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Radtech Buyers Guide UV Glossary Feature 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 is intended to “close the gap” in technical language, and is recommended for authors, suppliers and designers in UV-curing technologies. absorbance had small amounts of metal halide(s) wavelengths (IR) are called “cold An index of the light or UV absorbed added to the mercury within the bulb. mirrors,” while reflectors having by a medium compared to the light These materials will emit their character- enhanced reflectance to long transmitted through it. Numerically, it is istic wavelengths in addition to the wavelengths are called “hot mirrors.” the logarithm of the ratio of incident mercury emissions. [This term is diffuse spectral irradiance to the transmitted preferred over doped lamps.] A characteristic of a surface that spectral irradiance. It is unitless bandwidth reflects or scatters light or UV equally number. Absorbance implies monochro- The range of wavelengths between in all directions (often confused with matic radiation, although it is some- two identified limits, expressed in the spread reflectance.) times used as an average applied over a same units as wavelength (nm). In doped lamps specified wavelength range. radiometry, it is important that the basis Term applied to UV lamps having absorptivity (absorption coefficient) of the limits (i.e., % response of the metal halide additives to the mercury Absorbance per unit thickness of instrument used) be specified or known. to alter the emission spectrum of the a medium. Band (wavelength range) must be lamp. (Historically this term has been communicated with radiometric data. actinometer used by UV arc lamp manufacturers. It A chemical system or physical device cold mirror is an imprecise usage, as the added that determines the number of photons A type of dichroic reflector that has chemical does not alter the properties in a beam integrally or per unit time. low reflectance to IR compared to its of another.) [The preferred term is For example, solutions of iron (III) reflectance of shorter wavelengths, additive lamps.] oxalate can be used as a chemical visible or UV. Also see dichroic. dose actinometer, while bolometers, cosine response Energy absorbed per unit mass. thermopiles, and photodiodes are Description of the spatial response to A precisely defined term in EB curing devices whose electrical response can incident energy where response is and ionizing radiation technologies: be correlated with the number of proportional to the cosine of the incident 1 gray (Gy) = 1 J/kg = 0.1 Mrad. In photons detected. angle. A radiometer with a diffuser or a UV curing, the term is often used actinometry photo-responsive coating will exhibit instead of the preferred terms for Determination of the number of nearly cosine response. energy measured at a surface. photons in a beam per unit time, or Preferred UV terms are energy, dichroic integrated over time. Exhibiting significantly different effective energy density or exposure (for energy delivered additive lamps reflection or transmission in two different Medium pressure mercury vapor wavelength ranges. Dichroic reflectors to a surface per unit area, not UV lamps (arc or microwave) that have that have reduced reflectance to long energy absorbed). 14 RADTECH REPORT JANUARY/FEBRUARY 2007 dynamic exposure emission spectra incident beam, not scattered or Exposure to a varying irradiance, Radiation from an atom or atoms in reflected, irradiance and fluence such as when a lamp passes over a an excited state, usually displayed rate become identical. Units are surface, or a surface passes under a lamp as radiant power vs wavelength. typically W/cm2 or mW/cm2. or lamps. In that case, energy is the Emission spectra are unique to each Feature flux (radiant flux) time-integral of the irradiance profile. atom or molecule. The spectra may be The flow of photons, in einstein/ observed as narrow line emission (as dynamic range second; one einstein = one mole The span between the minimum in atomic emission spectra), or as of photons. irradiance and the maximum irradiance quasi-continuous emission (as in hot mirror molecular emission spectra). A to which a radiometer will accurately A dichroic reflector having a higher mercury plasma emits both line spectra respond. Expressed as a ratio, or in reflectance to IR than to visible or UV. 2 and continuum simultaneously. measured units (e.g., watts/cm ). See dichroic. energy density effective energy density intensity Radiant energy arriving at a surface Radiant energy, within a specified A generic