UV Curing Process

UV curing terminology

This useful guide includes most of the terminology you will find used in the field of arc and UV LED curing.

GEW are market leaders in UV curing and UV LED curing technology and this gives us the opportunity to dictate much of the discussion and language used within these fields. You can find our latest summary of terms below – we will keep this updated as the industry language evolves.

General UV terminology

absorption – a process in which matter acquires photonic energy from electromagnetic radiation as wavelengths travel through or reflect off matter.

additive lamp (bulb) – medium pressure mercury vapour lamp (arc or microwave) containing metal additives such as iron (Fe), gallium (Ga), lead (Pb), tin (Sn), bismuth (Bi), or indium (In). Additive lamps have UV spectral emissions that are different than those of standard mercury (Hg) lamps. Sometimes referred to as doped lamps or metal halide lamps.

adhesion – the bonding of an adhesive to an adherend (substrate) consisting of intermolecular forces, chemical bonds, and mechanical or interlocking interactions.  The strength of adhesion is dependent on the adhesive, adherend (substrate) and cure.

anode – positive terminal of a light emitting diode (LED).

arc lamp – see mercury arc lamp

arc length – distance between electrodes in a quartz lamp (bulb). See effective cure length.

ASTM D3359 – ASTM test method used to perform a cross-cut or cross hatch adhesion test. An X-cut or lattice pattern of 6 or 11 cuts are scratched through the UV cured material to the substrate. Special pressure-sensitive tape is then applied over the cuts and then pulled away. Pulling the tape away from the substrate reveals the degree of adhesion of the cured formulation to the substrate or media. If any material between the lines is removed with the tape, adhesion is poor. If the cured material remains, adhesion is good. The recommended guidelines for testing and evaluation are documented in ASTM spec D3359 under Methods A and B. Method A employs the X cut and is used in the field as well as for films greater than 5 mils. Method B calls for lattice cuts and is recommended for use in the lab and for films that are 5 mils or less in thickness. Adhesion is assessed qualitatively against a scale of 5A or 5B (best adhesion) to 0A or 0B (worst adhesion). This test method is similar in content but not technically equivalent to ISO 2409.

ballast – an inductive device that limits and stabilises the amount of current flowing through an arc lamp (bulb) so that power output remains constant. Sometimes referred to as a choke.

bandwidth – range of wavelengths between two identified limits and expressed in the same units of wavelength measure (nanometres in the case of ultraviolet and visible output).1

binning – sorting discrete UV LEDs according to characteristics of peak irradiance, wavelength, and forward voltage among others in order to ensure similarity of operation.

black light – any lamp or spectral emission predominantly consisting of longer UVA and minimal near visible output.

borosilicate – type of heat-resistant glass made from silica and boron trioxide that exhibits exceptional thermal shock resistance and transmits more ultraviolet energy than standard glass. Borosilicate is often used in UV curing systems for plates, windows, reflectors, lenses, and other optics.

bulb (lamp) – sealed quartz tube containing a mixture of mercury and inert gas under medium pressure. Electrode UV lamps (bulbs) are fitted with electrical connections at the ends of the lamp (bulb). Microwave UV lamps (bulbs) contain no electrical connections. The internal mercury and inert gas are vaporised into a UV emitting plasma by a high voltage arc or microwave energy. Lamp is the more commonly used term in Europe and Asia while North Americans and South Americans tend to use both lamp and bulb interchangeably. 

burn-in-period – (1) stage of the start-up process for a UV lamp (bulb) where the current and voltage inside the lamp (bulb) proceed from inrush through to stabilisation and reach steady-state operation, (2) time period to achieve steady-state operation during start-up, and (3) time period a new lamp (bulb) or new lamphead is powered in a performance test at the manufacture before shipment.

capacitor – corrects the power factor in the main power supply to reduce current draw in the UV system.

cassette / cartridge /cradle – assembly that supports the UV lamp (bulb) and reflector inside a lamp housing and often designed to be removeable for easier maintenance and quicker lamp changes.

cathode – negative terminal of a light emitting diode (LED).

cationic chemistry / cationic curing – a photopolymer curing mechanism whereby UV exposure generates cations that propagate a chemical crosslinking reaction. UV exposure is required to initiate cure, but propagation proceeds without continuous or total direct line-of sight UV exposure. Cationic curing processes require minutes, hours, or days to terminate. See free radical chemistry.

