retinoscopic artefact

retinoscopic artefact 

An error encountered in determining the refractive error by retinoscopy. This error is due to the fact that the retinal layer which reflects the retinoscopic light is not situated within the photoreceptors (where an image must be formed to appear in focus) but in front of them. Retinoscopic light is reflected by the vitreous-retina interface due to the difference in their refractive indices (that difference may not exist in some species and in old eyes). Consequently retinoscopic results tend to be more hyperopic or less myopic than is actually the case. The smaller the eye the greater the error because the thickness of the retina is nearly constant across species. This error is equal to about 0.25 D in young human adults, about 1 D in infant eyes and can reach some 8 D in very small animals. Syn. retinoscopic error; small eye artifact (so called because the effect is much greater for small eyes than for large eyes).