Optical Apertures

Optical apertures are structures which spatially limit the propagation of light. Some examples:

• The entrance aperture of a photographic objective or a telescope, for example, lets light enter only over a limited area and a limited angular range. Light propagation may be further restricted by internal limitations.
• Within an optical system, there may be optical components of limited size, such as mirrors and lenses with a certain diameter. Light outside that diameter can usually not get through the system.
• Various kinds of light sources have some limited output aperture, where usable light can emerge.
• One often intentionally uses so-called diaphragms or pinholes, i.e., structures with a hole, which allow light to propagate only through that opening. Outside that opening, there is often a material with a black coating which absorbs most incident light.
• There are other kinds of introduced optical apertures with different geometric shapes of the opening. Most frequently, one uses optical slits of various widths and lengths.

Optical apertures can have various helpful or detrimental effects on the performance of optical instruments. Some examples:

• When truncating laser beams, for example, hard apertures can cause features like diffraction rings and thus reduce the beam quality. See the article on diffraction for more details.
• Apertures also often limit the light throughput, for example to an image sensor, and need to be compensated with an increased exposure time. An aperture with that effect is called an aperture stop. It can also cause images to be vignetted, which means that the brightness is reduced more in the peripheral regions of the image.
• The entrance aperture of an optical telescope or photographic objective, for example, limits the achievable angular resolution through the effect of diffraction.
• In some situations, the image quality is increased by an optical aperture (e.g. a diaphragm in a photographic objective). For example, it may prevent light from propagating through the regions of a lens, where spherical aberrations would be excessive. Also, it may avoid light propagation with extreme angular directions. In other cases, it increases the depth of field and therefore the image quality, if objects at different distances are imaged simultaneously. See the article on imaging with a lens for more details.
• Apertures can also limit the field of view of an optical instrument. They are then called field stops. The field of view is not always sharply limited; there may be some vignetting.
• In a laser, an optical aperture, e.g. in the form of the limited size of the laser crystal, may limit the area over which laser radiation is generated. In some cases, an intracavity aperture suppresses laser operation on higher-order resonator modes and thus improves the laser beam quality.

Not all apertures in an optical system have an impact on the performance. It often happens that the finite size of an optical element is not relevant, since light can anyway not reach its outer regions due to the optical design.

Standard optical apertures, often made of blackened thin metal sheets or anodized aluminum, are unsuitable for high-power laser applications: The absorption of intense laser light can lead to thermal deformation, ablation, or melting of the aperture material.

High-power apertures are therefore designed to handle substantial heat loads. Strategies include:

• Reflective surfaces: Using highly reflective materials (e.g., copper, gold-coated metals, or dielectric coatings) to reject rather than absorb the blocked radiation.
• Thermal conductivity: Using substrates with high thermal conductivity (e.g., copper or ceramics) to dissipate absorbed heat effectively.
• Active cooling: Water-cooled aperture blocks are used in multi-kilowatt laser systems to safely dump the excess power.
• Conical geometry: Some pinholes or diaphragms are shaped conically to direct reflected light into a safe beam dump rather than back into the laser source.

While many apertures like pinholes have a fixed geometry, variable apertures allow for continuous adjustment of the opening size.

• Iris diaphragms: Modeled after the eye's iris, these use a set of overlapping metal leaves to create a roughly circular opening. Increasing the number of leaves typically improves the circularity. They are widely used in imaging and beam alignment but may have lower damage thresholds due to the thinness and thermal isolation of the leaves.
• Adjustable slits: These consist of two precision-ground blades (knife edges) moved by a micrometer screw. They are essential in spectroscopy to trade off resolution against light throughput.

Apertures of different kinds are used in many optical instruments. Some examples:

• As mentioned above, photographic objectives (as well as other types of objectives) often contain one or more apertures. A smaller aperture reduces the amount of incoming light within a certain exposure time, and it increases the depth of field.
• Small pinholes are often used for producing light with increased spatial coherence. For example, there are mode cleaners with a pinhole between two lenses.
• At the entrance and exit of a monochromator, e.g. of Czerny–Turner type, there is usually a narrow slit. It prevents the propagation of light on paths which would be possible for light outside the desired range of transmission wavelengths.
• Hard or soft apertures can be used for Kerr lens mode locking of lasers.

The term aperture is sometimes used for the size of an aperture rather than the object itself. Often, one specifies the diameter, or in the case of objectives the f-number (where larger values indicate smaller aperture diameters).