chip – layers of semiconducting material doped and processed to have p-n junction characteristics. Gallium nitride (GaN) is the material commonly used to generate longer 385, 395, and 405 nm wavelength LEDs as well as blue visible LEDs. Aluminium gallium nitride (AlGaN) is the material used for 365 nm LEDs. A chip is sometimes referred to as a diode, die, or semiconductor.

choke – an inductive device that limits and stabilises the amount of current flowing through an arc lamp (bulb), so power output remains constant. Sometimes referred to as a ballast.

cold mirror – a type of mercury lamp reflector coated with a dichroic material that passes or absorbs infrared wavelengths while reflecting UV wavelengths toward the cure surface. See dichroic.

coolant – (1) liquid substance such as water or a specific water-solvent solution that flows through a manifold in a liquid-cooled LED head to remove heat generated by electrical inefficiencies in the electroluminescence process and maintain the desired junction temperatures during system operation. (2) liquid substance such as water or a specific water-solvent solution that flows through some mercury arc lampheads to remove radiated heat generated by the UV lamp during system operation.

cross-cut / cross hatch test for measuring adhesion – see ASTM D3359 and ISO 2409.

crosslinking – process of forming covalent bonds amongst numerous disconnected molecules and short polymer chains and resulting in longer, homogenous, polymer chains.

cure length – see effective cure length.

cure / curing / cured – a general chemical reaction term applying to all polymerisation methods producing highly crosslinked covalent bonds.

DC power supply – device that supplies electric energy at constant voltage to power LEDs or other electrical components. DC means direct current. Direct current is an alternative to AC or alternating current.

depletion zone – insulating boundary between the positive and negative sides of an LED p-n junction.

devitrification – act of making transparent quartz crystalline or opaque through prolonged heat and UV exposure.

dichroic – a coating designed to transmit or absorb certain wavelengths while reflecting other wavelengths. In UV lampheads, dichroic coatings are sometimes applied to reflectors for the purpose of transmitting or absorbing infrared energy while reflecting UV energy toward the cure surface.

die / diode – layers of semiconducting material doped and processed to have p-n junction characteristics. Gallium nitride (GaN) is the material commonly used to generate longer 385, 395, and 405 nm wavelength LEDs as well as blue visible LEDs. Aluminium gallium nitride (AlGaN) is the material used for 365 nm LEDs. A die or diode is sometimes referred to as a chip or semiconductor.

doped lamp (bulb) – see additive lamp.

doped LED – semiconductors that have been intentionally impregnated with impurities during manufacturing for the purposes of changing the electrical, structural, or optical properties.

dose (dosage) – energy absorbed per unit mass.1 Dose is frequently and incorrectly used as a synonym for energy density which, for clarification, is total delivered energy as opposed to absorbed energy. See energy density.

dose rate – another term for irradiance (W/cm2 or watts per square centimetre) with equivalent units of J/s/cm2.

dose test strip – See UV energy density test strip.

driver / driver board – distributes, limits, and stabilises the DC voltage to the LEDs or modules in an assembly so that power output remains constant. Similar in concept to a ballast or choke used with mercury arc lamps.

dual cure – chemistry formulated to cure with UV LED systems AND also cure with conventional microwave or electrode arc lamps.

duty cycle – proportion of ON time in a pulse width modulation (PWM) cycle to the total cycle time (ON + OFF) expressed as a percentage. A low duty cycle corresponds to a low power because the LEDs are OFF most of the time. 100% is fully ON, and 0% is fully OFF. 50% means that the power is ON half the time and OFF half the time. Varying the duty cycle but not the input power changes the energy density while maintaining a constant irradiance. Not all LED systems incorporate a PWM duty cycle. Many are constant current or constant wattage.

