Pinhole Diffraction

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Confocal microscopes allow to optically separate slices of the sample’s response and record theses slices as images. Optimally, the slices cover just the depth of focus generated in the microscope.

To achieve this task, the sample is scanned by a tiny light point. The smallest shape that can be generated is a diffraction limited spot. A diffraction limited spot in the focal plane of the microscope is the image of a spot-shaped light source and therefore called “point spread function, psf”. The psf is the distribution of light in the focus of an optical device when imaging a dimensionless spot. As all conventional light sources are usually not spot-shaped but have a significant extension, the light source is projected on a tiny aperture, the pinhole, acting as a spot-shaped source.

A real Airy disk created by passing a red laser beam through a 90- micrometre pinhole aperture with 27 orders of diffraction Airy disk captured by 2000 mm camera lens at f/25 aperture. Image size: 1×1 mm.

On the detection side, the sensor must follow in an equal manner. That is, the sensing region should as well be a diffraction limited spot, at all times coinciding with the illumination spot. This is achieved by a similar arrangement: the emission light is fed through a tiny aperture, the detection pinhole, before recorded by a sensing element. It is this detection pinhole, which usually is referred to when we mention the “pinhole” in a confocal microscope.

In the case of a pinhole, the light rays do not shoot straight by the rim of the hole, but bend around the edge. This wave effect creates a diffraction pattern of rings on the screen which resembles a bull's eye. That's for a flat wave single light source. If the aperture is illuminated by a scene, it acts as a lens to image the scene on a screen. I'm trying to produce a diffraction pattern with pinhole using a laser of 632nm wavelength.The pinholes' diameter are ranging from 10um, 25um, 50 um, 0.1mm and so on. I also have good lenses for. The pinhole diffraction phenomena in DPM was described in Section 2.2e in Ref. Additionally, the use of an LCD does not create obvious power loss in our experiments, which indicates the filling ratio of the liquid crystal cell is high. Modeling Pinhole Diffraction within System. Diffraction effects are typically considered only at the exit pupil of a system and including intra-system diffraction (especially when it is caused by multiple truncations), as discussed by M. Mout et al., is a challenging task.With the innovations in Fourier transforms, you have direct and flexible control of the diffraction inclusion in VirtualLab.

The term “confocal” refers to exact this arrangement: both illumination and detection are focused to the same spot [2]. The foci coincide. The image is then generated by scanning the field of view spotwise, like the electron beam in a tv-screen of the last century. The scanning is usually performed by an arrangement of pivotable mirrors.

As illustrated in Figure 1 [3], the detection pinhole removes all emission not originating from the focal plane. It is therefore also referred to as “spatial filter”, filtering the depth of focus and blocking extrafocal signal. As the device cuts out sections from the sample by optical means, it is also called the “optical knife”.

  • Applied Optics
  • Vol. 57,
  • Issue 4,
  • pp. 781-787
  • (2018)
  • https://doi.org/10.1364/AO.57.000781
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Pinhole Diffraction
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    Chen Wang, Yongying Yang, Yao Li, Yuankai Chen, and Jian Bai, 'Characterization of the pinhole diffraction based on the waveguide effect in a point diffraction interferometer,' Appl. Opt. 57, 781-787 (2018)
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Abstract

The nearly ideal spherical wavefront generated by pinhole diffraction is the key factor determining the achievable accuracy in point diffraction interferometers (PDIs), as it is employed as the reference wavefront. A comprehensive characterization of the diffraction of a pinhole at the operating-wavelength scale that is normally adopted in PDI is given. The incident light is coupled into the pinhole, which functions as a cylindrical waveguide, and is diffracted in the end. The field in the pinhole is analyzed based on mode theory and the diffraction wave in the far field is derived from the field equivalence principle. The diffraction wave is characterized by the light transmittance, the polarization, and the wavefront aberration, which are all determined by the properties of the mode in the pinhole. The diameter of the pinhole should not be smaller than 0.6λ to make the transmittance sufficient. With a linearly polarized incident light, the diffraction wave is elliptically polarized, and the wavefront aberration is dominated by the astigmatic component. The method explicitly reveals the physical mechanism of pinhole diffraction. The analytic solutions are fast to compute, easy to analyze, and intuitively show the diffractive properties of the pinhole. The conclusions are significant for insight into the nature of pinhole diffraction and provide theoretical reference for analysis of numerical results and the design of PDI systems.

© 2018 Optical Society of America

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