A CCD can work with a single photon of light. Sadly, humans and all animals would be blinded if their eyes were that sensitive, so only technically work if TWO photons hit at almost the same time in the same place : http://webhome.phy.duke.edu/~hsg/264L/images/photons-on-retina.html Even then the retina filters out "noise" and looks for patterns in a multi layer neural net within the eye. Fun Fact : People with their lenses surgically removed (cataracts), can see more spectrum of light. Fun Fact 2 : Some females with vertical chimera of XX activating one X on one eye, and another X for the other eye, can have hypervision and see millions of more color shades of red than normal females. I cannot find a link, but it is true. Each X has to have the most divergent gene for red shade for each X. Humans have a variety of red chemical options, all close to true red.