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Dictionary of Light Technology Terms: A - M

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Absolute Refractive Index
The refractive index of a medium in relation to that of a vacuum.
In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).
The loss of light as it passes through a material, generally due to its conversion to other energy forms.
See Alternating Current.
Accelerated Graphics Port (AGP)
An interface specification that enables 3D graphics to display quickly on ordinary personal computers.
Acceptance Angle
The maximum angle to the axis (center line) of an optical fiber at which a light ray will enter the core and be propagated. Light striking the optical fiber at an angle greater than the Acceptance Angle will be lost. Also called acceptance cone.
A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.
Achromatic Lens
A lens typically composed of two separate lenses, one convex and one concave and generally of different materials, designed in such a way that the chromatic aberration produced by one is corrected by the other.
Acousto-Optic Tunable Filter
An optical filter that can be tuned by altering the refractive index using acoustic waves
Air Mass (Air Mass Ratio)
Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).
The ratio of light reflected from a surface, often used in reference to a celestial body. It is a more specific form of the term reflectivity.
Alternating Current
A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.
Amorphous Semiconductor
A non-crystalline semiconductor material that has only short-range order in its structure.
Amorphous Silicon
A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate.
A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.
Producing different optical imaging effects along mutually perpendicular radii. Anamorphic systems generate distorted images, such as those used in motion pictures, that compress a scene laterally in the camera and expand it again during projection.
Angle Of Incidence
The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.
A material such as a crystal having different physical properties in different directions. For example, light striking a crystal's surface at a 90° angle might not be reflected, while light striking it at shallower angles is reflected. The opposite of isotropic.
Antireflection Coating
A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.
An opening or hole which restricts the width of the light path through an optical system. In systems such as telescopes, this is the diameter of the objective lens.
An optical system that is corrected for both chromatic aberration and spherical aberration.
Area Camera
A camera covering an area at once rather than a single line at a time. Area Cameras are of two types: interlaced scan and progressive scan.
Aspect Ratio
The width of a picture (or screen) in relation to its height. Ratios are expressed in the form "width x height". Typical standard computer and television aspect ratio is 4 x 3; DVD/HDTV is 16 x 9.
A defect of a lens such that light rays coming from a point do not meet at a focal point so that the image is blurred.
Avalanche Photodiode
A photodiode which is operated with a relatively high reverse voltage (sometimes just below breakdown) to achieve internal photocurrent multiplication, thereby providing rapid light-controlled switching operation.
The angle between true south and the point on the horizon directly below the sun.



Ball Lens
A spherical optical component often used for improving signal coupling between fibers, emitters and detectors.
Band Gap
In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.
Beam Profiler
A beam profiler is a device which measures the whole optical intensity profile of a laser beam, i.e., not only the beam radius but also the detailed shape.
An optical device (typically a mirror) that reflects part of a beam of light falling on it and transmits part.
The separating of a ray of light into two parallel rays of perpendicular polarization by passage through an optically anisotropic medium.
Bit Depth
The number of bits used to code a value (such as a pixel) into an integer value. This is directly related to the number of levels that the value might have, such as 256 with an 8-bit depth or 1024 with a 10-bit depth.
When a CCD is exposed to very high intensity illumination, it is possible to exhaust the storage capacity of the CCD photodiode wells, causing a condition known as blooming. When this occurs, excess charge will overflow into adjacent wells, resulting in a corrupted image near the blooming site.
Board Level Camera
A camera not yet housed in an enclosure or connected to particular terminations. Ideal for OEM applications due to not having dedicated housing and mounting.
An optical instrument used for seeing inside confined spaces consisting of a rigid or flexible tube with an optical relay inside so that the view through a lens or window at one end of the tube may be seen from a lens or eyepiece in the opposite end of the tube.
The chemical element commonly used as the dopant in a photovoltaic device or cell material.
A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth.
BTU (British Thermal Unit)
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.



