your computer screen and television screen.
• 
  Tiny Red, green and blue sub-pixels (enlarged on left side of image) create the colors you see on your computer screen and TV.

Color of sunset[edit]

As a ray of white sunlight travels through the atmosphere to the eye, some of the colors are scattered out of the beam by air molecules and airborne particles due to Rayleigh scattering, changing the final color of the beam that is seen. Colors with a shorter wavelength, such as blue and green, scatter more strongly, and are removed from the light that finally reaches the eye.^[16] At sunrise and sunset, when the path of the sunlight through the atmosphere to the eye is longest, the blue and green components are removed almost completely, leaving the longer wavelength orange and red light. The remaining reddened sunlight can also be scattered by cloud droplets and other relatively large particles, which give the sky above the horizon its red glow.^[17]

Lasers[edit]

Lasers emitting in the red region of the spectrum have been available since the invention of the ruby laser in 1960. In 1962 the red helium–neon laser was invented,^[18] and these two types of lasers were widely used in many scientific applications including holography, and in education. Red helium–neon lasers were used commercially in LaserDisc players. The use of red laser diodes became widespread with the commercial success of modern DVD players, which use a 660 nm laser diode technology. Today, red and red-orange laser diodes are widely available to the public in the form of extremely inexpensive laser pointers. Portable, high-powered versions are also available for various applications.^[19] More recently, 671 nm diode-pumped solid state (DPSS) lasers have been introduced to the market for all-DPSS laser display systems, particle image velocimetry, Raman spectroscopy, and holography.^[20]

Red's wavelength has been an important factor in laser technologies; red lasers, used in early compact disc technologies, are being replaced by blue lasers, as red's longer wavelength causes the laser's recordings to take up more space on the disc than would blue-laser recordings.^[21]

Astronomy[edit]

• Mars is called the Red Planet because of the reddish color imparted to its surface by the abundant iron oxide present there.^[22]
• Astronomical objects that are moving away from the observer exhibit a Doppler red shift.
• Jupiter's surface displays a Great Red Spot caused by an oval-shaped mega storm south of the planet's equator.^[23]
• Red giants are stars that have exhausted the supply of hydrogen in their cores and switched to thermonuclear fusion of hydrogen in a shell that surrounds its core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is much lower in temperature, giving them an orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun due to their large size.
• Red supergiants like Betelgeuse, Antares, Mu Cephei, VV Cephei, and VY Canis Majoris one of the biggest stars in the Universe, are the biggest variety of red giants. They are huge in size, with radii 200 to 1700 times greater than the Sun, but relatively cool in temperature (3000–4500 K), causing their distinct red tint. Because they are shrinking rapidly in size, they are surrounded by an envelope or skin much bigger than the star itself. The envelope of Betelgeuse is 250 times bigger than the star inside.
• A red dwarf is a small and relatively cool star, which has a mass of less than half that of the Sun and a surface temperature of less than 4,000 K. Red dwarfs are by far the most common type of star in the Galaxy, but due to their low luminosity, from Earth, none are visible to the naked eye.^[24]
• Interstellar reddening is caused by the extinction of radiation by dust and gas

• 
  Mars appears to be red because of iron oxide on its surface.
• 
  Mira, a red giant
• 
  Artist's impression of a red dwarf, a small, relatively cool star that appears red due to its temperature

Pigments and dyes[edit]

• 
  Red ochre cliffs near Roussillon in France. Red ochre is composed of clay tinted with hematite. Ochre was the first pigment used by man in prehistoric cave paintings.
• 
  Vermilion pigment, made from cinnabar. This was the pigment used in the murals of Pompeii and to color Chinese lacquerware beginning in the Song dynasty.
• 
  Red lead, also known as minium, has been used since the time of the ancient Greeks. Chemically it is known as lead tetroxide. The Romans prepared it by the roasting of lead white pigment. It was commonly used in the Middle Ages for the headings and decoration of illuminated manuscripts.
• 
  The roots of the Rubia tinctorum, or madder plant, produced the most common red dye used from ancient times until the 19th century.
• 
  Alizarin was the first synthetic red dye, created by German chemists in 1868. It duplicated the colorant in the madder plant, but was cheaper and longer lasting. After its introduction, the production of natural dyes from the madder plant virtually ceased.

Food coloring[edit]

The most common synthetic food coloring today is Allura Red AC, a red azo dye that goes by several names including: Allura Red, Food Red 17, C.I. 16035, FD&C Red 40,^[25][26] It was originally manufactured from coal tar, but now is mostly made from petroleum.^[27]

In Europe, Allura Red AC is not recommended for consumption by children. It is banned in Denmark, Belgium, France and Switzerland, and was also banned in Sweden until the country joined the European Union in 1994.^[26] The European Union approves Allura Red AC as a food colorant, but EU countries' local laws banning food colorants are preserved.^[28]

In the United States, Allura Red AC is approved by the Food and Drug Administration (FDA) for use in cosmetics, drugs, and food. It is used in some tattoo inks and is used in many products, such as soft drinks, children's medications, and cotton candy. On June 30, 2010, the Center for Science in the Public Interest (CSPI) called for the FDA to ban Red 40.^[29]

Because of public concerns about possible health risks associated with synthetic dyes, many companies have switched to using natural pigments such as carmine, made from crushing the tiny female cochineal insect. This insect, originating in Mexico and Central America, was used to make the brilliant scarlet dyes of the European Renaissance.^{[citation needed]}

Autumn leaves[edit]