dynamic exposure – exposure to a varying irradiance and typically occurring when a lamphead passes over a cure surface without pausing or when a cure surface passes underneath a lamphead without pausing. As point locations on the cure surface approach, oppose, and move away from a UV source, the corresponding peak irradiance incident at each location and at each moment in time varies. The dynamic irradiance profile with respect to time generally assumes the shape of a bell curve with energy density being the time-integral of the irradiance profile or the area under the curve.1

dynamic range – span between the minimum irradiance and the maximum irradiance to which a UV radiometer will accurately respond. Expressed in measured units of W/cm2.1

effective cure length – portion of lamp (bulb) length that emits optimal and relatively uniform UV output. For electrode lamps (bulbs), the effective cure length is always slightly less than the arc length. For microwave lamps (bulbs), the effective cure length is the length of the lamp (bulb). UV LED lamps emit a bit less output towards the ends of the array.

electrode – (1) electrical fitting at each end of an arc lamp (bulb). The electrode consists of a tungsten pin surrounded by a tungsten coil and is used to maintain a voltage arc across the lamp (bulb). (2) Electrode also refers to the style of lamp (bulb) or system when differentiating between microwave and electrode arc lamps and microwave and electrode systems. Electrode lamps and systems are also referred to as arc lamps and arc lamp systems.

electrodeless – microwave powered mercury vapour UV lamps or UV systems, neither of which have electrodes.

electroluminescence – phenomenon where electromagnetic radiation of specific wavelengths is emitted when electric current flows through materials with inherent optical properties. 

electromagnetic spectrum – continuous range of all radiation in the universe; segmented into gamma rays, x-rays, ultraviolet, visible, infrared, and radio waves; and discretely quantified by wavelength, frequency, and photonic energy.

emitting window – flat rectangular piece of UV transparent quartz or borosilicate secured and often sealed at the base of an LED lamphead or mercury arc lamphead to physically protect internal components and limit foreign matter ingress. See quartz plate.

encapsulate – transparent material sometimes used around individual or small groups of LEDs to provide added protection and seal from dirt and moisture.

energy density – total radiant energy arriving at a surface per unit area and expressed in J/cm2 or mJ/cm2. Energy density is the integration of irradiance (W/cm2 or mW/cm2) over exposure time (line speed or dwell). Though technically incorrect, energy density is commonly referred to as dose. See dose.1

energy density test strip – see UV energy density test strip.

excimer lamp – specific type of quasimonochromatic emitting source that spontaneously generates UV emitting excimer or exciplex molecules. A typical method of emission generation is via dielectric barrier discharge (DBD). Commonly utilised excimer wavelengths are 172, 222, and 308 nm.

flood – an unfocused emission of UV energy that is evenly distributed across the width and length of a reflector. To generate the flood profile, lampheads use a parabolic reflector instead of an elliptical reflector or locate the lamp away from the focus when using an elliptical reflector.

focal distance (length) – perpendicular distance from the edge of a focused lamphead to the location where the UV energy emitted from the lamp (bulb) is concentrated. This is the location of maximum UV concentration. Only applies to arc and microwave systems. Does not apply to LEDs.

focus – narrow band that runs parallel to the lamphead where reflected UV energy is at its highest concentration. Only applies to arc and microwave systems. Does not apply to LEDs.

forward bias – occurs when the anode of an LED is connected to the positive terminal of a DC voltage supply and the cathode of the LED is connected to the negative terminal. The applied voltage pushes the positive holes and negative electrons from opposite sides of the LED towards the depletion zone. Current flows through the device as electrons on the n-side respond to attractive forces of the holes on the p-side and ultimately cross the junction boundary and drop into a state of lower energy. The energy differential is emitted from the semiconductor as photons of a given wavelength.

forward voltage – voltage drop across a semiconductor when the anode of an LED is connected to the positive terminal of a DC voltage supply and the cathode of the LED is connected to the negative terminal.

free radical chemistry / free radical curing – a photopolymer curing mechanism whereby UV exposure generates free radicals that propagate a chemical crosslinking reaction. Continuous and direct line-of-sight UV exposure is required to initiate and propagate free radical cure with termination often occurring in a fraction of a second. See cationic chemistry.

frequency – number of times a periodic wave cycle occurs in one second. Unit of measure is Hertz (Hz) or cycles per second and denoted by letters (f) or (v).

