What is a Collimator?

The collimator is a beam direction device for changing the diverging light or other radiation from a point source into a parallel beam as shown in the figure below. An optical collimator consists of a tube containing a convex lens at one end and an adjustable aperture at the other, the aperture being in the focal plane of the lens. Radiation entering the aperture leaves the collimator as a parallel beam so that the image can be viewed without parallax. Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight. Collimators can be used for reducing the spatial cross-section of a light beam, thereby, making it narrower. Collimators that are used to converge light beams are referred to as optical collimators, while those that are used to collimate energy particles are called neutron, gamma, or X-ray collimators.

The collimator may be a telescope with an aperture at the principal focal length of the lens. Light from the luminous source is focused on this slit by another lens of similar focal length, and the slit then serves as the luminous object of the optical system.

Beam collimators are beam direction devices used in the x-ray tube housing, along with an arrangement of mirrors and lights, in such a way that the light and x-ray fields match each other. They are made of lead shutters which completely absorb the photons, and thus reduce the patient dose as well as focus the radiation accordingly to the area of interest. They allow different projections of x-ray fields. It is used in the x-ray tube housing, along with an arrangement of mirrors and lights, in such a way that the light and x-ray fields match each other. They are made of lead shutters which completely absorb the photons, and thus reduce the patient dose as well as focus the radiation accordingly to the area of interest. They allow different projections of x-ray fields.

The primary collimator may be constructed of depleted uranium as this material is approximately 1.6 times dense than lead. The secondary motorized collimators which greater define the beam shape are constructed of lead or tungsten. Multileaf collimators which are now in widespread use in medicine consist of two collimator banks of thin tungsten leaves with each bank consisting of 40 to 80 leaves. This allows each collimator leaf to move independently under computer control. Multileaf collimators allow even more diverse field shapes to be created which can shield organs at risk whilst allowing the complex shape of the tumor bed to be irradiated to allow maximum cell kill.

Reflective Collimators

As we know a collimator is a device that narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction i.e., make collimated light or parallel rays, or to cause the spatial cross-section of the beam to become smaller. The reflective collimators are based on a 90° off-axis parabolic mirror, which has a focal length that remains constant over a broad wavelength range. They are available with either a UV-enhanced aluminum coating, for high reflectivity in the 250 nm to 450 nm wavelength range, or a protected silver coating, for high reflectivity in the 450 nm to 20 µm wavelength range. Compact reflective collimators with a protective silver coating are available for easy integration into cage systems. All reflective collimator options are available with FC/PC, FC/APC, or SMA connectors.

In lighting, collimators are typically designed using the principles of non-imaging optics.

Applications

Optical collimators can be used to calibrate other optical devices, to check if all elements are aligned on the optical axis, to set elements at proper focus, or to align two or more devices such as binoculars or gun barrels and gunsights. A surveying camera may be collimated by setting its fiduciary markers so that they define the principal point, as in photogrammetry. Optical collimators are also used as gun sights in the collimator sight, which is a simple optical collimator with a crosshair or some other reticle at its focus. The viewer only sees an image of the reticle. Where a reticle is a series of fine lines or fibres in the eyepiece of an optical device, such as a telescope or microscope, or on the screen of an oscilloscope, used as a measuring scale or an aid in locating objects. They have to use it either with both eyes open and one eye looking into the collimator sight, with one eye open and moving the head to alternately see the sight and the target or with one eye to partially see the sight and target at the same time. Adding a beam splitter allows the viewer to see the reticle and the field of view, making a reflector sight.

Collimators may be used with laser diodes and CO2 cutting lasers. Proper collimation of a laser source with a long enough coherence length can be verified with a shearing interferometer.

X-ray, gamma ray, and neutron collimators

In X-ray optics, gamma-ray optics, and neutron optics, a collimator is a device that filters a stream of rays so that only those traveling parallel to a specified direction are allowed through. Collimators are used for X-ray, gamma-ray, and neutron imaging because it is difficult to focus these types of radiation into an image using lenses, as is routine with electromagnetic radiation at optical or near-optical wavelengths. Collimators are also used in radiation detectors in nuclear power stations to make them directionally sensitive. Söller collimator is used in neutron and X-ray machines. The upper panel shows a situation where a collimator is not used, while the lower panel introduces a collimator. In both panels, the source of radiation is to the right, and the image is recorded on the gray plate at the left of the panels. Without a collimator, rays from all directions will be recorded; for example, a ray that has passed through the top of the specimen to the right of the diagram but happens to be traveling in a downward direction may be recorded at the bottom of the plate. The resultant image will be so blurred and indistinct as to be useless.

For industrial radiography using gamma radiation sources such as iridium-192 or cobalt-60, a collimator beam limiting device allows the radiographer to control the exposure of radiation to expose a film and create a radiograph, to inspect materials for defects. A collimator in this instance is most commonly made of tungsten and is rated according to how many half-value layers it contains, i.e., how many times it reduces undesirable radiation by half. For instance, the thinnest walls on the sides of a 4 HVL tungsten collimator 13 mm (0.52 in) thick will reduce the intensity of radiation passing through them by 88.5%. The shape of these collimators allows emitted radiation to travel freely toward the specimen and the x-ray film while blocking most of the radiation that is emitted in undesirable directions such as toward workers.

In radiation therapy

Collimators (beam limiting devices) are used in linear accelerators used for radiotherapy treatments. They help to shape the beam of radiation emerging from the machine and can limit the maximum field size of a beam.

The treatment head of a linear accelerator consists of both a primary and secondary collimator. The primary collimator is positioned after the electron beam has reached a vertical orientation. When using photons, it is placed after the beam has passed through the X-ray target. The secondary collimator is positioned after either a flattening filter (for photon therapy) or a scattering foil (for electron therapy). The secondary collimator consists of two jaws that can be moved to either enlarge or minimize the size of the treatment field.

New systems involving multileaf collimators (MLCs) are used to further shape a beam to localize treatment fields in radiotherapy. MLCs consist of approximately 50–120 leaves of heavy, metal collimator plates which slide into place to form the desired field shape.

Limitations

Although collimators improve resolution, they also reduce intensity by blocking incoming radiation, which is undesirable for remote sensing instruments that require high sensitivity. For this reason, the gamma-ray spectrometer on the Mars Odyssey is a non-collimated instrument. Most lead collimators let less than 1% of incident photons through. Attempts have been made to replace collimators with electronic analysis.