UV_IR Academy

Learn terminology.

Our glossary summarizes terms applicable to UV light, flash lamps and infrared heating. It may vary slightly in their usage from other sciences. It is recommenced for users, formulators, suppliers and researchers to “close the gap” in technical language.


An index of the light or UV absorbed by a medium compared to the light transmitted through it. Numerically, it is the logarithm of the ratio of incident spectral irradiance to the transmitted spectral irradiance. It is unitless number. Absorbance implies monochromatic radiation, although it is sometimes used as an average applied over a specified wavelength range.

Source:  Radtech

Absorptivity (absorption coefficient)

Absorbance per unit thickness of a medium.

Source:  Radtech


A chemical system or physical device that determines the number of photons in a beam integrally or per unit time. For example, solutions of iron (III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are devices whose electrical response can be correlated with the number of photons detected.

Source:  Radtech


Determination of the number of photons in a beam per unit time, or integrated over time.

Source:  Radtech

Additive lamps

Medium pressure mercury vapor UV lamps (arc or microwave) that have had small amounts of metal halide(s) added to the mercury within the bulb. These materials will emit their characteristic wavelengths in addition to the mercury emissions. [This term ispreferred over doped lamps].

Source:  Radtech

Back end assembly

is the name of the technology used to reliably combine all subsystems to make a micro-system operable. As part of microelectronics and micro-systems engineering, packaging comprises the entirety of the technologies and designing tools required to assemble microelectronic components (made using semiconductor technology) within the smallest space. Printed circuit board with integrated circuit and SMD components as an example of packaging.

Heraeus use the Chip-on-Board-Technology as specific type of packaging of integrated circuits.

(Source: Wikipedia)


The range of wavelengths between two identified limits, expressed in the same units as wavelength (nm). In radiometry, it is important that the basis of the limits (i.e. % response of the instrument used) be specified or known. Band (wavelength range) must be communicated with radiometric data.

Source:  Radtech


The term bonding is used as a generic term for various joining processes used in packaging: Wire bonding is the wiring of a semiconductor chip to its case. Chip or die bonding (sometimes also called bare chip bonding) means the connecting of a semiconductor chip to its housing and accordingly to its substrate

(Source: Wikipedia)

Chip-on-board technology

COB technology is the technique used to assemble uncased semiconductor chips on substrates to form an electronic module.

Cold mirror

A type of dichroic reflector that has low reflectance to IR compared to its reflectance of shorter wavelengths, visible or UV. Also see dichroic.

Source:  Radtech

Computer-Aided Engineering (CAE)

Generally CAE is the broad usage of computer software to aid in engineering tasks. It is also the headline for our numerical analysis (or computer simulation) activities. CAE can analyze the robustness and performance of systems and assemblies. Optimizing and testing a design using computer simulation reduces development time and costs.

Cosine response

Description of the spatial response to incident energy where response is proportional to the cosine of the incident angle. A radiometer with a diffuser or a photo-responsive coating will exhibit nearly cosine response.

Source:  Radtech


Exhibiting significantly different reflection or transmission in two different wavelength ranges. Dichroic reflectors that have reduced reflectance to long wavelengths (IR) are called “cold mirrors,” while reflectors having enhanced reflectance to long wavelengths are called “hot mirrors.”

Source:  Radtech


A characteristic of a surface that reflects or scatters light or UV equally in all directions (often confused with spread reflectance).

Source:  Radtech

Doped lamps

Term applied to UV lamps having metal halide additives to the mercury to alter the emission spectrum of the lamp. (Historically this term has been used by UV arc lamp manufacturers. It is an imprecise usage, as the added chemical does not alter the properties of another). [The preferred term is additive lamps].

Source:  Radtech


Energy absorbed per unit mass. A precisely defined term in EB curing and ionizing radiation technologies: 1 gray (Gy) = 1 J/kg. Different materials that receive the same exposure may not absorb the same amount of energy. In EB, The units of gray (Gy) replace previously used units of megarads (10 kGy = 1 Mrad). Because most EB-curable materials have density of approximately 1 g/cc, dose is typically expressed either J/g or J/cm3. In UV curing, the term is often used instead of the preferred terms, effective energy density or exposure.

Dynamic exposure

Exposure to a varying irradiance, such as when a lamp passes over a surface, or a surface passes under a lamp, or lamps. In that case, energy is the time-integral of the irradiance profile.

Source:  Radtech

Dynamic range

The span between the minimum irradiance and the maximum irradiance to which a radiometer will accurately respond. Expressed as a ratio, or in measured units (e.g.,watts/cm²).