Gallium (Ga) – a silvery blue metallic element used in additive mercury lamps (bulbs). Gallium additives produce a violet coloured UV output when vaporised. Gallium bulbs have a spectral peak around 417 nm and a spectral concentration between 400 and 450 nm. They are often used when deeper cure is required or with white formulations containing titanium dioxides. In some industries, microwave gallium bulbs are referred to as V bulbs.

germicidal – any lamp or spectral emission predominantly consisting of UVC output.

human machine interface (HMI) – touch screen controls and dashboards that enable user friendly operation of manufacturing equipment.  

igniter – see starter.

indium (In) – a silvery white metallic element used in additive mercury lamps (bulbs). Indium additives produce a violet coloured UV output when vaporised. Indium is used to shift the spectral output past 400 nm. In some industries, microwave indium bulbs are referred to as Q bulbs.

infrared – portion of the electromagnetic spectrum between 700 nm and 1 mm. Dominant contributor to radiated heat emitted by electrode and microwave UV curing systems. Infrared is not emitted by UV LEDs.

integral blower – refers to the system cooling fan or blower when mounted directly on or inside the lamphead.

integral shutter – a pneumatically or electrically actuated assembly integrated into mercury arc lampheads and used to block UV output when closed. Some shutters have a dual purpose with the internal surface serving as the reflector when open.

integrating sphere / Ulbricht sphere – a hollow spherical cavity with a diffuse white reflective coating on the internal surface. UV emitting sources are inserted or directed through small holes in the sphere. The UV output is then scattered or diffused by the internal coating and measured in Watts by a sensor array. This is an R&D device and not something used in the field. 

intensity – a generic term with several meanings but generally undefined. Intensity is frequently and incorrectly used as a synonym for irradiance. See irradiance.1

interlock – internal or external device to a UV curing system that enables or disables another system function. An internal interlock can be a temperature, pressure, or flow meter sensor designed into the cooling system to monitor proper conditions and adjust or switch off UV system components when conditions are not met. An external interlock is typically implemented by the integrator or machine builder. It is a safety feature that prevents the emitting source from turning on or forces a shut off under certain conditions such as when a machine door is open or when the web or parts stop moving.

irradiance – radiant power arriving at a surface from all forward angles per unit area. It is expressed in watts or milliwatts per square centimetre (W/cm2 or mW/cm2).1 Irradiance is independent of line speed and exposure time. It decreases at the cure surface as the distance between the cure surface and the emitting source increases. Though technically incorrect, irradiance is commonly referred to as intensity. Other commonly used terms for irradiance include dose rate, power density, and watt density.

irradiance profile – irradiance pattern of 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.1

irradiator – see lamphead.

ISO 2409 Standard – ISO test method used to perform a cross-cut or cross hatch adhesion test. A lattice pattern of 6 cuts are scratched through the UV cured material to the substrate. Special pressure-sensitive tape is then applied over the cuts and then pulled away. Pulling the tape away from the substrate reveals the degree of adhesion of the cured formulation to the substrate or media. If any material between the lines is removed with the tape, adhesion is poor. If the cured material remains, adhesion is good. Adhesion is assessed qualitatively against a scale of 0 (best adhesion) to 5 (worst adhesion). The recommended guidelines for testing and evaluation are intended for the laboratory but are suitable for field testing. Not suitable for coatings of thickness greater than 250 µm or for textured coatings. Similar in content but not technically equivalent to ASTM D3359.

joule – metric unit for measuring work or energy. One joule is equivalent to the work done by a force of one Netwon (N) acting through one meter (m) and alternatively expressed as newton-meter. A joule is the time-integral of power where one joule equals one watt-second and is abbreviated J or mJ for millijoule.

Lambert’s cosine law – radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal.2

Lambertian – when an emitting surface has the same radiance when viewed from any angle. In other words, it has the same apparent brightness or luminance.2

Lambertian source – optical source that obeys Lambert’s cosine law. LEDs approximate a Lambertian source in that they have a large beam divergence and a radiation pattern that approximates a sphere.2

lamp (bulb) – sealed quartz tube containing a mixture of mercury and inert gas under medium pressure. Electrode UV lamps (bulbs) are fitted with electrical connections at the ends of the lamp (bulb). Microwave UV lamps (bulbs) contain no electrical connections and are electrodeless. The internal mercury and inert gas are vaporised into a UV emitting plasma by a high voltage arc or microwave energy. Lamp is the more commonly used term in Europe and Asia while North Americans and South Americans tend to use both lamp and bulb interchangeably. 

lamphead – (1) assembly consisting of an outer housing or casing, a UV lamp (bulb), an integral or remote cooling fan, and/or liquid cooling pipe connections. An electrode arc system often contains a removeable cassette subassembly while a microwave system contains magnetrons and an RF screen. (2) refers to UV LED curing assemblies despite UV LED emitting sources not using traditional quartz lamps (bulbs).

