Commonly used words in the UV LED curing industry
The following are the high-level terms that are commonly used in the UV LED curing industry. This is not intended to be an exhaustive list but rather a first-level guide to understanding basic terminology.
Acrylate – A type of raw material used in UV curing formulations. It contains a double bond, which is the reactive species in free radical polymerizations.
Amine synergist (photoactivator) –Type of photoinitiator that contains a basic nitrogen atom with a lone pair, and abstractable hydrogens. Amine synergists are used together with photosensitizers to minimize oxygen inhibition and achieve surface cure.
Anode - positive terminal of an LED.
Beam Profile – a spatial mapping of the power density from an LED system delivered to the work surface. Typically provided in two dimensions to fully define the transverse and longitudinal shape of the power density.
Binders (for UV printing inks) - Monomers, prepolymers - usually acrylate compounds. With the help of the photoinitiators, the liquid acrylate compounds react under the influence of UV light to form a solid plastic film. Binders are produced synthetically.
Binning - process of sorting LED chips into groups according to peak irradiance, wavelength tolerance, and forward voltage.
Cathode - negative terminal of an LED.
Chip - a fully functioning, minute slice of a semiconducting material, such as silicon, germanium, and gallium arsenide doped and processed to have p-n junction characteristics. Specifically, gallium nitride (GaN) is used to generate longer ultraviolet and blue visible wavelengths. In referring to LEDs, chip is often used interchangeably with diode, die, and semiconductor.
Coolant - liquid circulation material that flows over the heat sink in the LED array or module in order to 1) remove wasted heat energy produced by the electroluminescence process and 2) maintain the correct operating temperature of the LED chip(s) and wire bonds.
Cure Speed – The rate at which the photo-polymerization occurs to achieve the desired coating or ink properties. The curing speed is a function of the irradiance of the UV source and the formulation of the UV material being cured.
Curing - The cationic or free radical photo-polymerization of UV sensitive formulations to provide a highly cross-linked polymer. This polymerization occurs in fractions of a second
Depletion Zone - the non-conductive boundary where the positive and negative sides of a p-n junction meet.
Die - a fully functioning, minute slice of a semiconducting material, such as silicon, germanium, and gallium arsenide doped and processed to have p-n junction characteristics. Specifically, gallium nitride (GaN) is used to generate longer ultraviolet and blue visible wavelengths. In referring to LEDs, chip is often used interchangeably with diode, chip, and semiconductor.
Diode- common semiconductor device that is added to a circuit as a means of restricting the flow of electricity. It can generically be thought of as a switch or a valve. A key property of a diode is that it only conducts electricity in one direction. In referring to LEDs, diode is often used interchangeably with chip, die, and semiconductor.
Doped - refers to an LED semiconductor material that has been impregnated with impurities to produce a specific n-type or p-type conductivity.
Dose – Energy delivered to the media (J/cm² or mJ/cm²), also referred to as energy density. Dose is a function of Irradiance (W/cm² or mW/cm²) and exposure time (line speed).
Driver Board- a printed circuit board (PCB) that distributes the DC voltage and current to the LED chips or modules in an array and provides ability for a variety of features.
Drop-On-Demand (DoD) - A drop-on-demand inkjet print head differs from the continuous ones in that a physical process is manipulated to momentarily overcome the surface tension forces and emit a drop cluster of drops. The in supply is not sufficiently pressurized to form a continuous stream of fluid. Print head systems utilizing piezoelectric technology and thermal technology operate in drop-on-demand mode.
Duty Cycle - the 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 low power because the power is 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.
Electroluminescence - an optical and electrical phenomenon inherent to LEDs in which a material emits light energy when an electric current is passed through it.
Emitting Window - translucent material typically secured at the base of an LED head to protect the LEDs while simultaneously transmitting ultraviolet wavelengths.
Encapsulate - a transparent material used to physically protect LEDs and block dust and moisture. It can either be flat or shaped into a convex lens. Modern LEDs often use encapsulates made of silicone while older LEDs were made of epoxy resins. Both silicone and epoxy resin encapulsulants fully surround the LED chip(s).
Energy Cure: Polymerization of a chemical system by the interaction of the system with incident radiation. The cationic or free radical chain polymerization of UV sensitive materials to provide a high molecular weight crosslinked polymer. This polymerization occurs in fractions of a second.
UV Curable Formulation – A mixture of materials that when cured, provide an ink or coating. This mixture consists of oligomers, monomers, additives, and photoinitiators.
