Bloch s Law of temporal summation is analogous to Ricco s Law of spatial summation. In other words, when luminance is halved, a doubling in stimulus duration is required to reach threshold. When luminance is doubled, threshold can be reached in half the duration. Temporal summation data plotted as log L vs log t, showing where Bloch s Law applies. Critical duration is shorter for stimulus of high luminance as threshold is reached faster and slower for stimulus of low luminance as a longer period of time is required to sum the quanta to reach threshold.
Temporal summation ceases beyond the temporal integration time.
Above this value, threshold is dependent only on luminance rather than the product of luminance and duration. Temporal summation is also affected by other test variables such as background luminance and the size of the stimulus size. Critical duration is longer for brighter background and smaller test stimuli. When temporal summation data is plotted as log L . Luminance-duration versus time curves.