Measuring Broadband UVA And Violet Led Light Sources For Industrial And Medical Applications

For the above-mentioned applications, the following measurements are typically used:

1. Irradiance measurement
2. Irradiance uniformity mapping
3. Dosage measurement of a static or moving source

With this type of measurement, we normally want to measure the irradiance at a certain distance from the source. The sensor should be able to collect light at steep angles and measure it correctly. This property is called 'cosine correction' because, by definition, the irradiance equals the power density multiplied by the cosine of the angle of incidence and is usually achieved using a PTFE diffuser as the input optic of the sensor (See Fig. 1).

With this type of measurement, we want to measure the irradiance distribution on the illuminated area (See Fig. 2). Consistent processing with UV light requires uniform illumination. Uneven illumination can lead to under exposure in some areas and over exposure and cracking in others.

For this application, the sensor should have a small aperture in order to allow mapping with good spatial resolution. However, achieving a small aperture of a few millimeters and good cosine correction is challenging, and users usually must choose one or the other. This is due to shadowing effects that occur when the thickness of the diffuser and aperture become comparable with the aperture diameter.

In the dosage measurement application, the irradiance is accumulated over time and the total energy density in [J/cm2] is measured. This is the true measurement of the number of photons reaching an area to perform the curing process. If the UV source is scanning over the work area, as is the case in UV 3D printers, or if the work area is a conveyor passing underneath a static UV source, as is often the case in large UV curing machines, the sensor should be able to sample at a high rate and track the changes in irradiation (see Fig. 3). Good cosine response is also important because the sensor will be illuminated from different directions due to the motion of the source or the sensor.

In printing, curing, and 3D printing of polymers, LEDs in the UVA range (mostly 365-395 nm) are used. Some applications also make use of violet LEDs at 405-425 nm. Unlike the fixed and narrow UV spectral lines of traditional mercury lamps at 254 nm, 365 nm, and 405 nm, LEDs have a spectral bandwidth of 10-25 nm. Their peak wavelength is specified with an uncertainty of several nanometers and tends to shift when they heat up.

The first graph in Fig 4 shows the spectra of three different LED sources with the same peak wavelength and different spectral bandwidths (a). The second graph shows three different LED sources with same spectral widths but different peak wavelengths (b). With the third graph, the spectra of three different LED sources are presented, each with a specific peak wavelength and different spectral bandwidth (c).

To see how this affects measurement accuracy, we need to consider three types of irradiance sensors.

uses a wideband filter for UVA (315-400 nm), UVB (280-315 nm), or UVC (200-280 nm). The sensitivity changes significantly within the spectral range of the sensor and the user has no way of 'telling' the sensor which wavelength is being measured. In the example shown above, an LED at 365 nm peak will produce a measurement 10 times higher than that of a 395 nm LED with the same power. See illustration in Fig. 5.

the user the possibility of entering the LED wavelength, thus adjusting the measurement accordingly. While this is a great improvement over the previous method, some limitations still exist, because as already mentioned, the wavelength is not exactly known, or can shift and LEDs have a finite bandwidth. Put together, these can still cause measurement errors of 15%. Additionally, the two types just described cannot handle measurement of several LEDs with different wavelengths simultaneously.

Taking the above aspects into consideration, a sensor with a flat spectral response will be the best choice when it comes to measuring irradiance without being sensitive to LED wavelength, bandwidth, and the combination of several LEDs.