pinholes (original) (raw)

Definition: circular holes, typically with diameters well below one millimeter

Category: general optics

• optical elements
  • optical apertures
    * field stops
    * diaphragms
    * pinholes
    * optical slits
    * aperture stops

Related: diaphragmsoptical aperturesoptical slits

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DOI: 10.61835/2qe Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Pinholes?

Optical pinholes are small circular holes through which light can be transmitted. The name of the term comes from the simple fabrication method of punching a thin metallic plate or foil with a pin. However, precision pinholes for optical applications are normally made by laser drilling. The hole diameter is typically well below 1 mm, sometimes even down to a few micrometers. Diaphragms typically have larger openings.

Applications of Pinholes

A pinhole can be used as a kind of small optical aperture:

• A pinhole is the simplest possible optical element for imaging; it can be used in a pinhole camera (camera obscura).
• A very small pinhole can be used for obtaining light with increased spatial coherence from incoherent light. Early experiments with interferometers often made use of that technique.
• In a mode cleaner (spatial filter), a pinhole is placed in the Fourier plane of a lens such that it can be used to suppress higher spatial frequency components. After a collimation lens, the beam profile is then smoother.
• In a laser resonator, a pinhole can cause substantial losses for higher-order resonator modes, while the losses for the fundamental mode may still be relatively small. The beam quality of the laser may therefore be improved.

For some of the applications, the power handling capability is essential. Such high-power apertures are usually made in the form of reflecting pinholes, which absorb only a minor portion of the laser radiation. They can be made of relatively resistant metals like stainless steel, molybdenum and tungsten. Ultra-thin pinholes are also made from platinum iridium. Some pinholes are equipped with a gold coating for improved power handling.

A relatively large thickness of the plate is beneficial in terms of heat conduction. However, the thickness may need to be reduced just around the hole to allow for operation with strongly divergent light. Note that even for diffraction-limited beams, the Rayleigh length becomes rather small in situations with tight focusing.

Pinholes for low-power applications often have a black coating so that they mostly absorb the non-transmitted part of the radiation.

Devices with Multiple Pinholes

There are various kinds of devices containing multiple pinholes:

Figure 1: A pinhole wheel.

• Pinhole wheels contain some number of pinholes of different sizes arranged along a circle, so that one can select one of the holes simply by rotating the wheel.
• Nipkow discs are rotating discs with hundreds or thousands of pinholes of equal size. They are used in confocal scanning microscopes.
• Two-dimensional pinhole arrays can contain a large number of pinholes on a rectangular grid. They may be used as Hartmann plates for analyzing optical systems.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is an optical pinhole?

An optical pinhole is a small circular hole, typically well below 1 mm in diameter, through which light can pass. While simple ones can be made with a pin, precision pinholes for optical applications are usually fabricated by laser drilling.

How does a pinhole work in a spatial filter?

In a spatial filter (or mode cleaner), a pinhole is placed in the focal plane of a lens. This setup blocks higher spatial frequency components of the light, effectively cleaning up the beam and resulting in a smoother beam profile after a second, collimating lens.

How can a pinhole improve the beam quality of a laser?

A pinhole placed inside a laser resonator can act as an aperture that introduces significant optical losses for higher-order modes. This forces the laser to operate on the fundamental mode, which has lower losses, thereby improving the output beam quality.

Why are some pinholes made of special metals or given a gold coating?

For high-power applications, pinholes need good power handling capability. They are often made of resistant metals like molybdenum or tungsten and can have a gold coating to be highly reflective, thus absorbing only a small fraction of the incident laser radiation.

What are Nipkow discs?

Nipkow discs are rotating discs that feature hundreds or thousands of pinholes of the same size. They are used in devices like confocal scanning microscopes.

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