LED (light emitting diode) – layers of semiconducting material doped and processed to have p-n junction characteristics. Gallium nitride (GaN) is the material commonly used to generate longer 385, 395, and 405 nm wavelength LEDs as well as blue visible LEDs. Aluminium gallium nitride (AlGaN) is the material used for 365 nm LEDs. When a forward bias is applied to LEDs, current flows from the p-side to the n-side (anode to cathode) emitting light in the process.

LED array – (1) sub-assembly, module, or package with one diode existing as a single point source or multiple diodes arranged in a row, in a matrix of rows and columns, or in another configuration and including the necessary wire bonds, electronics, and thermal transfer components. Occasionally micro reflectors, micro or macro optics, a protective lens, or encapsulation is integrated into an array or module. Additional system components are generally necessary to integrate, power, control, and cool the array. (2) curing assembly that includes one or numerous LED arrays or modules mounted in a casing or housing and equipped with a suitable air or liquid cooling system, internal heat sink, quartz window, and power and controls connection. This latter definition of an LED array is akin to lampheads and irradiators used in traditional UV curing systems.

LED lamphead / head / lamp / irradiator / light source / dryer / light engine – curing assembly that includes one or numerous LED arrays or modules mounted in a casing or housing and equipped with a suitable air or liquid cooling system, internal heat sink, quartz window, and power and controls connection.

LED module / package – array sub-assembly with one diode existing as a single point source or multiple diodes arranged in a row, in a matrix of rows and columns, or in another configuration and including the necessary wire bonds, electronics, and thermal transfer components. Occasionally micro reflectors, micro or macro optics, a protective lens, or encapsulation is integrated into a module or package. Additional system components are generally necessary to integrate, power, control, and cool the module or package.

lens – transparent micro or macro optical device often made of quartz or borosilicate and used to redirect or collimate UV output, increase irradiance over distance, and/or reduce stray light.

liquid chiller (cooler) – refrigerated or air blast cooling system used with some arc lamp systems and some UV LED curing systems to circulate liquid coolant through lamp housings and quartz filters if used to remove heat. See coolant.

louvre – part of some UV shutter assemblies or shielding consisting of closely spaced metal baffles and used to block UV light while allowing cooling air to pass.

magnetron – assembly located inside a microwave lamphead that converts high voltage electrical input into radio frequency (RF) energy.

mercury – a silver-white metallic element that is liquid at room temperature and emits bright white UV output when vaporised into a high temperature plasma. Mercury lamps (bulbs) have a peak spectral output around 365 nm and a concentration around 254 nm. In some industries, mercury lamps (bulbs) are referred to as H bulbs.

mercury arc lamp – a type of gas discharge lamp where an electric arc is struck between two electrodes contained within a quartz tube to vaporise mercury and emit UV spectral output.

mercury plus (H+) – refers to microwave lamps (bulbs) that emit a greater portion of UVC output.

mercury vapour lamp – a type of gas discharge lamp where mercury is vaporised to emit spectral output by 1) striking an electric arc between two electrodes contained within a mercury quartz tube or 2) energising an electrodeless mercury quartz tube with microwave energy.

metal halide – see additive lamp (bulb).

micrometre – metric unit of length equal to one millionth of a meter. Abbreviated µm and often referred to as micron(s).

microwave – (1) portion of the electromagnetic spectrum within the shorter radar end of radio waves and having wavelengths between one millimetre and one meter. (2) an electrodeless UV curing system where the mercury inside the quartz tube is energised with microwaves.

monochromatic – UV output consisting of a single wavelength or a narrow bandwidth. Excimer lamps are relatively monochromatic. UV LED sources are relatively monochromatic. Mercury lamps are broad-spectrum and not monochromatic. See polychromatic.

monomer – a type of resin molecule of relatively low molecular weight and simple structure capable of bonding with itself or other similar molecules to form UV crosslinked polymers. Monomers are reactive diluents used to adjust overall viscosity but also influence cured material properties.