Source:  Radtech

Effective energy density

Radiant energy, within a specified wavelength range, arriving at a surface per unit area, usually expressed in joules per square centimeter or millijoules per square centimeter (J/cm² or mJ/cm²). Is expressed in a specified wavelength range (without wavelength specification, it is essentially meaningless). Commonly accepted abbreviations are Wλ or Eλ. [An alternate term is exposure.]

Source:  Radtech

Effective irradiance

Radiant power, within a specified wavelength range, arriving at a surface per unit area. It is expressed in watts or milliwatts per square centimeter (W/cm² or mW/cm²) in a specified wavelength range (without wavelength specification, it is essentially meaningless). For brevity, when the wavelength range is clearly understood, the term is shortened to irradiance. Commonly accepted abbreviations are Eλ or Iλ. Compare spectral irradiance

Source:  Radtech


One mole of photons. Sometimes equated to the energy of one mole of photons – although this use is discouraged.

Source:  Radtech

Electromagnetic spectrum

An extremely wide range of radiation that travels at the speed of light, and characterized by wavelength. Extends from radio waves (~104 meters), through visible and UV (~10-4 meters), to gamma rays (~10-14 meters).

Source:  Radtech

Emission spectra

Radiation from an atom or atoms in an excited state, usually displayed as radiant power vs wavelength. Emission spectra are unique to each atom or molecule. The spectra may be observed as narrow line emission (as in atomic emission spectra), or as quasicontinuous emission (as in molecular emission spectra). A mercury plasma emits both line spectra and continuum simultaneously.

Source:  Radtech

Energy density

Radiant energy arriving at a surface per unit area, usually expressed in joules or millijoules per square centimeter (J/cm² or mJ/cm²). It is the time-integral of irradiance.

Same as exposure. For a parallel and perpendicularly incident beam, not scattered or reflected, energy density and fluence become identical. Compare fluence. [Not equivalent to dose.]

Source:  Radtech


Effective radiant energy density at a surface; the time-integral of irradiance within a specified bandwidth, expressed in J/cm² or mJ/cm². The time-integral of fluence rate, in J/m² or J/cm². Also radiant exposure. In solar UV exposure applications, larger units may be used – J/m² or even MJ/m². Compare fluence. [Not equivalent to dose.] The definition varies in radiation chemistry and in clinical chemistry.

Source:  Radtech


The total radiant energy of all wavelengths passing from all directions through an infinitesimally small sphere of cross-sectional area dA, divided by dA. Units are typically J/cm² or mJ/cm².

Source:  Radtech

Fluence rate

The radiant power of all wavelengths passing from all directions through an infinitesimally small sphere of cross-sectional area dA, divided by dA. For a parallel and perpendicularly incident beam, not scattered or reflected, irradiance and fluence rate become identical. Units are typically W/cm² or mW/cm².

Source:  Radtech

Flux (radiant flux)

The flow of photons, in einstein/second; one einstein = one mole of photons.

Source:  Radtech


A blend of ingredients (oligomers, monomers, photoinitiators, pigments and additives) selected to meet the required properties of the end application of the ink, coating or adhesive.

Goniometric measurement:

A detector is moved around the radiation source in such a way that its entire radiated power can be captured; result = radiant power

Gas catalytic infrared:

Gas catalytic infrared radiation is an exothermic chemical reaction.
Gas catalytic infrared emitters convert natural gas or propane into medium to long wave infrared by using a special platinum catalyst. By-products are water and carbon dioxide. This flameless catalytic reaction takes place at emitter surface temperatures of between 175°C and 480°C.

Hot mirror

A dichroic reflector having a higher reflectance to IR than to visible or UV. See dichroic.

Source:  Radtech

Infrared heating

Infrared heating is caused through heat transfer with radiation in the infrared spectrum. Infrared radiation may be generated by gas heated sources or electrically heated sources.


A generic term, with a variety of meanings; undefined, but sometimes used to mean irradiance. Generally mis-applied in UV curing. Its precise optical meaning is flux/steradian (W/sr), applied to emission of light; not useful in UV curing. [Compare irradiance or peak irradiance or effective irradiance].

Source:  Radtech


Radiant power arriving at a surface from all forward angles, per unit area. It is expressed in watts per square centimeter or milliwatts per square centimeter (W/cm2 or mW/cm2). Compare effective irradiance, spectral irradiance and fluence rate.

Source:  Radtech

Irradiance profile

The irradiance pattern a lamp; or, in the case of dynamic exposure, the varying irradiance at a point on a surface that passes through the field of illumination of a lamp or lamps; irradiance vs time.

Source:  Radtech

Joule (millijoule)

A unit of work or energy (a newton-meter). The time-integral of power. Abbreviated J or mJ. (Although derived from a proper name, the term is not capitalized, while its abbreviation is capitalized).

Source:  Radtech


A light emitting diode (LED) is an electronic semiconductor component. When a current flows through the diode in forward direction, it emits visible, infrared or ultraviolet light with a wavelength that depends on the semiconductor material and its doping.

