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. Be aware that the diffraction-like spreading of light is due to the limited diameter of a light beam, not the interaction with an aperture.The Rayleigh Criterion The Rayleigh Criterion The acuity of our vision is limited because light passes through the pupil, the circular aperture of our eye. There are many situations in which diffraction limits the resolution. This limit is an inescapable consequence of the wave nature of light. If they were closer together, as in Figure 1(c), we could not distinguish them, thus limiting the detail or resolution we can obtain. The pattern is similar to that for a single point source, and it is just barely possible to tell that there are two light sources rather than one. How does diffraction affect the detail that can be observed when light passes through an aperture? Figure 1(b) shows the diffraction pattern produced by two point light sources that are close to one another. (c) If they are closer together, they cannot be resolved or distinguished. (b) Two point light sources that are close to one another produce overlapping images because of diffraction. (a) Monochromatic light passed through a small circular aperture produces this diffraction pattern. The effect is most noticeable when the aperture is small, but the effect is there for large apertures, too. Light from different parts of the circular aperture interferes constructively and destructively. This pattern is caused by diffraction similar to that produced by a single slit. Instead of a bright spot with sharp edges, a spot with a fuzzy edge surrounded by circles of light is obtained. Figure 1(a) shows the effect of passing light through a small circular aperture. While this can be used as a spectroscopic tool-a diffraction grating disperses light according to wavelength, for example, and is used to produce spectra-diffraction also limits the detail we can obtain in images. Light diffracts as it moves through space, bending around obstacles, interfering constructively and destructively.
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