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CANDELA (CD). The luminous intensity, in the direction of the normal, of a blackbody surface 1/100,000 m2 in area, at the temperature of solidification of platinum (2042 K) under a pressure of 101,325 N/m2. (See CANDLE.)
CANDLE. (NEW UNIT). 1/60 of the intensity of 1 cm2 (100 mm2) of a blackbody radiator at the temperature of solidification of platinum (2042 K). To understand this a brief history may be in order: the first light standard was the standard candle, that was a candle of spermacel of which six weighted one pound with the candle burning at a rate of 6 grains per hour. Since this spermacel candle was not of sufficiently constant illuminating power to be of much scientific value, other standards were used.
In the USA, the Harcourt pentane lamp, in which air was drawn over the pentane liquid, wit the mixture burned in a standard burner and the flame adjusted to a definite height, with corrections made when atmospheric conditions varied from normal, became the standard. The illuminating power of such a lamp was equal to about 10 standard candles and was accepted as an international standard (defined as illuminating power equal to 1/10 of that of the Harcourt Lamp).
In Germany, the hefner lamp (constructed after a standard pattern and burning amyl acetate) was the standard and equal to 0.9 International ‘candles.
In France, the arcel lamp in which coiza oil was burned was the standard, and the Carcel Standard was 9.62 International Candles. For practical purposes, to measure the “quantity of light,” the intensity of illumination produced by a standard candle at a distance of 1 m was adopted and called a candlemeter. Analogously, there was the foot candle, with 1 candlemeter = 0.093 foot candles.
CANDLEPOWER. Light intensity of a light source. Also “candle” and a “lumer/steradian.” A source of one candle intensity at the center of a sphere emits a total radian flux of 12.57 (=4π) lumens. The total surface of the sphere is 12.57 steradians; thus one candle provides an intensity of 1 lumen/steradian.
CEILING EFFECT. Characteristic of behavioral data due to performance reaching its upper limit.
CENTRAL HEARING LOSS. Hearing impairment following damage in the auditory pathways or the auditory areas of the brain.
CHANNEL CAPACITY. By analogy to engineering usage, the concept of a theoretical upper limit on the amount of information that human beings can receive and process, usually within a given time. The term is most frequently applied to the sensory modalities and/or the central nervous system.
CHARACTER HEIGHT. Vertical distance, measured by physical length or number of visual picture elements (pixels), of a displayed alphanumeric symbol.
CHARACTER WIDTH. Horizontal distance, measured by physical length or number of visual picture elements (pixels), of a displayed alphanumeric symbol.
CHEMORECEPTOR. An end organ of the nervous system which triggers neural signaling activity in response to chemical stimuli, e.g., taste buds, olfactory end organs.
CHERNOFF DISPLAY. Display of multidimensional data using a facial representation.
CHOICE REACTION TIME. Amount of time required to make different responses to different stimuli. The time increment above what is needed for a response to a single stimulus is sometimes taken as a measure of the time to process the cognitive choice. (See REACTION TIME.)
CLO UNIT. A measure of the thermal insulation provided by clothing.
CLICK. Depress and release a button on a mouse or other computer input device.
CLOSED LOOP. System in which some function of system output is used as system input.
COGNITION. Higher mental activities typically involving the use of stored information or knowledge.
COGNITIVE ENGINEERING. The analysis and design of systems performing cognitive functions. Also, the application to cognitive psychology to the design of such systems.
COMMAND. Input to a computer or control system.
COMMAND LINE INTERFACE. Computer interface in which commands are entered as strings of alphanumeric characters.
COMMAND LANGUAGE. The elements and syntax of a set of computer commands.
COMPATIBILITY. How consistently the spatial movement or conceptual relationships of stimuli and responses meet human expectations. With reference to controls and displays, the naturalness of the movement of a control compared to what is displayed to the operator.
COMPENSATORY DISPLAY. In tracking tasks, a display on which either the input signal (target) or the output signal (cursor) is fixed and the other moves; the operator’s task in manipulating the controls is to eliminate or minimize system error by superimposing the cursor onto the target.
COMPUTER GRAPHICS. Manipulation and display of pictorial or graphical data on a computer display.
COMPUTER-AIDED INSTRUCTION. Use of computer technology to support learning and education.
COMPUTER SUPPORTED COOPERATIVE WORK (CSCW). Use of computer technology to support communication and collaboration in a group work environment.
CONCEPT TRAINERS. Aids used when the concepts to be learned are too complex to be absorbed in a few trials from verbal descriptions, and when the principles to be used in task performance can best be simulated by physical objects and real actions such as miniature representation of control components, and check points of a complex electronic system.
CONDUCTIVE HEARING LOSS. Hearing impairment due to interference with the transmission of sound waves through the external or middle ear.
CONES. Visual receptor cells found most densely toward the center of the retina. They are responsible for sharp vision and color discrimination, but cease functioning at low light intensities.
CONSISTENT MAPPING. An invariant relationship between stimuli and required response.
CONSONANCE. The phenomenon in which tones presented simultaneously produce a blended or pleasant sensation.
CONTINGENCY ANALYSIS. The part of a task analysis performed to identify non-routine situations with which a system may have to deal, in order to determine any special human performance required by such events.
CONTRAST. The brightness relationships of two non-specular surfaces adjacent to each other when the illumination of both the objects and the immediate surroundings are the same.
CONTROL. (1) A device, usually mechanical, electrical, or electronic, which directs the action of some mechanism or produces some change in the operation of a process or system. (2) The activity of bringing a system to a desired state, or maintaining a desired system state in the presence of external disturbances.
CONTROL LAYOUT. The spatial organization of controls at an interface or workplace.
CONTROL SYSTEM. A system performing a control function.
CONTROLLED SYSTEM. A system under the control of humans or automation.
CONTROL-DISPLAY RATIO. The ratio of movement of a control device to the movement of a display indicator.
CONTROL-FORCE CURVE. A function describing the relationship between the amount of force applied by the operator and the resulting displacement of a control.
CONVERGENCE. Coordinated directing of both eyes at the same point so as to obtain single vision by bringing the image of the object to the fovea of each eye.
COPPER-HARPER-SCALE. Ordinal scale used to indicate task difficulty or workload level.
CORIOLIS EFFECT. The difference in effective G force at different distances from the rotation axes in a short-axis rotating environment. When limbs or head move across a G force gradient, misperceptions of body orientation (tilting, rotation) or of limb position may be experienced, with concomitant physiological effects such as nausea or vertigo.
CREW STATION. Work Station within a vehicle cockpit.
CRITICAL FLICKER FREQUENCY. The minimum number of alternations per second at which a regularly intermittent light ceases to appear as flickering and fuses to yield a perception of steady light. Also called critical fusion frequency or flicker-fusion frequency.
CRITICAL INCIDENT TECHNIQUE. An approach to job analysis which calls for factual description of specific events or behavior. The technique is also applicable to other situations, such as accident investigation in which reports of errors or near-accidents may be collected.
CUE. A stimulus probabilistically related to a judgment or action.
CURSOR. A moving element on a display. On an instrument, it is used to show position discrepancy or error in reference to another “target” element. On a CRT display, it is used to show row and column positions or to indicate where the next activity will occur.
CUT AND PASTE. A text editing task involving the selection and removal of a text block, followed by insertion of the block at a new point in the text.
CYBERNETICS. Study of communication and control functions common to both living and engineered systems with the emphasis on attempting to understand organisms through making analogies to machines. (“I made up the word... from the Greek...'Steersman’...from the same root as 'governor’ and refers to control and communication in the animal and the machine.” - Norbert Wiener.)
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