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Packing Fraction

Considering a cross-section of an optical fiber bundle, it is the ratio of the area of the fibers to the space between the fibers. Bundles comprised of smaller individual fibers have larger packing fractions.

See also Optical Fiber


Perspective distortion caused by the change in magnification of an object with distance from a lens, making objects farther away seem smaller than those close up.


The specification for the deviation in alignment of an optical surface relative to another; it causes angular deviation in optical or mechanical systems.


In geometric optics, the design paradigm in which small-angle approximations (e.g. Sinθ=θ) remain accurate.

Paraxial Rays

The small bundle of rays that travel through an optical system on- or nearly on-axis. They are useful for first-order approximations and for defining basic system parameters.

See also Marginal Ray, Chief Ray


The ability of an imaging or objective lens to maintain focus as the focal length or magnification is changed.

See also Magnification


The orientation of an image after reflection from a surface or refraction through the surface. Also referred to as image handedness.

Peak Transmittance

The highest transmittance value for light passed by a filter.

See also Filter

Peak Wavelength

The wavelength of light that has the highest transmittance value.

Pellicle Beamsplitter

A type of beamsplitter constructed of an ultra-thin nitrocellulose membrane bonded to the lapped edge of an aluminum ring. It neither adds distance to the optical path length, nor creates ghost images due to back reflections. Although a pellicle beamsplitter is more sensitive to vibrations than a standard glass substrate beamsplitter, it does not add chromatic aberration or induce a focal shift.

See also Beamsplitter, Chromatic Focal Shift

Penta Prism

A five-sided prism that deviates the ray path by 90°. Since incident light rays are reflected twice to achieve a right angle turn, the image from a penta prism is of the same orientation as the object. It is useful for shortening the length of an assembly.

Periplan Eyepiece

An eyepiece consisting of one achromatic doublet plus one plano-convex lens (plano side facing the doublet) as the eye lens and a second plano-convex lens (plano side also facing the doublet) as the field lens. Designed for use with Plan and Semi-Plan objectives.

See also Kellner Eyepiece, Huygenian Eyepiece, Wide-Field Eyepiece, Plan Objective, Semi-Plan Objective


An electro-optic component comprised of two electrodes in a semiconductor material where the reverse current varies with illumination. It detects optical power and converts optical power to electrical power.

See also Detectivity, Quantum Efficiency (QE), Spectral Response, Breakdown Voltage (BDV), Amplifiers, Shunt Resistance


The measure of how likely a material is to react if exposed to light.

Photovoltaic Effect

Light with enough energy excites electrons from the valence to the conduction band, creating a difference in electric potential. This allows detectors to transform light energy into an electrical current.


In relation to movement, it is the angular rotation about the lateral axis, typically the Y-axis for X-Y-Z configurations. In relation to mechanical components, it is the spacing between consecutive threads on a threaded part. It is the inverse of TPI.

See also Gimbal, Moment Load, Yaw, Roll


Photosites, or potential wells, on a camera sensor. In a CCD, each pixel's charge is converted to voltage, buffered, and transferred through a single node as an analog signal. In a CMOS sensor, the charge-to-voltage conversion is done at the pixel level.

See also Complementary Metal Oxide Semiconductor (CMOS), Binning, Jitter, Nyquist Limit, Charge-Coupled Device (CCD), Pixel Clock, Pixel Depth

Pixel Clock

In a CCD camera sensor, it describes the speed of the complementary signals which are used to move the charge packets through the shift registers towards the read out amplifiers. This determines how long it takes to read out the entire sensor, but it is also limited by noise and spillover issues which occur when the packets are transferred too quickly.

See also Pixel, Pixel Depth

Pixel Depth

Often referred to as grayscale, the pixel depth represents the number of steps of gray in the image. It is closely related to the minimum amount of contrast detectable by a sensor.

See also Pixel, Pixel Clock

Plan Objective

A type of microscope objective that best corrects for color and spherical aberration. It has a flat field in approximately 95% of the field of view.

See also Achromatic Objective, Periplan Eyepiece, Semi-Plan Objective

Plano-Concave (PCV) Lens

A type of singlet lens with one planar (flat) surface and one inwardly curved (concave) surface. It has a negative focal length. Optimized for infinite/ infinite conjugates and ideal for image reduction and to spread light.

See also Singlet Lens, Cylinder Lens, Infinite/Infinite Conjugate, Double-Concave (DCV) Lens

Plano-Convex (PCX) Lens

A type of singlet lens with one planar (flat) surface and one outwardly curved (convex) surface. It has a positive focal length. Optimized for infinite/ finite conjugate imaging and ideal for autocollimators, light detection, laser collimation and infinity-corrected objectives.