(Source: Wikipedia)

LED emission spectrums

The emission spectrum of an LED can be controlled and adjusted specifically via the doping and the internal structure of the semiconductor materials used.


The visible range of the electromagnetic spectrum. (Even though commonly used, there actually is no such entity as “UV light.”)

Line emission

Narrow lines of emission from an atom in an excited state. These are the “spikes” observed in spectrometry. Low-pressure sources exhibit finely distinguished line emission; higher pressure sources generally exhibit more continuous spectra.

Source:  Radtech

Mean power density at the exit window

Total of radiated power (W) / window area (cm²); used to compare UV LED systems, resulting units are W/cm2

Measurement laboratory

Noblelight operates an independent measurement laboratory which is accredited as a test laboratory in accordance with DIN EN ISO/IEC 17025:2005. Customer-specific measurements can be taken within the laboratory or at the customers site. The team of specialists at the measurement laboratory made up of physicists, engineers and measurement technicians can rely on a wealth of experience.



Light or UV radiated from a source that is concentrated in only a very narrow wavelength range (bandwidth). This may be accomplished either by filters or by narrow-band emission.

Source:  Radtech


An instrument that separates incoming radiant energy into its component wavelengths for measurement. Two methods are used for dispersing the radiation: diffraction grating or prism. The typical resolution may be 1 nanometer or less.

Source:  Radtech


Unit of length. Abbreviated nm. Equals 10-9 meter, = 10-3 micron, = 10 Å (ångstrom). Commonly used unit to define wavelength of light, particularly in the UV and visible ranges of the electromagnetic spectrum. An older equivalent term, millimicron, is rarely used today.

Source:  Radtech


The light emitted by an LED can have the most varied irradiance distributions, depending on the type of construction of the LED. Typically, however, the light is divergent which means that the irradiance substantially decreases with distance. Therefore, it is advisable to deflect the light which is radiated to the sides, and thus does not contribute to illumination, in a directed manner to efficiently use the optical output power. This can minimise loss and maximise irradiance. With optics adjusted to the purpose, optimal process results can be obtained which would not be possible without optimised directing of the light, while at the same time using the LED efficiently.

Optical density

The logarithm of the reciprocal of reflectance or transmittance. A dimensionless number. In printing and color, it is the log of the ratio of visible light absorbed by an “absolute white” to the light absorbed by the measured ink.

Source:  Radtech

Optical output

The optical output of an LED light source primarily depends on the number of LEDs, their electro-optical efficiency, the operating current and the operating temperature. Furthermore, it is important whether or not the light emitted is directed by specially adjusted optics and how the housing of the light source is designed. To maximise the optical output, it is necessary to precisely match all these factors to each other.


In the broadest sense, the term optoelectronics encompasses all products and methods that enable the conversion of electric energy into light and vice versa based on semiconductor technology.

Peak irradiance

The intense, peak of focused power directly under a lamp. The maximum point of the irradiance profile. Measured in irradiance units, W/cm².

Source:  Radtech


Compounds that absorb photons when irradiated with light in the UV-visible spectral range. They produce reactive species (free radicals, cations or anions) which go on to initiate photo polymerisation of UV curable formulations.


An instrument for measuring visible light, usually filtered or corrected to match the human eye response.

Source:  Radtech

Power density (peak value):

Typical data to describe the power output of devices; means the max. irradiance directly at the exit window; unit: W/cm²

Power (radiant) see radiant power

The rate of radiant energy or total radiant power (W) emitted in all directions by a source.

Source:  Radtech

Power (UV lamp)

Tubular UV lamps are commonly described by their operating power in “watts per inch” or “watts per centimeter.” This is derived simply from the electrical power input divided by the effective length of the bulb. (It does not have a direct meaning to the output efficiency of a lamp system, to the spectral conversion efficiency, to the curing performance, nor to the UV irradiance delivered to a work surface).

Source:  Radtech

Polychromatic, or Polychromic

Consisting of many wavelengths.

Source:  Radtech

Quantum yield

A measure of the photon efficiency of a photochemical reaction. The ratio of the number of chemical events per unit time to the number of photons absorbed per unit time. It is a unitless measure. Note that it is based on the absorbed and not the incident photon flow.

Source:  Radtech


Exhibiting a change of color or optical density with exposure to light or UV. A character of films whose color or density change can be correlated to exposure to UV energy.

Source:  Radtech


Generally refers to the radiant output of a source. It is radiant flux per unit area per steradian (W/cm²/sr). In UV curing, it is used in a generic sense rather than as a precise optical term.

Source:  Radtech

Radiant energy

Energy transfer, expressed in joules or watt-seconds (J = W×sec).

