Understanding and quantifying energy emitted from UV curing sources

This article from GEW, Inc’s Vice President for Business Development, Jennifer Heathcote, outlines how professionals can deal with quantifying energy emitted from UV curing sources.

Understanding and quantifying energy emitted from UV curing sources

In UV curing processes, ultraviolet energy emitted by mercury vapour and UV LED lampheads is characterized by irradiance, energy density, spectral output, and spectral irradiance. Understanding these elements and how they are quantified enables users to better match UV curing sources to the needs of formulations, processes, and material handling. When UV output is suitably matched, successful and efficient photopolymerisation occurs at desired line speeds and required working or offset distances. In addition, when operating windows for irradiance and energy density are maintained over time, quality product is consistently and repeatably produced.

Commonly referenced elements that characterise UV output include:

• Irradiance (W/cm²) – This is the radiant power arriving at a cure surface from all forward angles, per unit area.¹ It is typically reported as the effective irradiance over a defined bandwidth and is most meaningful when referenced to a specified working distance for the measured UV curing system.
• Energy Density (J/cm²) – This is the radiant energy arriving at a cure surface per unit area.¹ It is typically reported as the effective energy density over a defined bandwidth. While it can be thought of as peak irradiance multiplied by time, calculating energy density is a little more involved for most UV curing set-ups and manufacturing line installations. This is discussed later in the article.  
• Spectral Output – This is the radiant output (W) of a lamp verses wavelength (nm). It is expressed in W/nm or W/10nm.¹
• Spectral Irradiance – This is the radiant power of a lamp per unit area (W/cm²) verses wavelength (nm). In other words, it is the irradiance per wavelength. It is expressed in W/cm²/nm.

Irradiance, energy density, spectral output, and spectral irradiance vary by lamp type, lamp supplier, lamphead design, applied electrical power, and hours of operation. In addition, the magnitudes of the irradiance and energy density that ultimately reach the cure surface are heavily influenced by how systems are integrated into manufacturing lines and how well systems are maintained following commissioning. Fortunately, once a UV curing system is suitably matched to a formulation and properly integrated into a production line, the curing process is incredibly repeatable and can be held in control with periodic measurement of irradiance and energy density along with regular system maintenance.

Irradiance

Irradiance (W/cm²) is a UV curing system’s power at an instant in time per unit area and is often referred to as lamp intensity. Since the units of W/cm² are equivalent to J/cm²/s, irradiance can be thought of as the rate at which energy density (J/cm²) is delivered to the cure surface. In other words, irradiance is the dose rate.

Throughout the entire universe, electromagnetic waves of energy diverge from one another as they travel away from their respective emitting source. As waves diverge with distance, the magnitude of the corresponding irradiance also decreases. According to the inverse square law, irradiance is inversely proportional to the square of the distance traveled. As a result, if the irradiance at a defined distance from an emitting source is known, the irradiance at a second distance can be calculated using the inverse square law.

Within proximity of UV curing lampheads, irradiance is only approximately proportional to the inverse square law. This is because engineers incorporate reflectors and other optics into lamphead designs to re-direct rays of UV light over short distances and minimise light’s natural spread. Furthermore, in the case of UV LED lampheads, emitting sources are composed of numerous tiny diodes where each diode serves as a separate point source of radiated light. A short travel distance is necessary for waves emitted from numerous LED point sources to evenly blend and ultimately form a single source of uniform radiation. Reflectors, optics, and the use of multiple diodes circumvent the inverse square law within short distances of UV curing lampheads.

Engineers employ optics and reflectors to focus or collimate the output from UV curing systems. Optics and reflectors concentrate light rays which subsequently increases the magnitude of the irradiance at the intended working distance or serves to maintain a more constant irradiance over a defined range of working distances. For conventional electrode arc and microwave lamps, the area of focused concentration is known as the focal point. Conversely, conventional and UV LED systems without a focal point or other optics result in light rays naturally diverging from one another as soon as they exit the lamphead assembly. This floods UV light over a much broader surface area. Mounting a lamphead such that the cure surface is beyond the focal point produces a similar result. In general, flood systems, systems mounted out-of-focus, and systems mounted at increasingly greater working distances result in decreased irradiance at the cure surface.

