Temporal Summation (Bloch's Law)

Camera buffs will recognize that where light is concerned that there is a certain reciprocity between how much light you let into the camera and for how long you allow it to enter. For example, to increase the depth of field in a picture (i.e. to make near and far objects in focus) you need to reduce the size of the input aperture. But if you do this you also need to increase the exposure time.

There is a similar reciprocity in the visual system, although it is not as broad as in photography. Under about 100 milliseconds (1/1000 of a second) stimulus duration it is possible to exchange the amount of light for the duration and maintain a constant effect. Chemists and physicists may recognize this as the Bunsen-Roscoe Law. In vision it is called Bloch's Law. Mathematically it is expressed as:

                                                                                 R = I x T

Where R stands for response, I for intensity and T for time.

Bloch's law is of great interest for the detailed, basic, scientific understanding of the visual process. It does not impact us very much on a daily basis. One way you can see how it can be important is with the use of lights of very short duration (e.g.. strobe lights). If we know, for example, that a steady, long duration light will be comfortably seen at a luminance of 100 cd/m2 then it probably would not be too visible if presented by a strobe light with a duration of a few milliseconds. Consequently, the intensity of the strobe would have to be increased considerably.

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