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Movements, leaders, victories and the continuing fight for equality.
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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
Katherine Johnson’s mathematical calculations helped guide some of America’s most important early space missions while she confronted the racial and gender barriers faced by Black women in twentieth-century America.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
A color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with a natural or standard light source. Color rendering, as defined by the International Commission on Illumination (CIE), is the effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference or standard illuminant. The CRI of a light source does not indicate the apparent color of the light source; that information is given by the correlated color temperature (CCT). The CRI is determined by the light source's spectrum. An incandescent lamp has a spectrum approximating that of a black body emitter, while a fluorescent lamp has an irregular emission spectrum with multiple relatively narrow peaks, which suggests that the incandescent lamp should have the higher CRI. The value often quoted as "CRI" on commercially available lighting products is properly called the CIE Ra value, "CRI" being a general term and CIE Ra being the international standard color rendering index. Numerically, the highest possible CIE Ra value is 100 and would only be given to a source whose spectrum is identical to the spectrum of daylight, very close to that of a black body (incandescent lamps are effectively black bodies), dropping to negative values for some light sources. Low-pressure sodium lighting has a negative CRI; fluorescent lights range from about 50 for the basic types, up to about 98 for the best multi-phosphor type. Typical white-color LEDs have a CRI of 80 or more, while some manufacturers claim that their LEDs achieve a CRI of up to 98. CIE Ra's ability to predict color appearance has been criticized in favor of measures based on color appearance models, such as CIECAM02 and for daylight simulators, the CIE metamerism index. CRI is not a good indicator for use in visual assessment of light sources, especially for sources below 5000 kelvin (K). New standards, such as the IES TM-30, resolve these issues and have begun replacing the usage of CRI among professional lighting designers. However, CRI is still common among household lighting products.
Color reproduction is an aspect of color science concerned with producing light spectra that evoke a desired color, either through additive (light emitting) or subtractive (surface color) models. It converts physical correlates of color perception (CIE 1931 XYZ color space tristimulus values and related quantities) into light spectra that can be experienced by observers. In this way, it is the opposite of colorimetry. It is concerned with the faithful reproduction of a color in one medium, with a color in another, so it is a central concept in color management and relies heavily on color calibration. For example, food packaging must be able to faithfully reproduce the colors of the foods therein in order to appeal to a customer. This involves proper color calibration of at least four devices: Lighting, which must have a high color rendering index and not give a color cast to the object. Camera, which measures the reflected spectrum of the object and converts to a trichromatic color space (e.g. RGB). Screen, which reproduces color so a designer can proof the captured image and make color corrections as necessary. Printer, which reproduces the final color on paper.
Color quality scale (CQS) is a color rendering score – a quantitative measure of the ability of a light source to reproduce colors of illuminated objects. Developed by researchers at NIST the metric aims to overcome some of the issues inherent in the widely used color rendering index (CIE Ra, 1974). The color space used in CIE Ra (CRI) is outdated and nonuniform, and CQS uses CIELAB as a replacement. The Von Kries chromatic adaptation transform used by Ra does not perform as well as other available models. CQS uses CMCCAT2000. CIE Ra is based on desaturated samples, and a lamp's performance in rendering these samples faithfully is not necessarily linked to how it may perform with samples of higher saturation. CQS uses higher saturation samples. 'Pure' fidelity (where all deviations are considered bad) does not account for desired chromaticity changes. Increased saturation might be preferred. CQS does not penalise against increases in saturation. In CIE Ra the arithmetic mean is taken of the color differences for the individual samples. In CQS the individual results are combined through a root mean square instead, so that a small number of poorly rendered objects reflects with greater strength in the overall result. Negative values of CQS are made impossible due to their potential for consumer confusion. CCTs of lower than 2800K are penalised so that the CQS is more representative of their actual color rendering as opposed to their fidelity. The CQS generates the general index Qa. Several manufacturers are beginning to publish data on CQS scores of their products, including some who claim light sources with CQS scores up to 97.
A ring flash is a photographic flash that surrounds a camera lens with a circular source of light. Unlike conventional flashes which provide a point light source, a ring flash is intended to illuminate a subject with minimal shadows by closely and evenly surrounding the optical axis of the camera lens. This makes it a popular choice for several types of photography, including macro, portrait, and fashion photography. Closely related to the ring flash is the continuous ring light, which has a similar annular shape surrounding the lens; in contrast to the ring flash, which provides an intense light for a short duration, a ring light produces constant illumination at lower intensity for video recording or to see a live preview before capturing photographs. Ring lights and flashes vary in diameter and thickness, as well as the amount, color temperature, and quality (imperfectly indicated by color rendering index, or CRI) of light output. Some ring lights allow for the adjustment of brightness and color temperature.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →The Greenwood District.