We can also see a candle in complete darkness from 50 km away so just a handful of photos are enough for a sensation. "The researchers found that the emission

We can also see a candle in complete darkness from 50 km away so just a handful of photos are enough for a sensation. "The researchers found that the emission of only 90 photons could elicit visual experience. However, only 45 of these actually entered the retina, due to absorption by the optical media. Furthermore, 80% of these did not reach the fovea. Therefore, the human eye can detect as few as nine photons." http://en.wikipedia.org/wiki/Absolute_threshold http://en.wikipedia.org/wiki/Absolute_threshold