nanometre – metric unit of length equal to one billionth of a meter and abbreviated nm. Visible light is generally considered to be within 400 to 700 nm and ultraviolet is within the range of 100 to 450 nm.

negative cooling – when cooling air for a lamphead is drawn from the area surrounding the substrate or part being cured through the lamphead.

nitrogen inerting / blanketing – when the surface of an applied formulation is flooded with inert nitrogen gas to prevent the surface chemistry from interacting with oxygen and oxidising before cure. Nitrogen inertion reduces oxygen inhibition in the chemistry.

oligomer – a type of resin molecule of relatively low molecular weight and simple structure capable of bonding with itself or other similar molecules to form UV crosslinked polymers. Oligomers build the backbone of the crosslinked material and influence many of the cured material’s properties.

optical device – a micro or macro lens or other component used to direct or collimate output emitted from an LED or array of LEDs, increase irradiance over distance, and/or reduce stray light.

out-of-focus – when a focused mercury vapour lamphead is located further away from the substrate or closer to the substrate than its focal distance. An out-of-focus lamp delivers less irradiance to the cure surface than when the surface is located at the focus.

oxidising – when formulations and particularly free radicals react with atmospheric oxygen or with oxygen dispersed within the chemical mixture. Exposure to oxygen slows photopolymerisation. The greater the ratio of exposed surface area to formulation mass, the more negative the impact oxygen has on cure.

oxygen inhibition – when oxygen within the atmosphere or dispersed with the formulation reduces the number and/or strength of free radicals and slows or impedes UV cure, particularly at the surface exposed to atmosphere.

ozone (O3) – unstable, colourless gas with a penetrating odour generated by the reaction of oxygen and UV wavelengths shorter than 240 nm.

ozone inhibiting / ozone free bulbs – lamps (bulbs) where the quartz is manufactured with an additive or coating that prevents the transmission of shorter ozone generating UV wavelengths.

parts per million (PPM) – commonly used unit of concentration comparing portions of two separate quantities. One part per million is one part of a smaller portion for every million parts of a larger portion. Parts per billion (PPB) and parts per trillion (PPT) are also used. The value is unitless and is not part of the International System of Units (SI) system. 

peak irradiance / peak power density – maximum irradiance or dose rate measured over a sample period or the maximum point on an irradiance profile. The units of measure of are W/cm2 or mW/cm2.

photoinitiator – a molecule that absorbs UV energy and drives a polymeric chemical reaction when exposed to wavelengths within a defined range of reactivity and above a minimum threshold irradiance.

photopolymerisation – chemical process where a UV formulated ink, coating, or adhesive is converted to a crosslinked polymer as a result of suitable exposure to a source of ultraviolet energy.

pinning – process used in UV digital inkjet printing where ink is partially cured after jetting to reduce dot gain and generate a sharper more vibrant image or to cure the under white before additional colours are jetted on top of the white. A secondary, full cure UV source is required following pinning.

planar shutter – a pneumatically or electrically actuated assembly that is externally attached to a lamphead. Louvred shutters typically move perpendicular to the face of the lamphead and block UV output when closed.

polychromatic or polychromic – UV output consisting of many wavelengths. See monochromatic.

polymer – a substance such as plastic consisting of large molecules or macromolecules that repeat in sequence.

positive cooling – when cooling air for a lamphead is blown into and through the lamphead. Positive cooling can be supplied through either an external blower ducted to the assembly or through an integral blower or fan mounted within or on the assembly.

positive-negative junction (p-n junction) – a semiconductor diode having a distinctly positive and distinctly negative side. The positive side is referred to as the anode or p-type region, and the negative side is referred to as the cathode or n-type region. Current flows from the p-side of the diode to the n-side when connected to a DC power source. Collectively, the device is referred to as a positive-negative junction or a p-n junction.

post cure – (1) any non-specific chemical or physical reaction occurring within photopolymers after UV exposure and crosslinking has ceased. (2) refers to the process of exposing LED cured chemistry, such as in 3D printing and additive manufacturing, to a secondary UV source containing UVC wavelengths.