term, with a variety of per unit area, usually expressed in wavelength range, arriving at a surface meanings; undefined, but sometimes joules or millijoules per square per unit area, usually expressed in joules used to mean irradiance. Generally centimeter (J/cm2 or mJ/cm2). It is per square centimeter or millijoules per misapplied in UV curing. Its precise 2 2 the time-integral of irradiance. Same square centimeter (J/cm or mJ/cm ). Is optical meaning is flux/steradian as exposure. For a parallel and expressed in a specified wavelength (W/sr), applied to emission of light; perpendicularly incident beam, not range (without wavelength specification, not useful in UV curing. [Compare scattered or reflected, energy it is essentially meaningless). Commonly irradiance or peak irradiance or λ λ density and fluence become accepted abbreviations are W or E . effective irradiance.] [An alternate term is exposure.] identical. Compare fluence. [Not equivalent to dose.] irradiance effective irradiance Radiant power arriving at a surface exposure Radiant power, within a specified from all forward angles, per unit area. It Effective radiant energy density at a wavelength range, arriving at a surface is expressed in watts per square surface; the time-integral of irradiance per unit area. It is expressed in watts or centimeter or milliwatts per square within a specified bandwidth, expressed milliwatts per square centimeter 2 2 2 2 centimeter (W/cm or mW/cm ). 2 2 in J/cm or mJ/cm . The time-integral (W/cm or mW/cm ) in a specified Compare effective irradiance, of fluence rate, in J/m2 or J/cm2. Also wavelength range (without wavelength spectral irradiance and fluence rate. specification, it is essentially meaning- radiant exposure. In solar UV irradiance profile less.) For brevity, when the wavelength exposure applications, larger units may 2 2 The irradiance pattern a lamp; or, range is clearly understood, the term is be used—J/m or even MJ/m . in the case of dynamic exposure, the shortened to irradiance. Commonly Compare fluence. [Not equivalent to varying irradiance at a point on a accepted abbreviations are Eλ or Iλ. dose.] The definition varies in radiation surface that passes through the field of Compare spectral irradiance. chemistry and in clinical chemistry. illumination of a lamp or lamps; fluence einstein irradiance vs. time. One mole of photons. Sometimes The total radiant energy of all joule (millijoule) equated to the energy of one wavelengths passing from all directions A unit of work or energy (a newton- mole of photons—although this use through an infinitesimally small sphere meter). The time-integral of power. is discouraged. of cross-sectional area dA, divided by dA. Units are typically J/cm2 or mJ/cm2. Abbreviated J or mJ. (Although electromagnetic spectrum derived from a proper name, the term fluence rate An extremely wide range of radiation joule is not capitalized, while its The radiant power of all wave- that travels at the speed of light, and abbreviation is capitalized.) characterized by wavelength. Extends lengths passing from all directions light from radio waves (~104 meters), through through an infinitesimally small sphere Radiant energy in the visible range visible and UV (~10-4 meters), to gamma of cross-sectional area dA, divided by of the electromagnetic spectrum. rays (~10-14 meters). dA. For a parallel and perpendicularly JANUARY/FEBRUARY 2007 RADTECH REPORT 15 line emission power (radiant) see radiant power radiant energy Narrow lines of emission from an The rate of radiant energy or total Energy transfer, expressed in joules atom in an excited state. These are radiant power (W) emitted in all or watt-seconds (J = W(sec). directions by a source. the “spikes” observed in spectrometry. radiant exposure Feature Low-pressure sources exhibit finely power (UV lamp) See exposure. distinguished line emission; higher- Tubular UV lamps are commonly radiochromic pressure sources generally exhibit more described by their operating power in Preferred term is radiachromic. continuous spectra. “watts per inch” or “watts per centime- ter.” This is derived simply from the radiometer monochromatic A device that senses irradiance Light or UV radiated from a electrical power input divided by the incident on its sensor element. Its source that is concentrated in only effective length of the bulb. (It does not construction may incorporate either a very narrow wavelength range have a direct meaning to the output a thermal detector or a photonic (bandwidth). This may be efficiency of a lamp system, to the detector.
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