Bloch's law observes that, for brief presentations, the product of luminance (or contrast) and duration at the detection threshold is constant.
Bloch's law states that detection occurs if the total luminance energy
is a constant that can vary with different viewing conditions, observer attributes, and adaptation levels.
Early measurements used single, isolated light flashes of varying duration and intensity to determine the boundary at which a viewer first reports seeing the flash.
When plotted against detection thresholds, these data typically exhibit a near-constant product of intensity and duration for short intervals.