power – UV curing systems constructed with quartz tubes (lamps) are nominally rated by the electrical input power divided by the effective length of the lamp. The value is reported in watts per centimetre (wpc) or watts per inch (wpi). Power does not capture the electrical efficiency of the curing system, the spectral conversion efficiency of the lamp, the curing performance, the irradiance, or the energy density.

power density – sometimes used to mean irradiance. See irradiance.

power supply / power supply unit (PSU) – can refer to an off-the-shelf DC power supply component or an entire electrical cabinet containing the DC power supply component(s), I/O interface, AC power connection, transformers, solid state ballasts, electronics, and other items. Sometimes referred to as the Controller if it contains the operator interface or HMI.

pulse width modulation (PWM) – modulating or varying the width and, therefore, the frequency of a pulse. This is a digital signal that employs a duty cycle to vary the ON time that power is delivered to an electronic component. Varying the duty cycle but not the input power changes the energy density while maintaining a constant irradiance. Not all LED systems incorporate a PWM duty cycle. Many are constant current or constant wattage. See duty cycle.

quartz filter – tube made from a silica material that is placed in front of a UV lamphead and flooded internally with circulating distilled water. The water absorbs infrared energy emitted by a UV lamp (bulb) and carries it away from the press or machine environment while allowing UV energy to penetrate through to the cure surface.

quartz plate / quartz window – flat rectangular piece of UV transparent quartz or borosilicate secured and often sealed at the base of an LED lamphead or mercury arc lamphead to physically protect internal components and limit foreign matter ingress. See emitting window.

quartz tube – (1) a sealed tube made from silica that is filled with a precise mixture of mercury and various inert gases and sometimes fitted with electrical connections. Vaporised mercury within the quartz tube emits ultraviolet, visible, and infrared wavelengths when energised by a voltage arc or microwaves. Quartz tube is often used to refer to the lamp (bulb). (2) tube made from a silica material that is placed in front of a UV lamphead or inside the lamphead assembly and flooded internally with circulating nitrogen. Parts traveling through the tube, such as fibre optics, are safeguarded from exposure to air and ozone which facilitates cure.

radiant power – rate of radiant energy or energy transfer expressed in Watts or the equivalent units of J/s.1

radiometer – an instrument for measuring irradiance and/or energy density.

reflector – reflects and concentrates UV energy onto a cure surface. Rolled from highly polished aluminium sheet metal or formed from borosilicate into elliptical or parabolic profiles. Elliptical profiles optimise the concentration of UV energy that is reflected off a lamp (bulb) by guiding the radiation into a tightly focused UV band of energy. A parabolic reflector produces a flood of UV light at a lower peak irradiance. Holes or slots in reflectors allow cooling air to pass and are engineered for size and location to provide both optimal and balanced airflow across the length of the bulb, reflector, quartz window, and lamphead.

remote blower – refers to the system cooling fan when it is mounted separate from the lamphead and ducted into the lamphead assembly.

RF – radio frequency between 20 KHz and 300 GHz including all frequencies between normally audible sound waves and infrared. Shorter radio waves at greater frequencies are designated microwaves and used to vaporise mercury in microwave lamps (bulbs). See microwave.

RF detector – monitors RF levels in the vicinity of a microwave UV curing system and signals the power supply to switch off the UV if RF levels exceed allowable limits.

RF screen – a wire mesh assembly attached to the UV emitting face of a microwave lamphead and allows UV energy to pass while preventing RF from leaking outside the unit.

semiconductor – a material that can be a conductor or insulator of electricity. In the case of LEDs, semiconductor conductivity and the narrow band wavelength emission depend on material construction, impurities (dopants), and concentration of dopants.

shutter – an assembly designed to block UV energy emitted from a lamphead while simultaneously allowing the flow of cooling air. Shutters allow electrode arc lamps to remain powered whenever production lines stop for short periods.

solarisation – effect of prolonged UV and heat exposure to a quartz lamp (bulb) that causes the quartz to devitrify or revert to a crystalline, cloudy, and opaque state that does not transmit UV energy well.

solid-state electronics – circuits or devices built entirely from solid materials and no moving parts.

spectral output – the radiant output of a lamp verses 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 normalising is to integrate the spectral power over 10-nanometer bands (W/10nm), to reduce the difficulty of quantifying the effects of line emission spectra.1

starter – used in electrode, ballast-based systems to vaporise the lamp mercury at start-up. Applies a several thousand-volt potential across a lamp (bulb) during start-up. An internal circuit disconnects the applied potential when current is established.

