Definition: (1) Of camera shutter: ratio of effective shutter time divided by actual time shutter is open. * Shutter efficiency drops with shorter shutter times and focal plane types are more efficient than betweeen-the-lens types. (2) Of appliances or devices, e.g. flash-guns, motor-drives: comparison of total power input to the usable power output. * For luminaires, light output is compared the electrical power input, therefore the units are lumens per watt. * By using suitable units, a ratio may be obtained.

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