Due to a lack of UV equipment specification standards and the need for different radiometry measurement devices, label and flexible packaging producers may be confused when comparing different UV LED curing systems. Understanding the UV LED equipment specifications and how to properly measure UV LED output can help flexographic and narrow web end users make legitimate comparisons and better decisions when choosing a UV LED curing system.

End users often compare UV LED curing systems based on electrical power input, radiant power output, and the efficiency calculated from them. However, what is useful for an optimal industrial process, such as the UV curing of label and flexible packaging inks, overprint varnishes or laminating adhesives, is information and comparison values about the intensity, homogeneity, and distribution of the UV radiation onto the substrate. Unfortunately, UV LED equipment specifications are not standardized and often imprecise. In addition, there are no standards for radiometry measuring equipment specifications and test methods, and little knowledge of what measurements are important for accurate comparison of UV LED curing systems and their production performance.

Mean power density is the important value
Manufacturers of UV LED curing systems usually specify peak power density at the exit or emission window. However, these values are misleading and unsuitable to obtain information about curing effectiveness. A UV LED curing system with high peak values may not have a very homogeneous radiation distribution, resulting in uneven curing. Moreover, peak values depend on the choice of measuring instrument. For example, measuring results for short distances (less than 25 mm) are very much determined by the instrument’s detector properties and hardly comparable at all.

CAUTION! Both peak values and mean power density are specified in W/cm², but the values refer to different output variables and are therefore not comparable.

To compare UV LED curing systems, it is better to use the mean power density at the emission window. Mean power density is determined by dividing the total radiated power (W) by the emission window area (cm2). The resulting units are W/cm2, but should not be confused with irradiance, whose units unfortunately are also W/cm2! That means both peak values and mean power density are specified in W/cm², but the values refer to different output variables and are therefore not comparable.

It is not sufficient to specify only the output
Heraeus’ independent and accredited measuring laboratory measured and compared the mean power density of three different UV LED curing systems. The total radiated power was determined by means of goniometry and the window area provided by the product technical data. Fig. 1 shows that major differences were found even though all three devices were from the “same” 8 W/cm² class.
 
In addition to the mean power density of the UV LED curing systems, the available UV irradiance is decisive for a process. Special micro-optics shape the beam to concentrate the radiant energy onto the substrate where it is needed for curing and reduces wasted or stray radiation. The result is higher useful irradiance and less loss for an optimal and efficient curing process. Systems with specifically adjusted micro-optics have very high irradiance values even at large distances from the working surface Fig. 2 illustrates the benefits of micro-optics on forward irradiance.

Measuring instruments for reliable and comparable measuring results
In spite of the limitations of various measuring devices and the variation in any individual measurement value taken with different instruments, with proper techniques we can use these values to set-up and monitor a UV LED curing process. The measurement instruments are simple to use and designed for various purposes. Nevertheless, certain requirements must be met to obtain reliable and, above all, comparable measuring results:

• The measuring instrument should have a flat spectral sensitivity, the red line shown in Figure 3, with an appropriate calibration. A device calibrated for mercury lamps is not suitable to measure UV LEDs. UV LEDs require special measuring instruments.
• The measuring instrument must have a cosine response to be able to also consider radiation impinging on a surface from all angles. The measuring instrument must be able to measure the high intensities and also withstand the high temperatures.
• The third important parameter is the precise determination of the measuring distance, i.e. the distance between UV LED source and measuring instrument detector.

For more detailed information about measuring UV LED system output and process specification and troubleshooting download Comparing Traditional UV Systems with LED UV Systems for UV Curing.

Tests carried out under real process conditions enable accurate comparisons
Now it is clear that even if 8 W is on the UV LED curing system label, 8 W are not always delivered. Uniform terms, appropriate measuring instruments and methods provide information about which UV LED curing system might be suitable for your process. The final UV LED curing system decision can be made based only on tests carried out under real process conditions.

The most important radiometry and metrological terms:

• Radiated power / radiant flux: Entire power emitted by radiation; unit: watt (W)
• Irradiance: Radiated power arriving on a unit of area; unit: W/cm²
• Radiant exposure / dose: Time integral of irradiance over a certain period of time; unit: J/cm²
• Mean power density at the exit window: Total of radiated power (W) / window area (cm²); used to compare UV-LED systems, resulting units are W/cm2
• Power density (peak value): Typical data to describe the power output of devices; means the max. irradiance directly at the exit window; unit: W/cm²

ATTENTION: W/cm ≠ W/cm²

• W/cm: Describes medium-pressure lamps in electrical watts divided by bulb length
• W/cm²: Describes the unit of irradiance or mean power density of UV-LEDs
• Goniometric measurement: A detector is moved around the radiation source in such a way that its entire 

Dawn Skinner is Heraeus Noblelight’s Process Development Manager for Europe.  Heraeus welcomes the opportunity to be of assistance. Experienced application experts are available at our development and application centers worldwide. The Heraeus Noblelight measuring laboratory measures various lamp types and devices and has decades of experience with technical light sources. Customers profit from the know-how of the measurement laboratory team and the equipment.