Luminance in the context of Chromaticity

In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor. It is determined by exposure time, lens f-number, and scene luminance. Exposure is measured in units of lux-seconds (symbol lx⋅s), and can be computed from exposure value (EV) and scene luminance in a specified region.

An "exposure" is a single shutter cycle. For example, a long exposure refers to a single, long shutter cycle to gather enough dim light, whereas a multiple exposure involves a series of shutter cycles, effectively layering a series of photographs in one image. The accumulated photometric exposure (H_v) is the same so long as the total exposure time is the same.

In digital recording, an audio or video signal is converted into a stream of discrete numbers representing the changes over time in air pressure for audio, or chroma and luminance values for video. This number stream is saved to a storage device. To play back a digital recording, the numbers are retrieved and converted back into their original analog audio or video forms so that they can be heard or seen.

In a properly matched analog-to-digital converter (ADC) and digital-to-analog converter (DAC) pair, the analog signal is accurately reconstructed, within the constraints of the Nyquist–Shannon sampling theorem, which dictates the sampling rate and quantization error dependent on the audio or video bit depth. Because the signal is stored digitally, assuming proper error detection and correction, the recording is not degraded by copying, storage or interference.

Lightness is a visual perception of the luminance ${\displaystyle (L)}$ of an object. It is often judged relative to a similarly lit object. In colorimetry and color appearance models, lightness is a prediction of how an illuminated color will appear to a standard observer. While luminance is a linear measurement of light, lightness is a linear prediction of the human perception of that light.

This distinction is meaningful because human vision's lightness perception is non-linear relative to light. Doubling the quantity of light does not result in a doubling in perceived lightness, only a modest increase.

In radiometry, photometry, and color science, a spectral power distribution (SPD) measurement describes the power per unit area per unit wavelength of an illumination (radiant exitance). More generally, the term spectral power distribution can refer to the concentration, as a function of wavelength, of any radiometric or photometric quantity (e.g. radiant energy, radiant flux, radiant intensity, radiance, irradiance, radiant exitance, radiosity, luminance, luminous flux, luminous intensity, illuminance, luminous emittance).

Knowledge of the SPD is crucial for optical-sensor system applications. Optical properties such as transmittance, reflectivity, and absorbance as well as the sensor response are typically dependent on the incident wavelength.

Color photography (also spelled as colour photography in Commonwealth English) is photography that uses media capable of capturing and reproducing colors. By contrast, black-and-white or gray-monochrome photography records only a single channel of luminance (brightness) and uses media capable only of showing shades of gray.

In color photography, electronic sensors or light-sensitive chemicals record color information at the time of exposure. This is usually done by analyzing the spectrum of colors into three channels of information, one dominated by red, another by green and the third by blue, in imitation of the way the normal human eye senses color. The recorded information is then used to reproduce the original colors by mixing various proportions of red, green and blue light (RGB color, used by video displays, digital projectors and some historical photographic processes), or by using dyes or pigments to remove various proportions of the red, green and blue which are present in white light (CMY color, used for prints on paper and transparencies on film).

Y′UV, also written YUV, is the color model found in the PAL analogue color TV standard. A color is described as a Y′ component (luma) and two chroma components U and V. The prime symbol (') denotes that the luma is calculated from gamma-corrected RGB input and that it is different from true luminance. Today, the term YUV is commonly used in the computer industry to describe colorspaces that are encoded using YCbCr.

In TV formats, color information (U and V) was added separately via a subcarrier so that a black-and-white receiver would still be able to receive and display a color picture transmission in the receiver's native black-and-white format, with no need for extra transmission bandwidth.

In photography, exposure value (EV) is a number that represents a combination of a camera's shutter speed and f-number, such that all combinations that yield the same exposure have the same EV (for any fixed scene luminance). Exposure value is also used to indicate an interval on the photographic exposure scale, with a difference of 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.

The EV concept was developed by the German shutter manufacturer Friedrich Deckel in the 1950s. Its intent was to simplify choosing among equivalent camera exposure settings by replacing combinations of shutter speed and f-number (e.g., 1/125 s at f/16) with a single number (e.g., 15).On some lenses with leaf shutters, the process was further simplified by allowing the shutter and aperture controls to be linked such that, when one was changed, the other was automatically adjusted to maintain the same exposure. This was especially helpful to beginners with limited understanding of the effects of shutter speed and aperture and the relationship between them. But it was also useful for experienced photographers who might choose a shutter speed to stop motion or an f-number for depth of field, because it allowed for faster adjustment—without the need for mental calculations—and reduced the chance of error when making the adjustment.

Darkness is the condition resulting from a lack of illumination, or an absence of visible light.