Forward Bias - occurs when the anode of an LED is connected to the positive terminal of a voltage supply, and the cathode of the LED is connected to the negative terminal. The effect of a forward bias is that the positive holes in the p region and the negative electrons in the n region of a p-n junction are pushed from opposite directions toward the depletion zone. This significantly reduces the width of the depletion zone causing the electrons on the n-side to respond to the attractive forces of the holes on the p-side. The end result is recombination, the flow of electricity, and the emission of photons.
Forward Voltage - the actual voltage across a semiconductor diode carrying a forward current.
Free Radical Cure – Polymerization through a free radical mechanism, using a system containing double bonds. Acrylate double bonds are most common.
Inkjet - Printing process that utilizes nozzles to break the ink into droplets, which are then deposited on the print substrate. . The two major types are continuous and drop-on-demand. The drop-on-demand can be further classified as piezoelectric or thermal. Using up to six colors is possible. The printing units use more than one hundred nozzles. The quality is comparable with four color printing.
Interconnect Cable - electrically connects the LED irradiator and the power supply unit (PSU).
Interlock – can be an internal or external device to the LED unit. An internal interlock is a temperature sensor and / or flow meter designed into the LED cooling system to monitor conditions at the outlet of the array. If the coolant outlet temperature exceeds a specified set point or the outlet flow rate drops below a specified set point, the interlock circuit switches off the LED array in order to avoid overheating and destroying the individual diodes and wire bonds. An external interlock is typically implemented by an OEM as a safety feature to prevent the light source from turning on in order to protect an operator when an emergency stop button is used, or a machine door is opened.
Irradiance -radiant power arriving at the surface from all forward angles. It is usually expressed in watts or milliwatts per square centimeter (W/cm2 or mW/cm2). It is independent of line speed, The energy (expressed in J/cm2 or mJ/cm2) delivered to the substrate depends on the line speed and the irradiance.
Joule – Unit of work or energy, time-integral of power – 1 Joule is equivalent to 1 Watt of power delivered in 1 second. Abbreviated J. Millijoule (0.001 J) is sometimes used in the case of small quanities of energy, abbreviated mJ.
LED (light emitting diode)- semiconductor device containing a p-n junction designed to emit specific narrow band wavelengths within the electromagnetic spectrum via a process known as electroluminescence. When a forward bias voltage is applied to the LED, current flows from the p-side to the n-side (anode to cathode). As the electrons cross the depletion zone and fill a hole, they drop into a state of lower energy. The excess energy is released in the form of a photon. The energy of the photon is directly related to the amount of excess energy while the wavelength of the photon is inversely related to the excess energy. In other words, the higher the excess energy the shorter the wavelength.
LED Array - 1) sub-assembly or module typically consisting of multiple LED diodes or chips that are individually wire bonded to a printed circuit board and then secured to a heat management system 2) also refers to a full curing assembly which includes numerous modules or LED chips as well as a cooling fan or tube fittings, a manifold block, an emitting window, and a sheet metal or plastic outer housing. In some cases, a complete array assembly will also contain the driver boards. The array is similar in concept to a lamp head or irradiator in traditional UV curing systems.
LED Irradiator, Head, Lamp, Light Source, LED Dryer, or Light Engine- a UV curing assembly which includes multiple LED chips or modules, a thermal heat management system, a cooling fan or tube fittings, a manifold block, an emitting window, a sheet metal or plastic outer housing, and sometimes the driver boards.
LED Package - an assembly containing one or several chips physically and electrically assembled together with a means of electrically connecting the entire assembly to another device.
Lens– a transparent optical device used to collect and redirect light. In LED systems, LEDs also act to physically protect LED chips, block air and moisture, and evenly manipulate or spread the emitted UV radiation. Often made of silicone, borosilicate, or quartz. See encapsulate.
Liquid Chiller / Cooler - cooling system that circulates the coolant through the light source, used to 1) ensure the LED chips and wire bonds remain at the correct operating temperature and 2) to remove wasted heat energy from the electroluminescence process.
Module- packaged assembly consisting of one or multiple LED diodes that are individually wire bonded to a printed circuit board (PCB) which is then secured to a heat sink. A module often includes a silicone encapsulate or lens over the chips for protection and to block dust and moisture. A module is an array, but several modules can also be assembled together to form a larger array known as a head or irradiator.
Monomer – Generally a low molecular weight acrylated material that is used to lower the viscosity of oligomers in a UV curable formulation. Also known as reactive diluent.
Nanometer - a metric unit of length equal to one billionth of a meter (abbreviated nm). It is the most common unit used to describe the wavelength of light, with visible light falling in the region of 400–700 nm. Ultraviolet light falls within the range of 200–400nm.
Oligomer – Generally a medium molecular weight acrylated material that provides the bulk of the coating or ink properties in a formulation. Also known as prepolymer.