For electrode arc lamps, the irradiance is greatest at the focal point. For UV LED curing systems, which do not have a focal point, as well as conventional lamps with a flood profile, the irradiance is greatest near the exit of the lamphead or quartz window. Due to inherent differences in technology, UV LED curing systems can be designed to emit less, similar, or substantially greater irradiance values compared to medium-pressure mercury vapor lamps. UV LED systems, however, emit this irradiance over a much narrower band of wavelengths which makes it difficult to directly compare conventional and LED curing systems. As previously stated, irradiance values for both technologies are highly impacted by the distance light travels. As a result, the offset distance between a lamphead and top surface of a UV measurement tool or between a lamphead and cure surface is significant and should always be noted in set-up and record keeping.

In addition to irradiance being dependent on lamphead configurations such as focused or flood for electrode arc lamps and collimated or flood for UV LEDs, the emitted irradiance of a UV curing system increases and decreases with corresponding changes in lamp power and offset distance. Provided lamp power and lamphead position with respect to the cure surface do not change, and ignoring for gradual lamp degradation over time, irradiance remains constant at each point on the irradiance profile regardless of how fast or slow webs, sheets, or parts pass or dwell in front of a UV curing system.

In practice, because most UV curing processes incorporate some form of material handling or lamphead automation, the cure surface typically moves in relation to the emitting source. As a result, irradiance delivered to a small moving area on the cure surface is not constant over exposure time. This dynamic exposure is the result of slight variations in emitted output, fluctuations in working distance due to web bounce or shapely part profiles, time delays in shutter actuation, a cure surface that passes in front of a stationary lamphead, or a lamphead that passes in front of a stationary cure surface. Dynamic exposure refers to any process where the cure surface experiences a variable irradiance over the duration of the formulation’s reaction time.

Imagine a small area on a much larger web, sheet, or part moving toward, under, and away from a fixed UV curing source. As the selected area approaches the light, the peak irradiance arriving at the area rapidly increases. The irradiance at the area continues to increase until it achieves a maximum value at the point when the area passes through the focal point or center of the lamphead. As the same small area on the cure surface proceeds to move away from the focal point or center of the lamphead, the peak irradiance arriving at the area rapidly decreases. UV irradiance profiles graphically illustrate how peak irradiance delivered to a cure surface varies with respect to time. For the scenario just described, the UV irradiance profile resembles a bell-shaped curve.

By contrast, static exposure refers to any process where the cure surface experiences a constant irradiance over the formulation’s entire reaction time. This is possible in spot and area cure applications and with specially designed curing chambers. In each of these cases, the lamphead and curing surface do not move, the entire cure surface is evenly exposed throughout the reaction, and the delivered UV energy is instant ON and instant OFF. Static exposure is represented by the following rectangular irradiance profile.

Energy density

Energy density (J/cm²) is a system’s total delivered energy over time per unit area and is often referred to as dose. Mathematically, energy density is the integral of irradiance over time and is often estimated by multiplying peak irradiance over exposure or dwell time. Estimating with multiplication, however, generally yields too high of an energy density value as irradiance at the cure surface is rarely static. This is illustrated with the following static and dynamic irradiance profiles where the energy density is equivalent to the area under the curve.

The rectangular profile represents static exposure where irradiance is constant over time. By contrast, the bell-shaped profile represents dynamic exposure where either the cure surface or the lamphead is moving with respect to the other. The area under the rectangular profile is easily calculated by multiplying peak irradiance by total exposure time. In the case of dynamic exposure, multiplying peak irradiance, which occurs at the at the top of the bell-shaped curve, by total exposure time grossly overestimates energy density.

Energy density can be increased by increasing lamp power, slowing line speed, increasing dwell time, adding more lamps, or passing a cure surface multiple times in front of a light source. While there are exceptions related to atmosphere, integration, and lamp orientation, energy density is minimally affected by working distance for most applications. In the case of LEDs, wider lamps with optimally spaced diodes are also used to deliver additional energy density.

Spectral output and spectral irradiance

Spectral output is the radiant output of a lamp (W) verses wavelength (nm). Spectral output is expressed in W/nm or W/10nm.¹ Closely related is spectral irradiance which is the irradiance per unit wavelength (W/cm²/nm).¹ Both are measured using a spectroradiometer which is an instrument that combines the functions of a radiometer and a monochromator to measure irradiance in finely divided bandwidths.¹

Both ultraviolet and visible wavelengths are typically measured in billionths of a meter (0.000000001 m) or nanometre (nm). For reference, a sheet of paper is approximately 100,000 nanometers thick. The ISO² standard UV spectral range is defined as 10 to 400 nm while the visible range is 400 to 700 nm. For the purposes of UV curing, industry bodies and experts classify ultraviolet light as spanning 200 to 450 nm among other variations. As a result, different and sometimes overlapping ranges are often referenced and used in practice. The best way to illustrate and communicate wavelength distribution of a UV curing source as well as relative power across the distribution is with a spectral output or spectral irradiance graph.

