Glossary: R

Rack and Pinion

A mating gear pair. The rack is a flat, straight bar (track) with teeth to engage (mate) with the gear teeth of the pinion gear, in order to move in a straight-line orientation. Typically the gear teeth are parallel, however for a helical rack the teeth are angled. The pinion is the smaller gear of the two. 

  See also Helical Gear

Radiant Sensitivity

The electrical current generated per watt of incident optical power. Expressed in A/W.

Radius of Curvature

The directed distance from the vertex of a surface to the center of curvature.

Reference Wavelength

A wavelength, for example 632.8nm, for measuring the accuracy of an optical flat. A 1/20λ flat will have a maximum peak to valley deviation of 632.8/20 or 31.64nm. One wave is 632.8nm.


The change in direction of radiation by a surface without a change in the wavelength.

  See also Transmission , Parity

Reflection Factor (Pd)

Ratio of the directionally reflected flux to that reflected in the same direction by a perfect reflecting diffuser identically irradiated or illuminated.


The term given for the bending of light rays that occurs at the interface between two materials with different indices of refraction. A higher index of refraction results in a lower refracted angle.


An instrument used to measure the refractive index of solids and liquids.

Relative Brightness

A figure of merit corresponding to the amount of light seen by a viewer through binoculars, obtained by squaring the diameter of the exit pupil in millimeters. A higher number indicates a brighter image in low-light situations.

Relative Efficiency

Also referred to as groove efficiency of diffraction gratings, it expresses the energy diffracted into the required order as compared to the efficiency of the coating itself at the same wavelength. For example, a grating that yields 40% diffraction into a specific order at 500nm compared to a mirror with the same coating that reflects 80% of light at 500nm would have a relative efficiency of 50%. Theoretically, the relative efficiency for a grating can appear very high, yet, the actual piece could yield very low diffraction into the desired order if the coating reflectivity is low at the wavelength region of interest. 

Relative Illumination (RI)

A measure of a lens's uniformity of sensor illumination across the lens's full image circle for a uniform object illumination, defined radially.

Relay Lens

A lens or combination of lenses used to transfer an image from one point to another at a specified magnification ratio.


In positioning systems, the ability to return to the same position by repeating the same adjustments or by providing the same inputs.

  See also Ball Bearing Slide


The ability to distinguish object detail. It is often expressed in terms of line pairs per millimeter (lp/mm), or microns (μm). 

Resolving Power

The measure of an optical system's ability to produce an image which separates two points or parallel lines on the object.


A measure of the effectiveness of a detector in producing an electrical signal. The product of the light input in Watts and the responsivity result in the predicted output of the detector in Amps. It varies with temperature.


The degree of angular shift in the phase of incoming polarized light.

  See also Retarder (Waveplate)

Retarder (Waveplate)

A class of materials that exhibit the characteristic of birefringence. Orthogonal components of light that enter a retarder (or waveplate) will experience a relative phase shift upon output. This phase shift is dependent upon the thickness of the retarding medium and the degree of birefringence. The axes of the material exhibit different index characteristics and are generally referred to as the fast (low index) and slow (high index) axes.


A planar, or flat, element located at the image plane of an optical system designed to measure the object imaged. The element is typically made of glass, and can have a wide variety of lines, circles, or patterns either etched or chrome deposited on it. Microscope eyepieces accept reticles while magnifiers or comparators accept contact reticles; the former must be placed close to the eye or sensor, whereas, the latter close to the object under inspection.


An optical component designed to reflect light back in the direction from which it originated. A trihedral prism is often used as a type of retroreflector.

Right Angle Prism

An image reflection prism that deviates the ray path by 90°. It is the most popular type of prism and is often used for image/ beam displacement.

Ring Light Guide

An illumination component comprised of a bundle of optical fibers that terminates in a circular pattern, canted slightly inward towards the axis of the circle. It provides intense, near on-axis directional illumination for shadow-free viewing through the ring.


In relation to rugate notch filters, it is a small variation in the transmission flatness within a specified spectral region.

  See also Rugate Notch Filter

Rise Time

The time it takes for a signal to rise from a defined minimum to a fraction of the maximum signal. The low and high points are commonly defined as 10% to 90% of the maximum signal output.

RMS Noise

The unwanted part of electrical output that is not consistent with the radiation signal.


In relation to movement, it is the angular rotation about the longitudinal axis (plane of translation), typically the X-axis for X-Y-Z configurations.

  See also Gimbal , Moment Load , Yaw , Pitch

Rotating Focusing Mechanism

A mechanical method of adjusting the focus of an optical assembly, such as an imaging lens or laser beam expander, in which optical components are rotated during translation. Rotating focusing mechanisms are simple and inexpensive, but they can suffer from image runout or beam wander due to the rotation of the elements. Sliding focusing mechanisms, which translate optical elements without rotation, reduce image runout and beam wander but are typically more expensive due to their more complicated mechanics. Threaded focusing tubes are a common type of rotating focusing mechanism.

Rugate Notch Filter

A type of notch filter constructed with a single layer of thin film in which the refractive index varies continuously with position in the direction perpendicular to the substrate plane.

  See also Notch Filter , Filter , Ripple

Ruled Grating

A type of diffraction grating with a sawtooth profile that is generated by precision mechanical ruling engines. Replicas are then generated by depositing a coating, an epoxy layer, and polished substrate onto the master. Although the gratings are generally cut from larger pieces and have grooves across the entire face, incident light should be confined to 90% of the full face during use.


In a linear stage, any deviation from the desired translation across a flat, straight line.

  See also Ball Bearing Stage





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