The diffraction integral makes it possible to calculate the diffraction of light in optics through an aperture of any shape. In particular, the intensity of the light incident at a point on the observation screen is calculated on the basis of an incident elementary wave and the diaphragm function , which describes the light transmittance of the diaphragm.
Two borderline cases of the diffraction integral are the approximations for the far field ( Fraunhofer diffraction ) and for the near field ( Fresnel diffraction ). See the relevant subsections.
Experimental setup for diffraction of light at a diaphragm
The sketch opposite shows the experimental arrangement, consisting of a light source , a diaphragm , on which the incident light is diffracted, and an observation screen on which the incident light intensity is examined. The shape and properties of the diaphragm determine how the intensity distribution looks on the observation screen.
Has the aperture z. B. the shape of a double slit , the known interference pattern results as the intensity distribution . Further applications of the diffraction integral are e.g. B. Diffraction disks and Klotoids .
Kirchhoff's diffraction integral
Sketch for the Fraunhofer / Fresnel approximation of the diffraction integral
The Kirchhoff diffraction integral , also called Fresnel-Kirchhoff diffraction integral , reads
the amplitude of the source,
the magnitude of the wave vector ,
the wavelength of light,
an infinitesimal surface element of the diaphragm,
the aperture function ,
the slope factor and finally
the amplitude at the point on the observation screen.
Since the distances and in most applications are sufficiently perpendicular to the diaphragm, the inclination factor can be set equal to one in these cases. In this case, or are the angles between the lines marked with and and a perpendicular to the diaphragm plane at the intersection of the lines.
The intensity at the point results from the square of the absolute value of
Fraunhofer and Fresnel diffraction
The principle of Fresnel diffraction explained using a slit diaphragm
The principle of Fraunhofer diffraction explained using a slit diaphragm
The principle of Fraunhofer diffraction explained using a lens system and a slit diaphragm
The geometric relationships apply to the light paths and (see sketch)
Under the assumptions and , the roots can be approximated by a Taylor expansion.
This approximation corresponds precisely to the case that , i. That is, for these considerations, the inclination factor can be set approximately equal to 1. The diffraction integral is thus
Furthermore, because of the approximation, the denominator can be set. The exponent contains the phase information that is essential for the interference and must not be simplified in this way. It follows
The approximation for the expressions and , explicitly carried out up to the 2nd order, results in
Expressed by the coordinates and gives that
The Fraunhofer approximation corresponds to a far-field approximation, which means that not only the aperture is assumed to be small, but also the distance to the observation screen is assumed to be large. The Fourier transform of the diaphragm function essentially results as the diffraction integral . This is why one also speaks of Fourier optics in the context of Fraunhofer diffraction .
According to these assumptions, only terms that are linear in and , that is, are considered
In this case the diffraction integral is simplified to
If a new wave vector is defined , the integral is
This is just the Fourier transform of the aperture function .
The Fresnel approximation corresponds to a near-field approximation. It also takes account of quadratic terms in the exponent. The diffraction integral, brought into the form of a Fourier transform, can then generally no longer be solved analytically by an additional term, but only numerically .
Taking into account quadratic terms in and results
In this case the diffraction integral is
Introduction of with and then results in the diffraction integral in near-field approximation
From the source with amplitude at comes the spherical wave , the amplitude of which decreases reciprocally with distance ( ). Wave vector times distance gives the phase shift of the wave at the location , angular frequency times time the phase shift to time . The wave is described by the phase at the location at the time :
At the point at , the wave meets the diaphragm at a distance . Let it be the field distribution of the wave at the point .
According to Huygens' principle , the point is the starting point of an elementary wave, the secondary wave .
The amplitude of is proportional to the source amplitude and the aperture function . The aperture function indicates the permeability of the aperture. The simplest case is when the shutter is open and when the shutter is closed. is the infinitesimal surface element of the aperture at the point .
The secondary wave generates the wave intensity at point at on the screen . It is infinitesimal because only the contribution from and not all other points on the diaphragm are considered.
The time dependency can be neglected, as it disappears later when calculating with intensities anyway due to the time averaging. By inserting you get:
A secondary wave emanates from every point on the diaphragm. The intensity in the observation point is generated by the superposition of all individual contributions:
This equation is already very reminiscent of the diffraction integral given above. With the proportionality factor we get (inclination angle neglected):