The Rayleigh Criterion

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.

Single slit

Circular aperture

Application to vision

If all parts of an imaging system are considered to be perfect, then the resolution of any imaging process will be limited by diffraction. 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°. 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. 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.


Diffraction concepts
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What is 20/20 Vision?

My understanding is that the old 20/20 standard was based on being able to read 20 mm high letters at 20 meters distance. If that is the nominal 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. The angular separation of two points of light 20 mm apart at 20 meters is

The Rayleigh criterion for diffraction-limited vision for an iris diameter of 5 mm and a wavelength of 500 nm is:

So 20/20 is about 8x the Rayleigh criterion. Ackerman reports that the data show:

Another example of the remarkable nature of human senses; the most acute vision is within about a factor of two of the physical limits imposed by diffraction!


Vision concepts

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