A spectral output/irradiance graph is a line or bar chart with either a system’s radiant output or irradiance on the y-axis and corresponding wavelengths on the x-axis. Spectral output/irradiance is a factor of lamp type and is different for mercury, iron, and gallium medium pressure vapour lamps as well as 365, 385, 395, and 405 nm LEDs. Spectral output/irradiance is also dependent on mechanical and electrical system design features which influence how light is directionally emitted from a lamphead; physical properties of reflectors, windows and other features; the power at which the lamp is driven; and the effectiveness of the cooling system. A product’s spectral output/irradiance chart is a specification that is provided by lamp and system suppliers. It is meant for reference only and is not something that is commonly measured or recreated by field users of the technology. It is important to note that spectral output and spectral irradiance of individual UV lamps or LEDs differs significantly from the spectral output and spectral irradiance of the UV curing system once the source has been fully integrated with other components.

The spectral output/irradiance can be displayed in several ways including absolute value expressed in (W/nm) or (W/cm²/nm) or arbitrary, relative, and normalised (unitless) measures. The profiles commonly display the information as either a line or bar chart where bar charts typically integrate output over 10 nm bands. Using 10 nm bands makes the information easier to interpret and reduces the difficulty of quantifying the effects of line emission spectra¹. Relative and normalised spectral graphs are the most common representations.

What is measured, and how is it measured?

While spectral output and spectral irradiance are lamp specifications measured using specialized spectroradiometers, irradiance and energy density are field measurable using less expensive, off-the-shelf, portable radiometers.

In some cases, such as with GEW’s mUVm option, UV monitoring can be directly integrated into the lamphead and corresponding system controls. Anytime irradiance and energy density are measured with a radiometer, readings are always relative to a standard calibration source chosen by the radiometer vendor. Field measured values are never absolute values. Instead, they are relative values that correlate directly to the factory calibration source. The implication is that different meters tend to report different values.

As a result, radiometers are best used as process control devices where the same meter and measurement protocol are consistently used to monitor UV output over time for a given lab or production line. When measured irradiance or energy density values fall below minimum levels, system adjustments can be made to bring the process back in control. It should be noted that radiometers are designed to measure mercury vapour lamps or LEDs. The same type of meter does not measure both categories of emitting sources.

Radiometers sample a system’s irradiance numerous times per second over a specified range of wavelengths. The frequency at which samples are recorded is known as the sampling rate. Sampling occurs over time as the meter passes in front of a light source or sources. The system’s peak irradiance is reported as the single greatest value within the set of sampled data points. Separate peak irradiance values measured over different bandwidths such as UVC, UVB, UVA, and UVV are not additive as the definition of peak irradiance is the greatest measured value at a given wavelength or over a range of wavelengths. The range of wavelengths over which irradiance values are sampled is a fixed specification of the meter and driven by the sensitivity and range of the meter’s photodiodes.

The full set of peak irradiance data points collected as a radiometer passes in front of a UV source forms the irradiance profile. The integration of that profile, which is the area under the curve, is the energy density. For dynamic exposures such as the bell-shaped profile in the following image, the energy density is determined by calculating and adding numerous smaller areas. The area of each small rectangle is determined by multiplying each data point on the profile by the time between data points. This is a well-established method of integrating areas under non-linear profiles and is the reason radiometers that measure energy density are referred to as integrating radiometers.

Differences in spectral output for mercury and UV LED curing sources

The output of conventional UV curing systems spans ultraviolet (UV), visible, and infrared wavelengths. For this reason, both arc and microwave UV lamps are considered broadband or broad-spectrum. The output of standard medium-pressure mercury vapor systems covers ultraviolet, visible, and infrared in approximate equal proportions. The spectral distribution within the UV band can be altered slightly by adding small amounts of metal dopants such as iron (Fe), gallium (Ga), lead (Pb), tin (Sn), bismuth (Bi), or indium (In). Lamps with metals added to the base mercury and inert gas mixture are typically referred to as doped, additive, or metal halide.

