Reciprocity Relations

Heisenberg uncertainty relation (1927) Bunson-Roscoe law: In a phototropic reaction, the product of time t and intensity I (thus the energy amount of the used light) corresponds to the stimulus strength. R = I × t It is of no importance whether a light stimulus of low intensity is applied for a longer period of time or whether a stimulus of high intensity is applied for a short time. (Bunsen: 1811-1899).

The Bunsen-Roscoe law applies only in a limited sense, since a certain minimal amount of light has to be present in order to trigger the reaction (threshold value), and an increase in light intensity does not always cause an increase in the phototropic reaction, but may in contrast suppress the positive reaction. A continuous increase causes a second positive reaction with a new maximum, that decreases again and rises once more (= 1., 2., and 3. positive phototropic bending). See http://www.biologie.uni-hamburg.de/b-online/e32/32b.htm

• Bloch's Law: A similar reciprocity in the visual system.
  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.

  Some units in the cat visual cortex fail to respond to a briefly flashed bar and it has been suggested that such neurons function as visual integrators with a long time constant. To test this integrator hypothesis, a study was made using presentations of a bar, flashed over the receptive field for various durations and at different luminances. Some cortical cells indeed showed an increase in the time to peak latency and in the response amplitude when stimulus duration was prolonged up to 320 msec. Such units obeyed Bloch's law for durations over 100 msec.

  There is the corresponding rule in photography: the reciprocity between how much light you let into the camera and for how long you allow it to enter.
  Both: http://www.yorku.ca/eye/blochlaw.htm