Emission theory has at least two meanings:

• First, it refers to Newton's proposal that light is emitted from luminous objects in the form of particles or corpuscles . (See also Photon.)

• Second, it refers to the proposal that visual perception is accomplished by rays of light emitted by the eyes. This theory has been replaced by Intromission theory , which is that visual perception is accomplished by rays of light reflected from objects into the eyes. Emission theory in this second meaning is also called Extramission theory . It is attributed to Empedocles and was held by Plato and Euclid. Adherents of emission theory cited at least two lines of evidence for it. The custom of saluting is said by some to stem from the habit of Greek soldiers putting their hands up in front of their eyes to “shade” their eyes from the powerful “light” shining from the eyes of their commanders. The light from some animals’ eyes could also be seen in “darkness”. Adherents of intromission theory countered by saying that if emission theory were true, then someone with weak eyes should have his or her vision improved when someone with good eyes looks at the same objects. Some argue that Euclid's version of emission theory was purely metaphorical, highlighting only the geometrical relations between eyes and objects. Modern ray-tracing computer programs often incorporate emission theory, tracing lines of sight from the eyes to objects and thence to light sources to determine the colour and luminance of pixels in a simulated scene. Winer et al. have found recent evidence that as many as 50% of American college students believe in emission theory.

As a point of comparison and contrast, one should look at the function of most typical cameras, which percieve objects via the reception of light generated external to their own lenses.