Optical Device – a general term used to describe a system consisting of optical lenses, mirrors, filters or prisms used to collect, transfer and/or direct light.
Oxygen Inhibition – The retardation of free radical polymerization by the interaction of oxygen with photoinitiators or free radicals. It generally affects surface cure of the polymer.
Photoinitiator - Additive for UV curable materials.. Through absorption of UV light, photoinitiators form reactive products (radicals), which initiate the free radical polymerization reaction. There are two main types: photocleavage and photoabstraction. The photoabstraction type is bimolecular, and may utilize an amine synergist.
Pinning - allows the ink to be partially cured immediately after being jetted to reduce dot gain and provide a sharper more vibrant image on an inkjet printer.
Piezoelectric - A material that generates an electric charge when mechanically deformed. Conversely, when an external electric field is applied to piezoelectric materials they mechanically deform.
Polymer – A large molecule (macromolecule) composed of repeating structural units. It is high in molecular weight, and usually consists of monomers. It is used to form coatings and inks.
Polymerization - The free radical chain polymerization of UV sensitive materials to provide a high molecular weight polymer. This polymerization occurs in fractions of a second. Also known as curing.
Prepolymer – Generally a medium molecular weight acrylated material that provides the bulk of the coating or ink properties in a formulation. Also known as prepolymer.
Printed Circuit Board (PCB)- part of the LED module or array to which the individual LED chips or diodes are wire bonded. The PCB provides the electrical interface between the LED chip(s) and the driver board(s).
Plug-in-module - packaged assembly consisting of one or multiple LED diodes that are individually wire bonded to a printed circuit board (PCB) which is then secured to a heat sink. A module often includes a silicone encapsulate or lens over the chips for protection and to block air and moisture. A module is an array, but several modules can also be assembled together to form a larger array known as a head or irradiator.
Positive-Negative Junction (p-n junction) - a specially engineered diode made by forming layers of semiconductive materials. Impurities or dopants are impregnated or doped into the semiconductor layers to create p and n-type regions. These regions can be made from the same or different semiconductor materials. The two sides of the diode are referred to as the anode (+) and the cathode (-) respectively. Current is able to flow from the p-side of the diode to the n-side, but it cannot flow in the reverse direction.
Power Supply Unit (PSU)- a generic term often used to describe an electrical cabinet containing the DC voltage supply, I/O interface and AC power connection for an LED array. The power supply unit may also contain the driver boards. Alternatively, the driver boards can be mounted on the array.
Pulse Width Modulation (PWM)- a digital technique for varying the amount of power delivered to an electronic component by using a duty cycle.
Radiation Cure: Polymerization of a chemical system by the interaction of the system with incident radiation. The catioinic or free radical chain polymerization of UV sensitive materials to provide a high molecular weight crosslinked polymer. This polymerization occurs in fractions of a second.
Radiometer – A device used to measure incident irradiation. 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 proportionally to radiant flux, and will depend on incident wavelengths. The resulting characteristic response to irradiance versus wavelength is called responsivity.
Reactive diluent – Generally a low molecular weight material that is used to lower the viscosity of oligomers in a UV curable formulation. Also known as monomer.
Semiconductor - a substance that can be made to conduct electricity or be an electrical insulator depending on its chemical composition. The conductivity of the semiconductor varies depending on the impurity (or dopant) concentration created during the manufacturing processes. Common semiconductor base materials include silicon, gallium nitride, gallium arsenide, and gallium phosphide.
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.
Total UV Power - The sum of the UV energy delivered to the working surface..
Ultraviolet - (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 100 nm to 400 nm, and energies from 3eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet.
UV Cure – Polymerization of a chemical system by the interaction of the system with incident radiation.The reaction can be either cationic or free radical based. The free radical chain polymerization of UV sensitive materials to provide a high molecular weight crosslinked polymer. This polymerization occurs in fractions of a second.
Wavelength – 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 (nm). Designations of UV wavelength ranges, originally for distinction of physiological effects of UV, and establishment of safe exposure limits are identified as the following generally accepted ranges:
V-UV: 100-200 nm
UV-C: 200-280 nm (also known as short-UV)
UV-B: 280-315 nm
UV-A: 315-400 nm (also know as long-UV or near-UV)
UV-V: 400-450 nm (also known as visible UV)
Wire Bond - refers to the electrical connection between the LED chip and the printed circuit board (PCB). There are two wire bonds between each LED chip and the PCB. These wire bonds are made at the anode and the cathode of the chip.
Wire Bonding- the method of making an electrical connection between the LED chip and the printed circuit board (PCB) using ultrasonic welding or thermal compression.