Updated: Aug 7, 2019
Properly characterizing output from an ultraviolet curing source requires knowledge of spectral output (nm), irradiance (Watts/cm2), and energy density (Joules/cm2). Knowing just one or two is insufficient because an optimal combination of all three parameters is necessary to drive successful photopolymerization (cure) at line speed or cycle time, keep a UV production line operating in control, and consistently produce quality product. A challenge faced by formulators, integrators, and end-user is that spectral output, irradiance, and energy density vary dramatically by lamp type, supplier, power setting, and lab or production line set-up. In addition, due to the nature of lamp technology, the values drift over time. The good news is that once a UV curing system is matched to a formulation and the line is properly commissioned and maintained, the process is incredibly repeatable and can be kept in control with periodic measurement and inspection.
In general, spectral output is the distribution of UV wavelengths emitted from the curing source. Irradiance can be thought of as the curing system’s delivered power at an instant in time (intensity), and energy density is the system’s total delivered energy over exposure time (dose). Spectral output is measured and communicated by the lamp or UV system supplier and is not something that is easily measured or monitored in the field. Irradiance and energy density are both measurable and sometimes specified by system suppliers and formulators; however, there is a lack of consistency in measurement protocol across vendors. Furthermore, different meters report different values, and the measurement source for the data points is not always referenced in data sheets. The good news is that both irradiance and energy density can be measured in the lab or field with your preferred meter. The bad news is that it is not always easy to measure UV curing sources installed on a press or manufacturing line. In the following paragraphs, I will address spectral output for conventional mercury lamps and LEDs. I’ll follow-up later with separate posts that expand on irradiance and energy density.
Conventional UV curing systems emit ultraviolet (UV), visible, and infrared output when a very small amount of mercury is vaporized into a plasma gas inside a sealed quartz tube. For this reason, both arc and microwave UV lamps are considered broadband or broadspectrum. Standard mercury spectral output is roughly one third UV, one third visible, and one third infrared. The distribution of wavelengths within the UV band can be altered slightly by adding small amounts of metal such as iron, gallium, lead, tin, or indium to the inside of the quartz tube. Lamps with metals added to the base mercury and inert gas mixture are typically referred to as doped, additive, or metal halide.
By contrast, the output of UV LED curing systems is mostly in the ultraviolet band with negligible visible output and no infrared. UV LEDs emit light whenever current flows through an arrangement of fabricated solid-state diodes. The spectral output is based on complex material science where hundreds or thousands of diodes are grown layer by layer on 4, 6, 8, and 10” diameter wafers in clean rooms and then individually diced or extracted following fabrication. The spectral output of UV LEDs is not something that can be changed or tuned following production. Discrete diodes, which are roughly 1 mm square, are packaged into a single row, series of rows and columns, or other configuration to produce the desired length and width curing lamp. A complete UV LED curing system will incorporate tens, hundreds, or even thousands of LEDs in the overall design.
Spectral output is defined within the entire electromagnetic spectrum according to the magnitude of its wavelength. For ultraviolet energy, wavelengths are on the order of a billionth of a meter (0.000001 m) or nanometer (nm). The term ultraviolet derives from ultra which is an adjective meaning very or extremely and violet which refers to the color of the shortest portion of the visible spectrum. The band of light classified as ultraviolet includes all light energy having wavelengths between 200 and 400 nm. It should be noted that the distribution of wavelengths and the peak irradiance of those wavelengths are not uniform across the entire UV band. The best way to illustrate and communicate the relative distribution as well as the relative power across the distribution for a particular lamp or system is with a spectral output graph.
Both UV equipment suppliers and lamp (bulb) manufacturers provide spectral output graphs for the various products they offer. These charts plot wavelength on the x-axis against irradiance on the y-axis. The irradiance can be displayed in several ways including measured output (W/cm2), relative output (unitless), and normalized output (percentage). Relative output depicts the portion of UV irradiance at a given wavelength or band of wavelengths with respect to the entire output. Normalized output sets the greatest output value to 100% with the other irradiance values shown as a relative percentage of the maximum. Relative and normalized output graphs are the most common representations.
