lux second
unit of exposure expressed in the metric system for illumination; equivalent to that produced by the illumination intensity of 1 candela at a distance of 1 meter for 1 second. (English equivalent is footcandle/second).
ISO speed ratings of a digital camera are based on the properties of the sensor and the image processing done in the camera, and are expressed in terms of the luminous exposure H (in lux seconds) arriving at the sensor.
The log exposure can be calculated easily as follows: E ´ T = H 80,000 (lux) ´ 1/100 (set) = 800 lux seconds The log exposure = the log of 800 or 2.90 When you convert exposure to logarithmic form, both density and exposure are on the same scale.
For the ISO 100 film, this requires -1.5 log lux-seconds (10^(-1.5) or 0.032 lux-seconds). Hmmm... the ISO 100 film took twice as many lux seconds to reach the same level of transmittance. So it is perhaps half as sensitive to light.
 See also: Light, Time, Film, Exposure, Photograph
  
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