Glossary: T

Tangent Arm Drive

A type of drive mechanism for rotary stages in which angular rotation is controlled by three knobs. The release knob disengages the shaft from the drive, freeing the stage to be rotated by hand to a desired location. The release knob is then tightened to re-engage the drive mechanism and transfer control to the adjustment knob which, when rotated, produces precise angular positioning of the shaft and stage top.

Tangential Plane

The component of the spot (or ray bundle) that lies parallel to the tangent line of a circle in the z-plane with diameter of the image height at that field point.

  See also Sagittal Plane

Telecentric Lens

A type of imaging lens in which the exit pupil is located at infinity. As a result, within a certain range of working distances there is no viewing angle error or magnification error associated with features in the image that are at different heights or angles.

  See also Magnification


The ability of a lens to maintain a constant magnification under changes in working distance. Quantified by the maximum angular deviation of a chief ray from parallel to the optical axis.

Tempered Glass

Glass that is heat treated after fabrication in order to make it more durable and resistant to heat and thermal shock. Since heat absorbing filters are placed in front of intense light sources, they are typically tempered to increase their resistance to breaking.

Threshold Current

The minimum driving current corresponding to lasing threshold at a specified temperature.

  See also Dark Current


Determines the light collecting efficiency of a lens or optical system. High throughput means high transmitted intensity while low throughput means low transmitted intensity.

Thrust Capacity

For ball bearing stages, the maximum force or load that can be applied in the direction of travel without damage to positioning stage components. The mounting surface (stage carriage) is pressed against the drive mechanism by means of a spring. As a result, the maximum thrust which the stage assembly can maintain is different when pressing towards the spring versus away from it. When pressing towards the spring, the force is taken up by the drive mechanism (i.e. micrometer). While pulling away, the force is being held in place by the spring. It is specified as Ta and Tb: Ta refers to the load capacity against the micrometer and Tb is the spring load capacity.

Time Delay Integration (TDI)

A type of line scanning where multiple linear arrays are placed side by side. After the first array is exposed, the charge is transferred to the neighboring line. When the object moves the distance of the separation between lines, a second exposure is taken on top of the first, and so on. Thus, each line of the object is imaged repeatedly, and the exposures are added to each other.


In terms of mechanical components, it is a mounting thread denoted by M42 x 0.75mm. T-mount components have a 42mm diameter screw thread and a 0.75mm thread pitch; the outer diameter is 48mm. In terms of imaging, it is commonly called a T2 thread or simply denoted by M42 x 0.75mm. The flange distance varies.

  See also CS-Mount , F-Mount , C-Mount , S-Mount

Total Internal Reflection (TIR)

The reflection that occurs within a material because the angle of incidence of light striking the boundary surface is in excess of the critical angle.

Transformation Temperature (Tg)

The temperature at which glass changes from brittle to viscous-liquid due to thermal stress. Within a well-defined operating temperature, glass does not undergo major changes to its mechanical structure; however, after a specific transformation temperature, it will deform to the point where its original optical properties no longer apply.

Transistor-Transistor Logic (TTL)

Refers to a common digital logic family of devices used for control in most modern electronics. TTL signals have two distinct states; "low" and "high." A logic "low" corresponds to a maximum voltage threshold of 0.8V; a "high" corresponds to a voltage greater than 2.5V.


The amount of radiant energy that passes through a medium; energy that is not reflected or absorbed.

  See also Reflection

Transverse Chromatic Aberration (TCA)

A type of chromatic aberration where the size of the image changes with wavelength. In other words, when white light is used, red, yellow, and blue wavelengths focus at separate points in a vertical plane.

Trihedral Prism (Corner Cube Retroreflector)

Also known as a Corner Cube Retroreflector, it has three total internal reflections that reflect incident light back onto itself (to the accuracy specified), regardless of the direction the beam is coming from or the tilt of the prism.

  See also Retroreflector

Trinocular Scope

A visual device, such as a stereo scope or microscope, that incorporates two eyepiece ports and a vertical third port for alternative image pickup. 

Triplet Lens

A lens with three optical components such as steinheil triplet or hastings triplet lens.

Tube Lens

A lens used to focus an infinity corrected objective to a senor. Typically this is an achromatic lens with a 200mm focal length.

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