A chemical element used in making certain types of solar cells and batteries.
Cadmium Telluride
A polycrystalline thin-film photovoltaic material.
A communications link using a dedicated cable connection and a standardized communications protocol.
Candela (cd)
Unit of luminous intensity. 1 candela is equal to 1/60 of the luminous intensity per square centimeter of a blackbody radiating at the temperature of solidification of platinum (2,046°K). Also called candle.
Carbon Nanotube
Carbon nanotubes are large molecules, typically of pure carbon, that are long and thin and shaped like tubes, about 1-3 nanometers (1 nm = 1 billionth of a meter) in diameter, and hundreds to thousands of nanometers long. As individual molecules, nanotubes are 100 times stronger-than-steel and one-sixth its weight. Nanotubes are being investigated as semiconductors and for uses in nanotechnology.
Charge-Coupled Device (CCD)
A sensor used in digital cameras and video cameras to record still and moving images. The CCD captures light and converts it to digital data in such a way that each pixel (picture element) in the image is converted into an electical charge, the intensity of which is related to a color in the color spectrum.
Chemical Vapor Deposition (CVD)
A method of depositing thin semiconductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.
Chromatic Aberration
Image imperfection caused by light of different wavelengths following different paths through an optical system due to different degrees of dispersion of the optical materials of the system.
The aspect of color that includes consideration of its dominant wavelength and purity, independent of brightness.
Chrominance (C)
The difference between one color and a chosen reference color of the same brightness and chromaticity.
CMYK (Cyan, Magenta, Yellow, and Black)
A color space primarily used in color printing The CMYK color space is subtractive, meaning that cyan, magenta, yellow and black pigments or inks are applied to a white surface to remove color information from the white surface to create the final color. See also RGB.
Complementary Metal Oxide Semiconductor (CMOS)
A type of sensor used in scanners and digital cameras that is based upon a semiconductor process designed for digital electronics instead of analog electronics as in the CCD.
Color Temperature
Most light sources are not 100% pure white; but instead have a certain color temperature, expressed in Kelvin (K). Some examples include: candle light color temperature around 1500°K, Incandescent light around 3000°K, midday sunlight around 5600°K, and blue sky around 9000°K.
An instrument designed to determine the color of something, by comparison with standard colors or by spectroscopy.
Composite Video
A type of video signal in which the luminance (Y) and chrominance (C) portions of the signal along with the timing reference ("sync") information, using one of the coding standards such as NTSC, PAL or SECAM.
Computed Tomography
A specialized X-ray imaging technique that takes successive images of an object from different angles and uses a computer to build these images into cross-sectional images of the object.
Conduction Band
An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.
Contrast Transfer Function (CTF)
A mathematical function that expresses the ability of an optical or electronic device to transfer signals as a function of the spatial or temporal frequency of the signal. The CTF is the ratio of percentage contrast of a square wave signal leaving to that entering the object over the range of frequencies of interest. CTF is usually presented as a graph of Contrast versus Frequency (line pairs per mm - lp/mm).
Copper Indium
A polycrystalline thin-film photovoltaic material composed of copper and indium (sometimes incorporating gallium (CIGS) and/or sulfur or selenium (CIS)).
Czochralski Process
A method of growing a large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.



See Direct Current
Decibel (dB)
A logarithmic measure of relative power levels, used to specify the amount of attenuation in optical fibers.
Dichroic Filter
A filter used to selectively transmit light according to its wavelength.
Causing light to be split into two distinct beams of different wavelengths (colors).
Diffuse Radiation
Radiation received from the sun after reflection and scattering by the atmosphere and ground.
A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.
Direct Beam
Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.
Direct Current
A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.
A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).
Dynamic Range
The dynamic range of a sensor is defined by the largest possible signal divided by the smallest possible signal it can generate. Cameras with a large dynamic range can capture shadow detail and highlight detail at the same time.



Electric Current
The flow of electrical energy (electricity) in a conductor, measured in amperes.
Energy resulting from the flow of charge particles, such as electrons or ions.
Electrolytic process in which a metal is deposited at the cathode from a solution of its ion.
An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.
An instrument that is used to analyze elliptically polarized light. Typically used to investigate the surface layers of semiconductor wafers and thin-films.
Epitaxial Growth
The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.
The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23 (fall equinox).



Fermi Level
Energy level at which the probability of finding an electron is one-half. In a metal, the Fermi level is very near the top of the filled levels in the partially filled valence band. In a band gap.
Fiber Bragg Grating (FBG)
A type of reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. A fiber Bragg grating can be used as an in-line optical filter to block certain wavelengths, or as a wavelength-specific reflector.
Fill Factor
The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance.
An optical component that pass certain wavelengths or frequencies of light unchanged while attenuating (reducing) or absorbing all others.
An x-ray imaging device equipped with a fluorescent screen on which the internal structures of an optically opaque object, such as the human body, may be continuously viewed as images formed by the differential transmission of x-rays through the object. Also called roentgenoscope.
Focal Length
The distance from the surface of a lens or mirror to its focal point. The focal length of a lens determines how much magnification it provides.
Focal Point
The point at which all radiation coming from a single direction and passing through a lens or striking a mirror converges. Also called focus.
Frame Grabber
A computer card that samples and digitizes analog video signals so that the information may be processed, stored, or operated on by the computer. Used with analog cameras.
Frame Rate
The frequency (rate) at which an imaging device (i.e. camera, sensor or computer graphics system) produces unique consecutive images called frames. Frame rate is most often expressed in frames per second (FPS)
The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).
Fresnel Lens
An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.
Full Sun
The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).



A chemical element, metallic in nature, used in making certain kinds of photovoltaic cells and semiconductor devices.
Gallium Arsenide
A crystalline, high-efficiency compound used to make certain types of photovoltaic cells and semiconductor materials.
A crystalline element that is a semiconducting metalloid (resembling silicon) used in semiconductor devices.



