Autocollimators are optical instruments that measure angular displacements with high sensitivity. They are used to align optical components and measure optical and mechanical deflections.
Autocollimators consist of a telescope and a collimator, a device that narrows a beam of particles or waves. They project a beam of collimated light to an external reflector that reflects all or part of the beam back into the instrument. This beam is then focused and sensed by a photodector. The difference between the emitted beam and the reflected beam is measured.
In this diagram of an autocollimator, the angle of the y-axis mirror displacement ( α ) is calculated using the formula α = Δy/2f where f = focal length of the autocollimator.
Image Credit: Vermont Photonics
Autocollimators are used by the optical industry and mechanical engineers in a variety of applications. Their specific functions include precision alignment, the detection of angular movement, the verification of angle standards, and angular monitoring over long periods.
Autocollimators can be used in the testing of
- parallelism with a collimator and telescope
- paque wedges and plane parallel plates
- transparent wedges
- angles in glass prisms
- the accuracy of rotary tables and index tables
- camera objectives
These optical instruments are also used in the flatness testing of reflecting surfaces, the control of wedges and plane parallel plates, and parallel measurement of cylindrical bores.
In addition to testing applications, autocollimators can be used to measure the
- radius of concave and convex spherical surfaces
- back focal of lenses and optical systems
- centration error of spherical surfaces
- centration error of lenses in transmission
- parallelism or perpendicularity of two surfaces
Additional applications include the
- radius measurement of concave and convex spherical surfaces
- relative measurement of the angular error of prisms
- pitch and yaw measurement of slides
- parallel setting of rolls
Finally, autocollimators can be used to measure straightness, flatness, roll angle, and the squareness between a vertical angle and a machine bed.
Types of Autocollimators
The Engineering360 SpecSearch database provides information about these types of autocollimators.
Visual autocollimators rely on the operator’s eye as the photodetector. This can result in varying resolution among operators, but most visual autocollimators can resolve from 3-to-5 arc-seconds. Visual autocollimators may also be used for measuring multiple surfaces at the same time, which makes them ideal in alignment applications such as checking parallelism among optics. With this type of autocollimator, operators view the reflected image through an eyepiece.
Application: Measurement of squareness of an outside corner by aperture sharing
Image Credit: Micro-Radian
Digital or Electronic autocollimators use an electronic photodetector to detect and reflect the beam. These instruments process the signal, which creates a calibrated angular output that can commonly be retrieved using a digital display and interface, or with analog outputs if available. Digital or electronic autocollimators are typically used for calibrating rotary tables, checking angle standards, providing angular feedback, and more. They are up to 100 times more accurate than a visual autocollimator. They increase repeatability and decrease measurement time, but they have difficulty resolving out-of-focus images.
Digital Autocollimator Diagram Image Credit: Micro-Radian
Laser Autocollimators use laser light sources. They are specialized instruments, which are ideal any time very low noise measurements are desired. Many are designed for measuring small test parts such as silicon components, miniature mirrors and fiber optic components.
Specialized autocollimators may also be available. These specialized tools include laser autocollimators, servo-controlled systems, and more. Other, unlisted autocollimator options may also be available.
For more information on how the fundamentals of collimation, please click here