![]() ![]() If the E on the chart (20/200) is 88mm high, then the 20/20 line would have letters of height 8.8 mm. If the above describes the standard for normal vision, what are the factors which limit the resolution of human vision? To examine whether diffraction is the limiting factor, it is interesting to compare this standard of resolution with the limits imposed by diffraction. A visual acuity of 20/200 with the best possible correction with lenses is a nominal condition for being considered legally blind. If you could just resolve the letters two rows up from the normal vision line at 20 feet, your acuity would be labeled 20/40 and if you could resolve two lines down it would be labeled 20/10.Īnother way of saying it is that if you vision is 20/40, you can just resolve at 20 feet what a person with normal vision could resolve at 40 feet. The nominal designations of visual acuity as a number ratio could be based on which lines you could read. The basic scheme is that the letters two rows down are half the size, and two rows up twice the size. The E on the chart has a standard height of 88 mm and the other letters are scaled accordingly. Where such units are used, normal vision came to be characterized by the fraction 20/20, which corresponded to being able to distinguish the letters on the fourth line up from the bottom at a distance of 20 feet. It was originally used at a standard distance of 6 meters, which in U.S. Hermann Snellen, a Dutch Ophthalmologist, in 1862. Visual acuity is typically measured with the use of a standard eye chart called the Snellen chart. This leads to the simplified statement that the limit of resolution of any imaging process is going to be on the order of the wavelength of the wave used to image it. This means that the wave is spread all the way to the plane of the slit and will not contain resolvable information about the source of the wave. Considering the single slit expression above, then when the wavelength is equal to the slit width, the angle for the first diffraction minimum is 90°. If all parts of an imaging system are considered to be perfect, then the resolution of any imaging process will be limited by diffraction. The Rayleigh criterion is the generally accepted criterion for the minimum resolvable detail - the imaging process is said to be diffraction-limited when the first diffraction minimum of the image of one source point coincides with the maximum of another. The Rayleigh Criterion The Rayleigh Criterion
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