By contrast, UV LED output is concentrated within the ultraviolet band with some visible output and no infrared. UV LEDs emit light when current flows through an arrangement of fabricated solid-state diodes. Numerous discrete diodes are packaged into a single row, series of rows and columns, or another configuration. The diode arrangement forms the length and width of the emission source. The spectral output of UV LED systems is based on complex material science where hundreds or thousands of diodes are grown layer by layer on wafers in clean rooms and then individually diced or extracted following fabrication. The emitted wavelengths of an LED are not something that can be changed or tuned following production; however, the magnitude of its irradiance is highly adjustable for a given curing system and generally has a greater range than electrode arc and microwave lamps.

The following spectral irradiance chart illustrates the general relationship between a broadband mercury lamp and commercially available UV LEDs. Standard mercury output is represented by the numerous green shaded peaks while UV LED output is represented by the taller purple bell curves. Approximately one third of mercury output falls in the infrared region (700 nm to 1 mm) located to the right of the visible band and not shown in the illustration. By contrast, UV LED systems have an absence of infrared which means they transfer considerably less total heat to the cure surface than conventional mercury lamps. Ultraviolet wavelengths, however, are still a form of radiated energy, and some UV energy is ultimately converted to thermal heat when it reaches a surface.

What important information does this spectral irradiance chart communicate?

The spectral irradiance chart clearly illustrates the difference between mercury (Hg) broadband output and the quasi-monochromatic output of UV LED technology across UVC (200 to 285 nm), UVB (285 to 315 nm), UVA (315 to 400 nm), UVV (400 to 450 nm), and visible (400 to 700 nm) wavelengths. Secondly, it demonstrates how the relative magnitude of irradiance varies by wavelength for broadband lamps as well as the fact that greater irradiances are possible with UV LED than with mercury. Finally, while both mercury lamps and UV LED systems emit UV energy, clearly there are significant differences in wavelength and irradiance that must be factored into system, formulation, and application development.

It should be emphasized that this chart is a general illustration of a typical GEW electrode arc lamp and GEW’s commercially available LED systems. The distribution of the arc lamp would be slightly different for another product and significantly different for an additive lamp. From a procurement perspective, UV LEDs are supplied and priced by semiconductor fabricators according to wavelength tolerance and output with a typical tolerance being ±5 nm. As a result, there is always some slight deviation in diode stack-up which affects the spectral profile and wavelength at which the LED curves peak. With respect to LEDs, minor wavelength shifts generally do not produce much of a difference in cure. For most UV LED curing applications, it is the magnitude of the irradiance and the corresponding energy density at a given UV LED wavelength that play a greater role in crosslinking.

How are spectral output/irradiance charts used in practice?

Spectral output/irradiance charts are a tool primarily used to compare different curing lamps or system designs and correctly pair them with the photoinitiator packages and pigment loading of existing UV formulations. Formulators and raw material suppliers also rely on spectral output/irradiance charts to develop new chemistry. Not all UV sources cure all formulations, and certain spectral emissions are better suited to some applications than others. This is because formulators select from a range of commercially available photoinitiators. The photoinititator is the part of the chemistry that absorbs UV light and drives crosslinking within the polymer. Even though photoinitiators absorb UV light over a wide range of wavelengths, a given photoinitiator is always more reactive to certain wavelengths and requires a minimum threshold irradiance to initiate. Different photoinitiators also produce different aesthetic and functional polymer properties depending on their design, their reaction with UV energy, and their reaction with the rest of the chemistry. Formulators evaluate available photoinitiator absorption curves against spectral output charts and make trade-off and blending decisions based on the needs of manufacturing lines and presses as well as the final product’s requirements of use.

Wavelength penetration for electrode arc and UV LED curing systems

As the following image illustrates, longer UVA and UVV wavelengths penetrate deep into inks, coatings, and adhesives while shorter UVC wavelengths are absorbed at the surface of the chemistry. Based on this information as well as the spectral output and spectral irradiance of commercial curing units, formulators recommend which sources and lamp types are better suited to their inks, coatings, and adhesives. These recommendations come in the form of lamp specifications (mercury, iron, gallium, etc.) or LED wavelength preferences (365, 385, 395, or 405 nm). Ultimately, formulators are tasked with making sure their products work across a wide range of UV curing systems that do not necessarily emit the same output, which is not always an easy task.

