Q&A: Where, under a UV lamp, should a radiometer be located for an accurate measurement?

By R.W. Stowe, UV applications engineering consultant, Heraeus Noblelight America LLC
Figure 1. For medium-pressure projection lamps, the radiometer can be placed on the plane of the work surface or the surface of interest.

We’ve heard this question several times. UV radiometers come in many shapes and sizes. Some are used only for measurement at a single point, while others are designed to make measurements while passing at a distance under the UV source.

For many years, the majority of UV lamps have been projection lamps, focused or unfocused, employing a shaped reflective surface behind the bulb, designed to deliver UV to a work surface. Typical measurements of the UV energy at the surface include irradiance (W/cm²), exposure (“dose”) (J/cm²) or irradiance profile – a “map” of the flat irradiance pattern on a surface.*

The object, of course, is to determine the UV irradiance at the surface to be cured. Integrating radiometers will electronically calculate the dynamic exposure (J/cm²) as the instrument passes (for example, on a conveyor) under the lamp. Most radiometers are a filter-detector type, with each model having its own wavelength band response, limited by the filter and detector set incorporated in the instrument. The differences in response bands is one reason the model of radiometer must be reported with data.

Medium-Pressure Mercury UV Lamps

Figure 2. Typical irradiance profiles of MP Hg lamps, illustrating highly or less focused lamps, or on a plane nearer or farther from the focus zone of the lamp.

To measure medium-pressure (MP) projection, or focused lamps, the work surface – and correspondingly the radiometer – would normally be a few inches from the lamp, or farther in the case of “far-field” exposure, as in 3D surface curing.

Radiometers that are popular for use in MP measurements are approximately ¼ inch to ½ inch thick – raising the question of where and how they should be positioned for measurement of lamp output. The argument arises from the fact that a diffuser is typically located at the top surface of the radiometer, while the actual sensor is located nearer the bottom of the instrument, beyond optical components such as an aperture and filters. So, where does the “peak” of a focused lamp occur? Actual measurements reveal that this point is somewhere in the “middle” of the thickness of the radiometer – and this will vary depending on the convergence or divergence of the UV radiance. Raising the lamp a little from the working plane to “correct” for this little difference may be impractical or cumbersome.

Figure 3. Typical radiance patterns of UV-LEDs and the near-field work plane.

As a practical solution, for consistent and reproducible measurements, the radiometer is placed on the surface of interest, and the “distance” is noted from the lamp face to the surface on which the radiometer is placed (or the bottom of the radiometer). This also makes conveyorized process measurements much simpler.

What About LEDs?

Figure 4. Irradiance vs. distance for several different UV-LEDs.

The same practice may not be possible for UV-LEDs. The LED array, essentially a cluster of point-sources, exhibits a divergent radiation pattern with irradiance diminishing as a function of distance. Also, the LED array is separated from the work region by a protective window. Various optical treatments, including macro-optics and micro-optics, have been applied to reduce the divergence and increase irradiance at a distance. For UV curing, because the UV-LED lamp generally is placed as close to the work surface as practical, it is difficult to make dynamic measurements because there is little space to accommodate a radiometer. Consequently, most reported data are measured with the lamp alone. For UV-LEDs, it has become a usual practice to note the distance from the lamp face (window) to the radiometer top surface, where the diffuser is located. The distance reported typically is the distance from the window to the diffuser of the radiometer.

We can conclude that, when reporting irradiance data, the best practice is always to report the distance from the MP lamp face to the bottom of the radiometer or from the UV-LED window to the diffuser face of the radiometer. Whichever distance is reported, it should be clearly stated. Consistency and reproducibility of data are important to process specification or troubleshooting when necessary.

*We discussed the spectral response and application of radiometers for use with medium-pressure mercury lamps and with UV LEDs in previous columns (UV+EB Technology, Vol. 1, No. 2 and Vol. 2, No. 1).