The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone’s individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the latter type is achromatic and detects light-dark variation, or luminance).[1] Responses to one color of an opponent channel are antagonistic to those to the other color.
While the trichromatic theory defines the way the retina of the eye allows the visual system to detect color with three types of cones, the opponent process theory accounts for mechanisms that receive and process information from cones. Though the trichromatic and opponent processes theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for the opponent process receive signals from the three types of cones and process them at a more complex level[2].
The three types of cones, S, M, and L, respond best to short-, medium- and long-wavelength light, respectively. Information from the cones is passed to bipolar cells in the retina, which may be the cells in the opponent process that transform the information from cones. The information is then passed to ganglion cells, of which there are two major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers. Parvocellular cells, or P cells, handle the majority of information about color, and fall into two groups: one that processes information about differences between firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones. The first subtype of cells are responsible for processing red-green differences,and the second process blue-yellow differences. P cells also transmit information about intensity of light (how much of it there is) due to their receptive fields.

2/29/08
A Lab color space is a color-opponent space with dimension L for luminance and a and b for the color-opponent dimensions, based on nonlinearly-compressed CIE XYZ color space coordinates.
The coordinates of the Hunter 1948 L, a, b color space are L, a, and b.[1][2] However, Lab is now more often used as an informal abbreviation for the CIE 1976 (L*, a*, b*) color space (also called CIELAB, whose coordinates are actually L*, a*, and b*). Thus the initials Lab by themselves are somewhat ambiguous. The color spaces are related in purpose, but differ in implementation.
Both spaces are derived from the “master” space CIE 1931 XYZ color space, which can predict which spectral power distributions will be perceived as the same color (see metamerism), but which is not particularly perceptually uniform.[3] Strongly influenced by the Munsell color system, the intention of both “Lab” color spaces is to create a space which can be computed via simple formulas from the XYZ space, but is more perceptually uniform than XYZ.[4] Perceptually uniform means that a change of the same amount in a color value should produce a change of about the same visual importance. When storing colors in limited precision values, this can improve the reproduction of tones. Both Lab spaces are relative to the white point of the XYZ data they were converted from. Lab values do not define absolute colors unless the white point is also specified. Often, in practice, the white point is assumed to follow a standard and is not explicitly stated (e.g., for “absolute colorimetric” rendering intent ICC L*a*b* values are relative to CIE standard illuminant D50, while they are relative to the unprinted substrate for other rendering intents).[5]
CIELAB is calculated using cube roots, and Hunter Lab is calculated using square roots.[6][clarify] Except where data must be compared with existing Hunter L,a,b values, it is recommended that CIELAB be used for new applications.[6]
Unlike the RGB and CMYK color models, Lab color is designed to approximate human vision. It aspires to perceptual uniformity, and its L component closely matches human perception of lightness. It can thus be used to make accurate color balance corrections by modifying output curves in the a and b components, or to adjust the lightness contrast using the L component. These transformations are difficult or impossible in the RGB or CMYK spaces, which model the output of physical devices, rather than human visual perception.
Because Lab space is much larger than the gamut of computer displays, printers, or even human vision, a bitmap image represented as Lab requires more data per pixel to obtain the same precision as an RGB or CMYK bitmap. In the 1990s, when computer hardware and software was mostly limited to storing and manipulating 8 bit/channel bitmaps, converting an RGB image to Lab and back was a lossy operation. With 16 bit/channel support now common, this is no longer such a problem.
Additionally, many of the “colors” within Lab space fall outside the gamut of human vision, and are therefore purely imaginary; these “colors” cannot be reproduced in the physical world. Though color management software, such as that built in to image editing applications, will pick the closest in-gamut approximation, changing lightness, colorfulness, and sometimes hue in the process, author Dan Margulis claims that this access to imaginary colors is useful going between several steps in the manipulation of a picture.[7]

CMYK (short for cyan, magenta, yellow, and key (black),[1] and often referred to as process color or four color) is a subtractive color model, used in color printing, also used to describe the printing process itself. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the acronym.[2]
The CMYK model works by partially or entirely masking certain colors on the typically white background (that is, absorbing particular wavelengths of light). Such a model is called subtractive because inks “subtract” brightness from white.
In additive color models such as RGB, white is the “additive” combination of all primary colored lights, while black is the absence of light. In the CMYK model, it is just the opposite: white is the natural color of the paper or other background, while black results from a full combination of colored inks. To save money on ink, and to produce deeper black tones, unsaturated and dark colors are produced by substituting black ink for the combination of cyan, magenta and yellow.