starter bulb – used in the start-up of a microwave system to ignite the mercury vapour in the lamp (bulb).

static exposure – exposure to a constant irradiance over a controlled time period.1

striking – initiating the start-up process in which (1) a high voltage arc is applied between the electrodes in a UV lamp (bulb) to vaporise the mercury or (2) a starter bulb is used to ignite the mercury in a microwave lamp.

surface cure – refers to cure or degree of cure that occurs on the outermost material surface directly exposed to ultraviolet energy.

tape test for measuring adhesion – see ASTM D3359 and ISO 2409.

Threshold Limit Value (TLV) – the maximum level of a chemical substance a worker can be exposed to during a normal day’s shift, over a normal work week, and over a working lifetime without harm. Often reported in mg/m3 or parts per million (ppm). See parts per million.

through cure – refers to cure or degree of cure that occurs within formulations down to and including the material / substrate interface layer. Good through cure does not necessarily mean good adhesion. 

Time Weighted Average (TWA) – see threshold limit value (TLV).

total energy – see energy density.

total power / total radiant power – rate of energy transfer in all directions, expressed in Watts or J/s and measured with an integrating sphere.

transmittance – ratio of the radiant energy passed through matter to the total radiant energy absorbed by matter.

ultraviolet (UV) – electromagnetic radiation shorter than visible light, longer than X-rays, and spanning roughly 100 to 400-450 nm. The boundary between UV and visible is not precisely defined and is generally regarded as lying somewhere between 400 and 450 nm.1 UV wavelengths are just beyond what the human eye sees as the colour violet.

UV energy density test strip – a special ultraviolet sensitive material cut into strips (approximately 15 to 50 mm x 25 to 50 mm) and passed underneath a UV source. The colour of the strip is measured before and after UV exposure using a densitometer. The change in colour density of the photochromic strip is compared to an established calibration curve and used to approximate the total incident UV energy density. Energy density strips are commonly used for UV production machines that are not suitable to passing radiometers.

UVA (315 – 400 nm) – portion of the electromagnetic spectrum between 315 and 400 nm. UVA represents the largest portion of UV energy and is commonly referred to as long UV. UVA is at the lower limit of what the human eye sees as colour.

UVB (280 – 315 nm) – portion of the electromagnetic spectrum between 280 and 315 nm. UVB is invisible to the human eye.

UVC (200 – 280 nm) – portion of the electromagnetic spectrum between 200 and 280 nm. UVC is commonly referred to as short UV or germicidal UV and is invisible to the human eye.

UVV (400 – 450 nm) – portion of the electromagnetic spectrum between 400 and 450 nm. The V stands for visible since these wavelengths are visible to the human eye and overlap a small portion of the visible spectrum.

vacuum UV – portion of the electromagnetic spectrum between 100 and 200 nm. Vacuum UV does not transmit in air. Lamps that emit vacuum UV are only effective when operated in a nitrogen inerted environment.

viscosity (dynamic)– measure of a fluid’s resistance to deformation by shear stress. In the graphic arts industry it is typically measured in centipoise (cp or cps). Water is 1 cp. Blood is 10 cp. Honey is 2,000 cp.

vitrification – act of changing pure opaque quartz into glass through fusion.

watt – unit of power and equivalent to one joule per second. Abbreviated as W or mW for milliwatt.

watt density – sometimes used to mean irradiance. See irradiance.

waveguide – directs microwaves toward the lamp (bulb) in microwave systems.

wavelength – distance between corresponding points on a wave. Wavelengths in the ultraviolet and visible spectrum are expressed in nanometres (nm).

wire bond – electrical connection or solder joint at the anode or cathode of an LED.


Citations

1RadTech North America. (2005). Glossary of Terms – Terminology Used for Ultraviolet (UV) Curing Process Design and Measurement. RadTech UV Measurements Group. pp. 1 – 6. https://www.radtech.org/images/pdf_upload/UVGLOSS_rev4-05.pdf

2Smith, Warren J. (2007) Modern Optical Engineering. McGraw-Hill Education, 4th Edition.

Share this: