A compound eye is a visual organ found in certain arthropods (some insects and crustaceans). The compound eye consists of 12-1000 ommatidia, little dark/bright sensors. The image perceived by the arthropod is "recalculated" from the numerous ommatidia which point in slightly different directions. In contrast to other eye types, there is no central lens or retina. Though the resulting image is poor in resolution, it can detect quick movments and, in some cases, the polarization of light.
Each ommatidium consists of a lens and a rhabdom. The rhabdom consists of several visual receptor cells parallel to each other or slightly twisted. In some species which can see polarization, the cells undergo a half twist along their length, except for the polarization sensor, which is shorter.
There are three types of compound eyes. The typical eye has a lens focusing light from one direction on the rhabdom; light from other directions is absorbed by the dark wall of the ommatidium. The second type has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used by nocturnal insects. Nocturnal crustaceans use a third kind, which also has a transparent gap but uses corner mirrors instead of lenses.
Strepsiptera have a different kind of eye, in which each lens does form an image, and the images are combined in the brain.