Source:  Radtech

Radiant exposure / dose

Time integral of irradiance over a certain period of time; unit: J/cm²

Radiant intensity

Power per unit of solid angle from a source, expressed in watts/steradian (W/sr).

Source:  Radtech

Radiated power / radiant flux

Entire power emitted by radiation; unit: watt (W)


Preferred term is radiachromic

Source:  Radtech


A device that senses irradiance incident on its sensor element. Its construction may incorporate either a thermal detector or a photonic detector. The instantaneous signal output will usually have a linear proportionality to radiant flux, and will depend on incident wavelength(s). The resulting characteristic response to irradiance versus wavelength is called responsivity.

Source:  Radtech

Responsivity (spectral sensitivity)

The response or sensitivity of any system in terms of incident wavelength. In radiometry, it is the relative output of a device versus wavelength. In photochemistry, it may also be applied to the spectral response of photoinitiators.

Source:  Radtech

Spectral output

The radiant output of a lamp versus wavelength. It is displayed in a variety of ways, but commonly a graph or chart of output watts plotted against wavelength. The appearance of the plot will vary dramatically, depending on the wavelength resolution used. A technique of normalizing is to integrate spectral power over 10-nanometer bands, (W/10nm), to reduce the difficulty of quantifying the effects of line emission spectra.

Source:  Radtech

Spectral absorbance (absorbance spectrum)

Absorbance described as a function of wavelength.

Source  Radtech

Spectral irradiance

Irradiance at a given wavelength per unit area per unit wavelength interval. Expressed in W/cm²/nm. Usually measured with a spectroradiometer. Compare effective irradiance.

Source:  Radtech


An instrument that combines the functions of a radiometer and a monochromator to measure irradiance in finely divided wavelength bands.

Source:  Radtech

Static exposure

Exposure to a constant irradiance for a controlled period of time. Contrast with dynamic exposure.

Source:  Radtech

Thermal management

Thermal management describes the directed dissipation of heat, e.g. heat generated by the operation of an LED light source. Such dissipation is necessary to prevent the LEDs from overheating which may lead to inefficient operation, shortened life and even destruction. Heat dissipation is influenced and optimised by means of a purposeful selection of the materials used to construct the LEDs and the cooling mechanisms applied. In most cases, LEDs are cooled with air or water.


Ultraviolet. Radiant energy in the 100 nm to 450 nm range. 100 nm to 200 nm is generally called vacuum UV (VUV), because it does not transmit in air. There is no precisely defined boundary between UV and visible light, and may be considered about 400-450 nm.

Source:  Radtech

Ultraviolet radiation

Ultraviolet radiation is electromagnetic radiation in the wavelength range from 100 nm to 380 nm. This range is the short-wave portion of the optical spectral range. As the wavelengths of UV radiation are shorter than the wavelengths of visible violet light, it is invisible to the human eye. Due to the assignment to optical radiation, the term "UV light" is commonly used. As with light of other wavelengths, UV radiation can be refracted, diffracted, reflected and absorbed. The UV range is divided into three classes, UV-A, UV-B and UV-C, each having different biological effects and technological areas of application.


Designations of UV wavelength ranges, originally for distinction of physiological effects of UV, and establishment of safe exposure limits. The generally accepted ranges are:
VUV: 100-200 nm
UVC: 200-280 nm
UVB: 280-315 nm
UVA: 315-400 nm

UVA is commonly referred to as long UV wavelengths; while UVC is considered short UV wavelengths. VUV stands for „vacuum UV.“ Measurement of specific ranges may be defined by the responsivity of a radiometer. It should be made clear, when referring to these ranges, exactly what wavelengths they represent. Specific manufacturers of radiometers will use uniquely specified ranges.


A vacuum is an enclosed space from which matter, especially air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere. Infrared radiation is transferred without contact and hence optimally suited for heat processes in a vacuum.

Source:  Dictionary.com


Describes medium-pressure lamps in electrical watts divided by bulb length.


Describes the unit of irradiance or mean power density of UV-LEDs

Watt (Milliwatt)

The absolute meter-kilogram-second unit of power equal to the work done at the rate of one joule per second or to the power produced by a current of one ampere across a potential difference of one volt : 1/746 horsepower. Abbreviated W or mW. In optics, a measure of radiant or irradiant power. (Even though the term is derived from a proper name, it is not capitalized, while the abbreviation is capitalized)

Source:  Radtech


A fundamental descriptor of electromagnetic energy, including light. It is the distance between corresponding points of a propagated wave. It is the velocity of light divided by equivalent frequency of oscillation associated with a photon. UV wavelengths are currently measured in nanometers (10-9 meter). An older term, Ångstroms (Å= 10-10meter) is rarely used today. The typical symbol for wavelength is λ (lambda).

Source:  Radtech