Emission theory (vision)

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Emission theory or extramission theory was 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 This theory has been replaced by intromission theory, which is that visual perception is accomplished by rays of light reflected from objects into the eyes. 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. The confusion over the direction of propagation is likely because light moves far too quickly for human perception or any other physical process known in classical times to directly observe its motion.

Emission theory is attributed to Empedocles and was held by Plato and Euclid [1]. 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.^{[citation needed]}

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.)

Winer et al. (2002) have found recent evidence that as many as 50% of American college students believe in emission theory [2].

Modern ray-tracing computer programs 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 cannot be "seen".

References

Winer, G. A., Cottrell, J. E., Gregg, V., Fournier, J. S., & Bica, L. A. (2002). Fundamentally misunderstanding visual perception: Adults' beliefs in visual emissions. American Psychologist, 57, 417-424.

See also

• Ray casting