Conditions For Observation
Light from a uniformly illuminated circular aperture (or from a uniform, flattop beam) will exhibit an Airy diffraction pattern far away from the aperture due to Fraunhofer diffraction (far-field diffraction).
The conditions for being in the far field and exhibiting an Airy pattern are: the incoming light illuminating the aperture is a plane wave (no phase variation across the aperture), the intensity is constant over the area of the aperture, and the distance R from the aperture where the diffracted light is observed (the screen distance) is large compared to the aperture size, and the radius of the aperture is not too much larger than the wavelength of the light. The last two conditions can be formally written as .
In practice, the conditions for uniform illumination can be met by placing the source of the illumination far from the aperture. If the conditions for far field are not met (for example if the aperture is large), the far-field Airy diffraction pattern can also be obtained on a screen much closer to the aperture by using a lens right after the aperture (or the lens itself can form the aperture). The Airy pattern will then be formed at the focus of the lens rather than at infinity.
Hence, the focal spot of a uniform circular laser beam (a flattop beam) focused by a lens will also be an Airy pattern.
In a camera or imaging system an object far away gets imaged onto the film or detector plane by the objective lens, and the far field diffraction pattern is observed at the detector. The resulting image is a convolution of the ideal image with the Airy diffraction pattern due to diffraction from the iris aperture or due to the finite size of the lens. This leads to the finite resolution of a lens system described above.
Read more about this topic: Airy Disk
Famous quotes containing the words conditions and/or observation:
“What is Americanism? Every one has a different answer. Some people say it is never to submit to the dictation of a King. Others say Americanism is the pride of liberty and the defence of an insult to the flag with their gore. When some half-developed person tramples on that flag, we should be ready to pour out the blood of the nation, they say. But do we not sit in silence when that flag waves over living conditions which should be an insult to all patriotism?”
—Anna Howard Shaw (1847–1919)
“Let observation with extensive view;
Survey mankind, from China to Peru;”
—Samuel Johnson (1709–1784)