Human vision is unable to distinguish colors in conditions of very low luminance because the hue-sensitive photoreceptor cells on the retina are inactive when light levels are insufficient, in the range of visual perception referred to as scotopic vision.

A spectroradiometer is a light measurement tool that is able to measure both the wavelength and amplitude of the light emitted from a light source. Spectrometers discriminate the wavelength based on the position the light hits at the detector array allowing the full spectrum to be obtained with a single acquisition. Most spectrometers have a base measurement of counts which is the un-calibrated reading and is thus impacted by the sensitivity of the detector to each wavelength. By applying a calibration, the spectrometer is then able to provide measurements of spectral irradiance, spectral radiance and/or spectral flux. This data is also then used with built in or PC software and numerous algorithms to provide readings or Irradiance (W/cm2), Illuminance (lux or fc), Radiance (W/sr), Luminance (cd), Flux (Lumens or Watts), Chromaticity, Color Temperature, Peak and Dominant Wavelength. Some more complex spectrometer software packages also allow calculation of PAR μmol/m/s, Metamerism, and candela calculations based on distance and include features like 2- and 20-degree observer, baseline overlay comparisons, transmission and reflectance.

Spectrometers are available in numerous packages and sizes covering many wavelength ranges. The effective wavelength (spectral) range of a spectrometer is determined not only by the grating dispersion ability but also depends on the detectors' sensitivity range. Limited by the semiconductor's band gap the silicon-based detector responds to 200-1100 nm while the InGaAs based detector is sensitive to 900-1700 nm (or out to 2500 nm with cooling).

Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and relies on the context of the viewing environment (for example, see White's illusion).

Brightness is a subjective sensation of an object being observed and one of the color appearance parameters of many color appearance models, typically denoted as ${\displaystyle Q}$. Brightness refers to how much light appears to shine from something. This is a different perception than lightness, which is how light something appears compared to a similarly lit white object.

Contrast is the difference in luminance or color that makes an object (or its representation in an image or display) visible against a background of different luminance or color. The human visual system is more sensitive to contrast than to absolute luminance; thus, we can perceive the world similarly despite significant changes in illumination throughout the day or across different locations.

The maximum contrast of an image is termed the contrast ratio or dynamic range. In images where the contrast ratio approaches the maximum possible for the medium, there is a conservation of contrast. In such cases, increasing contrast in certain parts of the image will necessarily result in a decrease in contrast elsewhere. Brightening an image increases contrast in darker areas but decreases it in brighter areas; conversely, darkening the image will have the opposite effect. Bleach bypass reduces contrast in the darkest and brightest parts of an image while enhancing luminance contrast in areas of intermediate brightness.

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ideal diffuse reflecting surface is said to exhibit Lambertian reflection, meaning that there is equal luminance when viewed from all directions lying in the half-space adjacent to the surface.

A surface built from a non-absorbing powder such as plaster, or from fibers such as paper, or from a polycrystalline material such as white marble, reflects light diffusely with great efficiency. Many common materials exhibit a mixture of specular and diffuse reflection.

Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 10 cd/m). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision.

The human eye uses three types of cones to sense light in three bands of color. The biological pigments of the cones have maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green), and 564 nm (yellowish-green). The color of the pure signal of the cones could be described as violet, blue-green, and scarlet red, respectively, but, in their wavelengths of maximum absorption other cones are activated as well. The sensitivity ranges of the cone cells overlap to provide vision throughout the visible spectrum. The maximum efficacy is 683 lm/W at a wavelength of 555 nm (green). By definition, light at a frequency of 5.4×10 hertz (λ = 555.17. . . nm) has a luminous efficacy of 683 lm/W.

In colorimetry, CIECAM02 is the color appearance model published in 2002 by the International Commission on Illumination (CIE) Technical Committee 8-01 (Color Appearance Modelling for Color Management Systems) and the successor of CIECAM97s. It has since been superseded by CIECAM16.

The two major parts of the model are its chromatic adaptation transform, CIECAT02, and its equations for calculating mathematical correlates for the six technically defined dimensions of color appearance: brightness (luminance), lightness, colorfulness, chroma, saturation, and hue.

Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called saturation, chroma, intensity, or excitation purity. This number of parameters follows from trichromacy of vision of most humans, which is assumed by most models in color science.

The Hunt effect or luminance-on-colorfulness effect comprises an increase in colorfulness of a color with increasing luminance. The effect was first described by R. W. G. Hunt in 1952.

Hunt noted that this effect occurs at low luminance levels. At higher luminance, he noted a hue shift of colors to be more blue with higher luminance, which is now known as the Bezold–Brücke effect. The Hunt effect is related to the Helmholtz–Kohlrausch effect, where a saturated stimulus is seen to be brighter than less saturated or achromatic stimuli.