See also Singlet Lens, Cylinder Lens, Infinite/Finite Conjugate, Double-Convex (DCX) Lens

Plate Beamsplitter

A type of beamsplitter fabricated from glass plates with a partially reflective metallic or dichroic coating. The thin glass substrate means small beam deviation and a very low absorption.

See also Beamsplitter, Cube Beamsplitter, Dichroic Beamsplitter, Anti-Reflection (AR) Coating, Dichroic Coating

Pointing Accuracy

Measure of the angular difference between the propagating axis (where the laser light is pointing) and the mechanical axis (where the housing is pointing). To measure, spin the housing of the laser and measure the distance and radius to find the angle of error.

See also Bore Sighting, Static Alignment

Poisson's Ratio

Ratio of the transverse strain to the axial strain when a material is being stretched. Measure of the Poisson effect.


A property arising from the wave nature of light. Polarized light consists of waves which oscillate in a defined and predictable manner. The direction in which the electric field vector points as it propagates through space defines the polarization state. If it oscillates in a single plane, the polarization state is linear. If it rotates about the axis of propagation, the polarization state is elliptical or circular.

See also Polarizing Efficiency, Linear Polarizer, Unpolarized, Wire Grid Polarizer, S-Polarization, Circular Polarizer, P-Polarization, Extinction Ratio, Non-Polarizing Beamsplitter


Polarizing filter placed over the light source in an imaging system to polarize light, used in conjunction with an analyzing filter.

Polarizing Beamsplitter

A type of beamsplitter that splits unpolarized light into S- and P- Polarization states.

See also Beamsplitter, S-Polarization, P-Polarization

Polarizing Efficiency

A common measure of the polarizing characteristics of a polarizer. 100% efficiency is ideal though not feasible due to manufacturing limitations. 95 - 99% polarizing efficiency is typical.

See also Polarization


Refers to multiple wavelengths as opposed to monochromatic which refers to only one specific wavelength.

See also Monochromatic

Porro prism

A 45-90-45 prism which inverts an image using the hypotenuse face. This will maintain a right-handed image because two reflections occur.

Positional Repeatability

The precision with which a positioner can reproduce a given movement or move sequence over a number of attempts. For a repeatability curve, it is the width of the dispersion about the mean value for a large number of positioning trials.


A type of surface accuracy specification that applies to curved optical surfaces. It is tested by comparing a curved surface against a reference surface with a highly calibrated radius of curvature. Using the same principle of interference caused by the air gaps between the two surfaces, the interferences pattern of fringes is used to describe the deviation of the test surface from the reference surface. A deviation from the reference piece will create a series of rings, known as Newton’s Rings.

See also Irregularity, Interferometry, Optical Flat, Interference Fringe

Power Density

Light intensity per unit area. This measurement is especially important for lasers and high intensity illumination sources.

See also Laser, Laser Damage Threshold


The linear polarization state which is parallel to the plane of incidence.  The plane of incidence is normal to the surface or interface upon which light is incident.  If the light is incident upon the interface at an angle, the light will be oscillating in such a way as to appear to be “plunging” into the surface.

See also Polarization, Linear Polarizer, Wire Grid Polarizer, S-Polarization, Polarizing Beamsplitter

Primary Magnification (PMAG)

Ratio of the sensor size on the camera being used to the field of view that the system produces.

Principal Plane

A hypothetical plane where incident light rays can be considered to bend due to refraction. Effective Focal Length (EFL) is specified from a principal plane location whereas Back Focal Length (BFL) is specified from the back surface of a lens.

See also Center Thickness (CT), Back Focal Length (BFL), Effective Focal Length (EFL), Conjugate Distance


A solid glass optic ground and polished into geometrical and optically significant shapes. A prism is used for "bending" light within a system, "folding" the system into a smaller space, changing the orientation of an image, as well as combining or splitting optical beams with partial reflecting surfaces.

See also Dove Prism, Right Angle Prism, Porro prism, Penta Prism, Amici Prism, Littrow Dispersion Prism, Trihedral Prism (Corner Cube Retroreflector), Axicon, Equilateral (Dispersing) Prism, Parity

Progressive Scan

A scanning method in which the sensor scans each lines of the field sequentially (rows 1, 2, 3, 4..., etc.). All image data is recorded in a single exposure making this ideal for high speed applications.

See also Frame, Interlaced Scan, Field