The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.
The term RGBA is also used to mean Red, Green, Blue, Alpha. This is not a different color model, but a representation; the Alpha is an additional channel (not component) used for transparency.
The RGB color model itself does not define what is meant by ‘red’, ‘green’ and ‘blue’ colorimetrically, and so the results of mixing them are not specified as exact, but relative.
When the exact chromaticities of the red, green, and blue primaries are defined, the color model then becomes an absolute color space, such as sRGB or Adobe RGB; see RGB color spaces for more details.

Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, the word is sometimes used to mean electromagnetic radiation of all wavelengths.[1] The elementary particle that defines light is the photon. The three basic properties of light (i.e., all electromagnetic radiation) are:
• Intensity, or alternatively amplitude, which is related to the perception of brightness of the light,
• Frequency, or alternatively wavelength, perceived by humans as the color of the light, and
• Polarization (angle of vibration), which is only weakly perceptible by humans under ordinary circumstances.
Due to its wave–particle duality, light can exhibit properties of both waves and particles. The study of light, known as optics, is an important research area in modern physics.

Main article: Speed of light
The speed of light in a vacuum is exactly 299 792 458 m/s (fixed by definition). Although this quantity is sometimes referred to as the “velocity of light”, the word velocity refers to a vector quantity, which has a direction (and speed refers to the magnitude of the velocity vector).
The speed of light has been measured many times, by many physicists. Though Galileo attempted to measure the speed of light in the 1600s, the best early measurement in Europe was by Ole Rømer, a Danish physicist, in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io’s orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth’s orbit. If he had known the diameter of the orbit (which he did not) he would have deduced a speed of 227 000 km/s.
The first successful measurement of the speed of light in Europe using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313 000 km/s.
Léon Foucault used rotating mirrors to obtain a value of 298 000 km/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault’s results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299 796 km/s. This was close to the modern value of 299 792 458 m/s.

There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to “red hot” or “white hot”. The blue color is most commonly seen in a gas flame or a welder’s torch.
Atoms emit and absorb light at characteristic energies. This produces “emission lines” in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.
Acceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.
Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.
Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in fluorescent lights. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.
Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions.
Certain other mechanisms can produce light:
• scintillation
• electroluminescence
• sonoluminescence
• triboluminescence
• Cherenkov radiation
When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
• radioactive decay
• particle–antiparticle annihilation

Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. Spectrometry is the measurement of these interactions and an instrument which performs such measurements is a spectrometer or spectrograph. A plot of the interaction is referred to as a spectrogram, or, informally, a spectrum.
Historically, spectroscopy referred to a branch of science in which visible light was used for the theoretical study of the structure of matter and for qualitative and quantitative analyses. Recently, however, the definition has broadened as new techniques have been developed that utilise not only visible light, but many other forms of radiation.
Spectroscopy is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. Spectroscopy is also heavily used in astronomy and remote sensing. Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines.

Nature of radiation measured
The type of spectroscopy depends on the physical quantity measured. Normally, the quantity that is measured is an amount or intensity of something.
• Electromagnetic spectroscopy involves interactions with electromagnetic radiation, or light. Ultraviolet-visible spectroscopy is an example.
• Electronic spectroscopy involves interactions with electron beams. Auger spectroscopy involves inducing the Auger effect with an electron beam.
• Mechanical spectroscopy involves interactions with macroscopic vibrations, such as phonons. An example is acoustic spectroscopy, involving sound waves.
• Mass spectroscopy involves the interaction of charged species with a magnetic field, giving rise to a mass spectrum. The term “mass spectroscopy” is deprecated in favour of mass spectrometry, for the technique is primarily a form of measurement, though it does produce a spectrum for observation.
[edit] Measurement process
Most spectroscopic methods are differentiated as either atomic or molecular based on whether or not they apply to atoms or molecules. Along with that distinction, they can be classified on the nature of their interaction:
• Absorption spectroscopy uses the range of the electromagnetic spectra in which a substance absorbs. This includes atomic absorption spectroscopy and various molecular techniques, such as infrared spectroscopy in that region and nuclear magnetic resonance (NMR) spectroscopy in the radio region.
• Emission spectroscopy uses the range of electromagnetic spectra in which a substance radiates (emits). The substance first must absorb energy. This energy can be from a variety of sources, which determines the name of the subsequent emission, like luminescence. Molecular luminescence techniques include spectrofluorimetry.
• Scattering spectroscopy measures the amount of light that a substance scatters at certain wavelengths, incident angles, and polarisation angles. The scattering process is much faster than the absorption/emission process. One of the most useful applications of light scattering spectroscopy is Raman spectroscopy.

সূত্র: ইউকিপিডিয়া
তারিখ: নভেম্বর ২৭, ২০২১

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