Glossary

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Aberration:
A defect in the image forming capacity of a lens.

Absorbance:
The ability of a medium to absorb radiation dependent on temperature and wavelength. Stated as a negative logarithm of the transmittance.

Afocal:
Without a focal length. An optical system with its object and image point at infinity.

Angle of Incidence (AOI):
The angle formed by an incident ray of light and an imaginary line perpendicular to the plane of the component’s surface. When the ray is said to be “normal” to the surface, the angle is 0 degrees.

Angstrom (A):
A unit of measure for a wavelength of light equal to ten one billionths of a meter or one-tenth of a nanometer. 10^-10 meters.

Aperture:
An opening in an optical system that limits the amount of light passing through the system.

AR
Coatings
: Anti-Reflection coatings. Coatings designed to enhance the transmission of an optic by reducing loss due to reflected light off the surfaces.

Attenuation:
The suppression of signal before detection in an optical system. Usually, the wavelength or wavelength range is defined along with the degree of attenuation expressed in relative or absolute optical density.

Axis/Optical Axis:
The optical center-line of a lens or system. The line passing through the centers of curvature of the optical surfaces of a lens.

Bandpass(BP):
Transmits a band of color, the center of which is the center wavelength (CWL). The width of the band is indicated by the full width at half maximum transmission (FWHM), also known as the half bandwidth (HBW). It attenuates the light of wavelengths both longer and shorter that the passband.

Beam Diameter:
The diameter of that portion of the beam containing 86% of the output power.

Beamsplitter:
A device that optically splits a laser (or other) beam into two or more beams.

Blocking:
Attenuation of light, usually accomplished by reflection or absorption, outside the passband. Blocking requirements are specified by wavelength range and amount of attenuation. [Depth]

Brewster Angle Window:
A window set at brewster’s angle with respect to incoming radiation. The result is that P polarized light will not be reflected while most of the S polarized light is reflected. The light transmitted will be mostly P polarized.

Center Wavelength (CWL):
The arithmetic center of the passband of a bandpass filter. It is not necessarily the same as the peak wavelength.

Coherent Light:
Light or radiation composed of wave trains vibrating in phase with each other. Parallel rays of light.

Cold Filter:
Reflect the visible wavelengths, transmits the IR.

Collimated Light:
Divergent light rays rendered parallel by means of a lens or other device allowing a sharper image of an object to be focused at the focal plane.

Complete Blocking:
Blocking of light from the UV into the far IR. It is a specification of blocking range, not depth.

Complex Lens:
An assembly consisting of several compound lenses.

Compound Lens:
An assembly consisting of more than one simple lens elements.

Concave:
A solid curved surface similar to the inside surface of a sphere.

Cone Angle:
In a converging or diverging beam of light, the full angle between the extreme off-axis rays.

Convergence:
The bending of light rays towards each other as by a positive (convex) lens.

Convex:
A solid curved surface similar to the outside surface of a sphere.

Cut-Off:
This is the wavelength of transition from transmission to reflection. Often given as 50% of peak transmission, but can also be given a 5% or 10% peak transmission.

Cut-On:
This is the wavelength of transition from attenuation to transmission, along continuum of increasing wavelength. Often given as 50% of peak transmission, but can also be given a 5% or 10% peak transmission.

Dichroic:
In reference to thin-film interference coatings, selectively transmitting and/or reflecting light according to its wavelength.

Diffraction Limited Lens:
A lens having negligible residual aberrations.

Divergence:
The angle at which a beam spreads in the far field. The bending of rays away from each other as by a negative (concave) lens.

Effective Focal Length (EFL):
The EFL of a lens is the distance from the principal point to the focal point.

F/number:
The focal length divided by the diameter of the axial beam on the entrance pupil when the object is at infinity. F/# = f/d.

Fluorescence:
The glow induced in a material when bombarded by light of radiation.

Focus:
The spot where the wavefront originating at a point on the source is converged to form a point image.

Full Width at Half Maximum (FWHM):
Defines the wide of the passband of a bandpass filter. It is referenced to the points on the cut-on and cut-off edge where the transmission is one-half of the maximum transmission.

Half-Power Bandwidth (HBW or HPBW):
Same as FWHM

Hot Filters: Transmit visible wavelengths, reflects IR.

Hyperfocal:
The distance at which a lens may be focused to produce satisfactory image quality over an extended range of object distances.

Image:
The likeness of an object produced by an optical system or lens.

Incident Light:
A ray [or rays] of light that strikes an optical surface or other object. The angle of incidence is the angle made by the striking beam from perpendicular.

Joule:
One watt per second. Generally used as a measure of laser output.

Lens:
A component that converges or diverges an incident wavefront.

Meniscus Lens:
A lens with one side convex and the other side concave.

Micron(µm):
1µm = 1,000nm. 10^-6 meters.

Nanometer (nm):
Unit of length used to measure wavelength of light. One billionth of one meter.

Neutral Density Filters (ND):
Used to uniformly attenuate a light signal over an entire wavelength range. This can be done by reflection, absorption, or a combination of both.

Notch Filter:
Rejection band filter, typically has a transmission range on both sides of blocking range.

Objective Lens:
The component in a lens system initially responsible for collecting light from the source or object and forming an image of it.

Optical Density (OD):
Units measuring transmission usually in blocking regions. Conversion: -log10(T) = OD. Example: 1% transmission is 0.1 absolute, so -log10 (0.01) = OD 2.0.

Photon:
The elemental unit of light, having wave and particle behavior. It has motion, but no charge or mass.

Polarization:
As regards light radiation; the restriction of the vibrations of the magnetic or electric field vector to a single plane.

Power Density:
The amount of radiant energy concentrated at a point.

Pulse Energy:
The amount of energy contained in a single, pulsed emission from a laser programmed for pulsed operation. Pulsed energy can be several times greater then CW emissions.

Radian:
An arc in a circle equal to the radius in length. [An angle of 57.3 degrees at the center or a circle, formed by 2 radii cutting off such as arc — thus 1 radian = 57.3 degrees]

Radiant Energy:
Energy traveling as wave motion. Specifically, for electromagnetic waves.

Reflectance:
The return of radiant energy by a surface.

Refraction:
The bending of incident rays as they pass from one medium to another.

Resonator:
A volume, bound at least in part by highly reflective surfaces, in which light of particularly discrete frequencies can set up standing wave modes of low loss.

Signal-to-Noise(S/N):
The system ratio of the integrated energy within the passband envelope to the energy outside this envelope and within the free spectral range.

Slope:
The rate of transition from attenuation [defined as 5% of peak transmission] to transmission [defined as 80% peak transmission]. Slope = (lambda 0.80 – lambda 0.05) divided by lambda 0.05.

Spectral Response:
The response of a device to material or to monochromatic light as a function of wavelength.

Transmission:
The passage of radiant energy through a medium.

Transmittance:
The ratio of transmitted radiant energy to incident radiant energy.

Wave:
The undulation or vibration [a form of movement] by which all radiant energy in the electromagnetic spectrum is thought to travel.

Wavelength:
The length of light wave, which determines it’s color. Common units of measurement are; angstroms, nanometers or microns.

Window:
A piece of material with plane parallel surfaces which may be used to transmit, reflect or block all or part of a beam.