Harmonic Content
The number of frequencies in the output waveform in addition to the primary frequency (such as 50 or 60 Hz in an electrical circuit). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.
A mirror which tracks the sun’s movement during the day and reflects the sun’s heat on a central receiver located on top of a tower. Used in tower-type solar thermal plants.
A region of electrical contact between two different materials.
The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle



Incident Light
The direct light that shines onto a surface, such as the face of a solar cell or module.
Indium Oxide
A wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. Often used as a front contact or one component of a heterojunction photovoltaic cell.
Industry Standard Architecture (ISA)
The original 16-bit PC bus architecture. Extended ISA (EISA) is a bus architecture that extended the 16-bit ISA bus to 32 bits.
Infrared Radiation
Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effectively than visible light.
The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour.
An instrument that uses the interference of waves to determine wavelengths and wave velocities, determine refractive indices, measure small distances, temperature changes, stresses, and many other useful measurements. Interferometers are used extensively for testing optical components.
Interlaced Scan
An image scanning method in which the sensor is divided into two fields: the odd field (rows 1, 3, 5..., etc.) and the even field (rows 2, 4, 6..., etc.). Odd rows are scanned first, then even rows. These fields are then integrated to produce a full frame. See also progressive scan.
An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.
The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.
A material whose physical properties are independent of direction. The opposite of anisotropic.
A graphical presentation of the current versus the voltage from a photovoltaic or semiconductor device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes device performance.



A metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 Btu equals 1,055 joules.
A region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).



Kelvin (K)
A unit of measurement for temperature, the Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero. The kelvin is defined as the fraction 1⁄273.16 of the thermodynamic temperature of the triple point of water (273.16 K (0.01 °C; 32.02 °F)). See also color temperature.



Langley (L)
Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.
An acronym that stands for "light amplification by stimulated emission of radiation". A laser is based on an active medium (either a gas or semiconductor) contained between two reflectors. A laser's reflectors contain light by oscillating it through a medium repeatedly allowing the energy to build up with each pass. Highly monochromatic, coherent laser radiation is emitted through a partially reflecting mirror in the assembly.
Laser Beam Profiler
See Beam Profiler.
The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.
An object, usually made of glass, that focuses or defocuses the light that passes through it.
Light Emitting Diode (LED)
A semiconductor diode that emits incoherent narrow-spectrum light when a voltage is applied.
Light Induced Defects
Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.
Light Trapping
The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.
Line Lighting
Lighting configured as a line, providing narrow, intense illumination.
Line Pair
Line pairs are a measurement of image resolution, defined as 1 cycle of an attenuating mark followed by a non-attenuating mark with equal spacing which produces a square wave, i.e. a dark line and an adjacent light line. 1 lp/mm would then be a 500 micron attenuation followed by a 500 micron transmission.
Line Scan Camera
A camera that use sensors that consist of a single row of photodectors. Also called a linear array camera.
Linear Fresnel Reflector (LFR)
A solar thermal concentrating technology using long, thin segments of mirrors to focus sunlight onto a fixed absorber located at a common focal point of the reflectors. These mirrors are capable of concentrating the sun’s energy to many times its normal intensity.
Lumen (Lm)
The derived unit of luminous flux; the amount of light that falls on a unit area at unit distance from a source of one candela.
Luminance (Y)
The intensity of light, measured in Lumens, emitted from a surface per unit area in a given direction.
A unit of illumination equal to the direct illumination on a surface; equal to one lumen per square meter.



Machine Vision
Systems that use cameras, sensors, lighting, computers and robots or other automation devices to provide imaging-based automatic inspection, process control and robot guidance in industrial and manufacturing applications.
Macro Lens
A lens that can focus sharply very close to an object for close-up photography.
Maximum Power Point Tracking (MPPT)
Operating a photovoltaic array at the peak power point of the array's current-voltage (I-V) curve, where the product of current and voltage is maximized (and therefore where maximum power is obtained). For a typical silicon photovoltaic cell, this is at about 0.45 volts.
Micro Electro Mechanical System (MEMS)
A microfabrication technology that uses manufacturing processes similar to that of semiconductors and integrated circuits to create discrete or integrated microdevices such as mechanical structures, microsensors, microactuators, and circuitry on a substrate material including silicon, glass or ceramic.
A technique for producing micron-size structures on surfaces by using short-wavelength light or electron beams to transfer a pattern or image from one medium to another. Typically used in semiconductor manufacturing to transfer a pattern from a mask to a wafer.
Modulation Frequency
The rate at which a signal (such as optical radiation) is varied through the use of a mechanical or electronic chopper. Also called chopping frequency.
Modulation Transfer Function (MTF)
A measure of resolution (image sharpness) that an imaging system (camera, video system, microscope, etc.) is capable of. As the name suggests, it is a measure of the transfer of modulation (or contrast) from the subject to the image, by predicting how faithfully the lens reproduces (or transfers) detail from the object to the image produced by the lens.
Having only one color (optical wavelength) with detail represented by differing hues and tints. For example, a black and white television.
Multicrystalline Silicon
See Polycrystalline Silicon
Multijunction Device
A A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.


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