A rod is one of the photoreceptors that actually process the packets of light energy or photons that impact on the retina. There are two types of photoreceptor cells which, because of their shapes, are called rods and cones. Rods are sensitive enough to respond to a single photon, the basic unit of light, but together they create only one coarse, gray image, which is just adequate for seeing in poormycontentbreak light. Inside the human eye, there are eighteen times more rods than cones.