exposure(redirected from threshold exposure)
Also found in: Dictionary, Thesaurus, Medical, Financial.
in photography, the quantity of illumination H (a photometric quantity), which serves as an evaluation of the surface density of the luminous energy Q. It determines the effect of optical radiation on the photographic material used.
In the general case, H = dQIdA = ∫Edt, where A is the illuminated area, E is the illuminance, and I is the duration of irradiation (exposure time). If E is a constant, then H = Et. In the SI system (seeINTERNATIONAL SYSTEM OF UNITS), exposure is expressed in lux-seconds (lx-s). Beyond the limits of the visible portion of the radiation spectrum, the quantity used is the energy exposure, which is the product of the irradiance and the duration of irradiation; it is expressed in joules per m2 (J/m2).
It is convenient to use the concept of exposure if the effect of radiation is cumulative over time (in photography as well as, for example, in photobiology). The concept is widely used in work with nonoptical and even corpuscular radiation, such as X rays and gamma rays (where the exposure is defined as the product of the surface density of the radiation flux and the duration (), as well as streams of electrons and other particles (where the exposure is equal to the product of the radiation dose rate and t). (See alsoSENSITOMETRY and CHARACTERISTIC CURVE.)
A. L. KARTUZHANSKH
ii. The act of exposing a light-sensitive material to a light source.
iii. One individual picture of a strip of photographs, usually called a frame.
exposure(1) The degree to which information can be accessed using authorized or unauthorized methods. See penetration test and risk analysis.
(2) In a camera, the amount of light that reaches the film (analog) or CCD or CMOS sensor (digital). The exposure is achieved by the sum of the shutter speed, aperture (f-stop) and ISO setting. See shutter speed, f-stop and ISO speed.