For detection of light, minimum amount of light (quanta) reaching retina should be:

"Visual threshold" is the minimum amount of light detected by retina. Threshold for a single rod is 1 quantum of light. To distinguish light from this discharge, at least 5-6 rods should be excited synchronously. Hence, about 5-6 quanta of light is the 'visual threshold' for retina. For 5-6 rods to be excited, about 100 quanta of light should reach the eye (cornea). Many of the quanta are absorbed, reflected, or refracted in the intervening ocular media.