Emission theory or extramission theory is the proposal that visual perception is accomplished by rays of light emitted by the eyes. This theory has been replaced by intromission theory, which states that visual perception comes from something representative of the object (later established to be rays of light reflected from it) entering the eyes. Modern physics has confirmed that light is physically transmitted by photons, from a light source such as the sun, to visible objects, and finishing with the detector, such as a human eye or camera.
In the fifth century BCE, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.
Around 400 BCE, emission theory was held by Plato.
In about 300 BCE, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asked how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.
In 55 BCE, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:
The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove. – On the nature of the Universe
Despite being similar to later particle theories, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye.
Ptolemy (c. 2nd century) wrote about the refraction of light, and developed a theory of vision that objects are seen by rays of light emanating from the eyes.
The dispute over the emission and Aristotelian intromission theories of vision began to settle during 11th century when the experiments of the Arabian physicist, Ibn al-Haytham (Alhazen, 965-1039) lent support to the intromissionist theory in his Book of Optics.
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 the eyes of some animals (such as cats, which modern science has determined merely have highly reflective 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.
Most argue that Euclid's version of emission theory was purely metaphorical, highlighting only the geometrical relations between eyes and objects. The geometry of classical optics is equivalent no matter which direction light is considered to be moving in, since light is modeled by its path, not as a moving object. (Direction of propagation is important, however, in the modern theory of special relativity.)
Measuring the speed of light was one line of evidence that spelled the end of emission theory as anything other than a metaphor.
Winer et al. (2002) have found recent evidence that as many as 50% of American college students believe in emission theory.
Modern computer graphics programs that use ray tracing often trace lines of sight from "eyes" to objects and thence to light sources to determine the colour and luminance of pixels in a simulated scene. This avoids the extra computation that would be required to trace rays which do not intersect with "eyes", and which by definition do not contribute to the simulated image.