Glossary of Terms


Terminology Used for Ultraviolet (UV) Curing  Process Design and Measurement

This glossary of terms has been assembled in order to provide users, formulators, suppliers and researchers with terms that are used in the design and measurement of UV curing systems.  It was prompted by the scattered and sometimes incorrect terms used in industrial UV curing
technologies.  It is intended to provide common and technical meanings as used in and appropriate for UV process design, measurement, and specification.   General scientific terms are included only where they relate to UV Measurements.  The object is to be "user-friendly," with
descriptions and comments on meaning and usage, and minimum use of mathematical and strict definitions, but technically correct.  Occasionally, where two or more terms are used similarly, notes will indicate the preferred term. For historical and other reasons, terms applicable to UV Curing may vary slightly in their usage from other sciences.  This glossary isintended to 'close the gap' in technical language, and is recommended for authors, suppliers and designers in UV Curing technologies.


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.

absorptivity (absorption coefficient)

Absorbance per unit thickness of a medium.


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.


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

additive lamps

Medium pressure mercury vapor UV lamps (arc or microwave) that have had smallamounts 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 is

preferred over doped lamps].  


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.

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.

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.


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.” 


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

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].


Energy absorbed per unit mass.  A precisely defined term in EB curing and ionizing radiation technologies: 1 gray (Gy) = 1 J/kg = 0.1 Mrad.   In UV curing, the term is often used instead of the preferred terms for energy measured at a surface.  Preferred UV

terms are energy, effective energy density, or exposure (for energy delivered to a surface per unit area, not energy absorbed).

dynamic range

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

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 or

E specification, it is essentially meaningless).  Commonly accepted abbreviations are .  [An alternate term is exposure.]

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 E.  Compare spectral irradiance.


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

electromagnetic spectrum

An extremely wide range of radiation that travels at the speed of light, and characterized

by wavelength.  Extends from radio waves (~10 4 meters), through visible and UV (~10meters), to gamma rays (~10-14 meters).

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 molecularemissionspectra).  A mercury plasmaemits bothline spectra and continuumsimultaneously.

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.]


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.


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.

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.

flux (radiant flux)

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

hot mirror

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


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 isflux/steradian (W/sr), applied to emission of light; not useful in UV curing.   [Compare irradiance or peak irradiance or effective irradiance].


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.

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.

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 joule is not capitalized, while its abbreviation is capitalized).


Radiant energy in the visible range of the electromagnetic spectrum.

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. 


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.


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. nanometer 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.

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.

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.


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

power (radiant)  see radiant power

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

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).

polychromatic, or polychromic

Consisting of many wavelengths.

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.


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.  


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.

radiant power

Rate of energy transfer, expressed in watts or joules/second (W = J/sec).

radiant intensity

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

radiant energy

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

radiant exposure

 See exposure.


Preferred term is radiachromic


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.

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.

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.

spectral absorbance (absorbance spectrum)

Absorbance described as a function of wavelength.

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.


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

static exposure

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


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.


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 wavelengthsVUV should not to be confused with the very long UV range called

UVV (400-450 nm). 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.

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, the term watt is not  capitalized, while the abbreviation is capitalized).


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 meter) is rarely used today.  The typical symbol for wavelength is λ (lambda).-10