As the test flash area increases, the number of quanta required for detection remains the same up to what size?

• A. 1 minute of arc
• B. 5 minutes of arc
• C. 20 minutes of arc
• D. 10 minutes of arc

Answer: (D)

Central idea: spatial summation in vision. When a flash is small, the visual system pools photons over the area that stimulates the retina, so the threshold amount of light is governed by Ricco’s law: the product of stimulus intensity and area stays constant at threshold. That means the total number of quanta needed to detect the flash remains the same as you increase the spot size, as long as you stay within the range where complete summation occurs. This range is about ten minutes of arc for typical human vision under dim-light conditions. So up to roughly ten minutes of arc, increasing the flash area doesn’t change how many photons are required for detection. Beyond that size, summation isn’t complete and the threshold quanta rise with area.