The following spectral output chart illustrates the general relationship between a broadband mercury lamp and five commercially available UV LEDs. Standard mercury output is represented by the blue shaded area, and UV LED output is represented by the purple bell curves. Approximately one third of mercury output falls in the infrared region located to the right of the visible band and not shown in this illustration. To address the infrared component, UV system manufactures, press OEMs, and integrators have incorporated lamp head and production line features that absorb and carry away unwanted heat generated by mercury lamps. UV LED systems and their absence of infrared have an advantage in that they transfer considerably less total heat than conventional mercury lamps, but ultraviolet wavelengths are still energy which is ultimately converted to some heat at the cure surface.
What important information do spectral output charts communicate? First, this chart very clearly illustrates the difference between mercury broadband output (UVC, UVB, UVA, visible, and infrared) and the relatively monochromatic output of UV LED technology. Secondly, it demonstrates how the magnitude of irradiance varies significantly 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 both 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 the chart in this post is only a general illustration for one specific mercury lamp and five commercially available LEDs with normal bell curve representations. The distribution of the mercury lamp would be slightly different for another vendor and significantly different for an additive lamp. Examples of spectral output graphs for mercury, iron, and gallium can be found on the Eminence UV website: www.eminenceuv.com/sources. With respect to the LEDs, the peak irradiances as well as the shape of the profiles vary significantly by product and supplier, but due to the monochromatic nature of LEDs, minor shifts generally do not produce much of a difference in cure. 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 exact spectral profile and wavelength at which the LED curves peak.
How are spectral output charts used in practice? To start, raw material suppliers and formulators design their products to react to the spectral output of a given source or category of sources. Not all UV sources will cure all formulations, and certain spectral outputs are better suited to some applications than others. This is because, despite photoinitiators absorbing UV light over a wide range of wavelengths, a given photoinitiator is always more reactive to certain wavelengths, requires a minimum threshold irradiance to initiate, and produces different aesthetic and functional photopolymer properties depending on its design and interaction with both UV energy and the rest of the chemistry. Formulators evaluate the various photoinitiator absorption curves against the UV system’s spectral output chart and make trade-off and blending decisions based on the needs of the manufacturing line or press and the final product’s requirements of use.
As the following image illustrates, longer UVA and UVV wavelengths penetrate deep into inks, coatings, and adhesives while shorter UVC wavelengths activate the surface chemistry. Based on this information as well as the spectral output of commercial curing units, formulators recommend which sources 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 (275, 365, 385, 395, or 405 nm). While it should be standard practice, only some specify a minimum peak irradiance and energy density requirement for full cure or disclose which UV unit was used in their lab to develop the chemistry. Ultimately, formulators are tasked with making sure their products work across a wide range of UV curing systems, which is not always an easy task.
There is no UV LED source that directly mimics a broadspectrum 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 utilize longer (near visible) wavelengths to penetrate deep into the 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 sufficient crosslinking at the surface, and photoinitiators that react to longer UV LED wavelengths tend to 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 in penetration (UVA and UVV) but can struggle with surface cure (UVC). The result is that UV LED curing leaves some formulations tacky or greasy to the touch. Optimizing chemistry, properly selecting a UV LED source, and thoughtful integration can often eliminate these surface cure issues. Adding UVC diodes to an LED curing device may be necessary for more problematic industrial coatings; however, despite the fact that UVC LEDs at 275 nm have made significant improvements in peak irradiance, reliability, and life, they are still significantly more expensive than UVA LEDs and not yet economically viable for most applications.
For almost 60 years, the UV curing industry has been formulating to the spectral output of conventional mercury and mercury doped lamps. All products utilized raw materials specifically designed to respond to mercury's broadspetrum. Dedicated development work in narrow band UVA LED chemistry among a few formulators started between 2005 and 2010, but most of the curing industry took a wait and see approach. More formulators entered between 2010 and 2015, and many are just now getting started.
In general, conventional UV chemistry designed for broadspectrum UV does not cure well with longer wavelength and relatively 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 start to develop their own 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 mercury lamps. This is known as dual cure chemistry.
Over the coming years, more and more formulations will offer dual cure capability, and the mercury only formulations will be made redundant and ultimately discontinued. This does not mean everything designed for mercury lamps will disappear tomorrow as many industries, particularly those using highly functional industrial inks and coatings as well as those engaged in high speed, wide web applications and complicated 3D part profiles, still require development work on formulations, lamps, and integration. This statement simply means that it is necessary to pay attention to what is happening in your specific industry in order to understand the impact of LED on your own product portfolio. The transition to UV LED will be calculated and gradual based on each industry and application, but it will happen. For those new to curing and UV LED, a baseline understanding of spectral output followed by irradiance and energy density is a great place to start.