Definition: (1) Measure of darkness, blackening or 'strength' of image in terms of its ability to disperse or absorb light i.e. its opacity. * It is logarithm of ratio between light incident on the sample to the light transmitted or reflected by the sample; also the logarithm of opacity. * In practice, a change in density of 0.3 equals approximately a doubling in opacity i.e. the sample appears 'darker' by one stop. * Density measured in variety of ways i.e. illuminated with diffuse or direct, parallel, light and then all or only part of emerging light is measured. (2) The number of dots per unit area given by a print process such as laser or ink-jet printer. NB: This is always smaller than the printer's actual output resolution. (3) The number of sensors packed into a unit area e.g. CCDs per square millimetre.
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