There is no UV LED source that directly mimics a broad-spectrum mercury lamp, but longer wavelengths emitted by LEDs result in the spectral distribution being more similar to the upper portion of an iron or gallium lamp which also emits some output in the 385 to 405 nm range. LEDs at 385, 395 and 405 nm as well as iron and gallium doped lamps all utilise longer, near visible wavelengths to penetrate deep into chemistry and produce better through cure particularly with thicker, opaque white, and highly pigmented formulations. For UV LED clear coatings, achieving a hard, chemical and scratch resistant surface cure without yellowing has been the primary challenge. This is because most coating formulations rely on shorter UVC wavelengths emitted by broadband lamps for crosslinking at the surface, and photo initiators that react to longer UV LED wavelengths can yellow or cloud during exposure. While this slight discoloration can be easily masked with pigments in ink, it can be more noticeable with clear chemistry.

In general, UV LED systems have an advantage over conventional systems in terms of deeper through cure. This is due to the concentration of UVA and UVV wavelengths; however, UV LEDs can struggle with surface cure when formulations are not optimized for emitting sources that do not emit UVC. When poorly matched, UV LED curing can leave some formulations tacky or greasy to the touch. Optimizing chemistry, properly selecting a UV LED source, utilising a higher irradiance, and thoughtful integration can often eliminate surface cure issues. Adding UVC diodes to an LED curing device may ultimately prove necessary for more challenging industrial coatings; however, despite the fact that UVC LEDs between 275 and 285 nm have made significant improvements in peak irradiance, reliability, and lifetime, the technology is behind that of UVA LEDs and not yet economically viable for most applications. The majority of UV LED curing systems installed and operating on manufacturing lines today are 395 nm, and most inks, coatings, and adhesives used in graphic printing applications are designed to cure at this wavelength.

Irradiance, energy density, and wavelength

Irradiance, energy density, and wavelength all play a critical role in UV curing.  First, a minimum level of irradiance at wavelengths easily absorbed by the photoinitiator package must be delivered to the cure surface. Under these conditions, photoinitiators absorb ultraviolet energy, generate free-radicals, and drive crosslinking within the chemistry. In dynamic UV curing processes, the outermost tails of a bell-shaped irradiance profile often fall below the minimum threshold irradiance and do not produce enough crosslinking within the chemistry, but as the cure surface moves closer to the lamphead, a more suitable irradiance is quickly established.  Second, the minimum irradiance threshold or greater must be maintained over the length of the reaction. Finally, once the necessary irradiance is delivered and maintained at the cure surface, energy density becomes the driving factor for cure and the limiting factor for fastest possible line speed or shortest possible cycle time. In other words, energy density is a significant contributor to a manufacturing line’s maximum material handling speed and the degree of photopolymerisation that is achievable in UV inks, coatings, and adhesives.

Evolution of UV chemistry

For almost 70 years, depending on the market and application, the UV curing industry has formulated chemistry to the spectral emissions of conventional mercury and mercury doped lamps. All historic chemistry utilises raw materials specifically designed to respond to mercury’s broad-spectrum output. Dedicated development work in narrow band UVA LED chemistry among a few formulators started between 2005 and 2010. Most of the established curing industry, however, delayed involvement until end users became more interested and UV LED curing feasibility and economics improved. More formulators entered between 2010 and 2020, and many others, particularly in industrial coatings, are just now getting started as the calendar approaches 2021. The same can be said of many conventional curing system suppliers who delayed releasing UV LED systems until market demand materialised.

In general, conventionally formulated UV chemistry designed for broad-spectrum mercury lamp systems does not cure well with longer wavelength and quasi-monochromatic UV LEDs. Due to the differences in spectral output, conventional chemistry must be reformulated to fully cure with a UV LED source. As more ink, coating, and adhesive companies develop UV LED offerings, they are increasingly designing chemistry such that a single formulation can be cured with LED while also being backwards compatible with conventional broadband lamps. This is known as dual cure chemistry and is meant to help reduce SKUs and ease the transition to UV LED technology.

Over the coming years, more and more formulations will have dual-cure capability, and the mercury only formulations will be made redundant and ultimately discontinued. This does not mean everything historically designed for electrode arc lamps will disappear tomorrow as many industries, particularly those using highly functional industrial inks and coatings as well as those running complicated 3D part profiles and curing across larger working distances, require further development work on formulations, lamps, and integration. This statement simply means that it is necessary to pay attention to what is happening in each specific industry in order to understand the impact of LED on existing UV manufacturing processes. The transition to UV LED is occurring; however, it is calculated and gradual relative to the needs of each industry and application. Paying attention to an application’s spectral output, spectral irradiance, irradiance, and energy density needs is key in determining whether a specific electrode arc lamp or a specific UV LED curing system will cure an ink, coating, or adhesive and which lamp type or LED wavelength is optimal for the overall process and chemistry.  

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 ²International Organization for Standardization.