I want to explain the issue of UV light before we start. UV radiation can’t be used for skin disinfection because it will damage the molecules and can cause serious health problems.
Secondly UV light is not the best term for what we are going to discuss here. The light refers to the visual part of the optical radiation. I guess that it is because the UV lamps are produced mainly by the lighting companies and the industry uses UV-light and UV-lighting expressions. Now, that it has been clarified, let’s dive into the UV radiation that depends on the wavelength and can be used for disinfection, skin treatment or activation of chemical processes in the industry.
Nowadays, due to the pandemic, we witness a huge increase in the demand for the UVC lamp measurements to verify the performance of germicidal UVC lamps (GUV lamps) and UVC LED sources for sterilization or disinfection of the surfaces. To control the quantity of the optical radiation, we can measure irradiance.
What is irradiance?
If you are working on a new project and want to be sure that there is enough signal to do the job, you need to check how much signal (UV radiation) falls on the contaminated surface. UV radiation, especially in the UVC range between the 200 and 280nm has a damaging effect on viruses and bacteria but critical is the power and time. It is the dose that is to guarantee the effectiveness of the sterilization process.
For this purpose, we can use the irradiance measuring device. To understand the irradiance [W/m2] we can say that it is the radiometric equivalent of illuminance [lx]. So, it is a part of radiant flux emitted by the source or lamp that falls on a specific surface. The table below shows some information about the critical levels to kill the novel coronavirus. Please note that the available data referes to previously recognized coronaviruses and we will need to wait before there are new research available for SARS-CoV-2. However it is a very good reference point.
How to measure?
Similar to the lighting, the UV radiation is electromagnetic radiation produced by the source. UV lamp turns part of the electrical power into the optical radiation. The irradiance can be measured with specially designed optical measurement probes. Such probes (or measurement heads) can be integrated with the measuring device or connected with a cable. The important point is that such a measuring device, including the probe, must be properly calibrated for the irradiance measurements outside the visual part of the spectrum. Unlike the relative UV measurements for scientific research, when working on UV disinfection or medical treatment with UV lamps, it is critical to measure the absolute irradiance value. There are not many devices available on the market which are actually ready to measure absolute irradiance below 350nm. Many of the available meters are relative. The picture below shows the spectral irradiance measuring device GL SPECTIS 5.0 touch with an integrated measuring head and the fiber optic model.
The distance changes everything!
The physics of the optical radiation stays the same regardless of the wavelength range. It means that the inverse square law works for the UV lamps too. In principle, the signal reduces as the distance from the source increases. In practice, the further from the UV source, the smaller signal level we get. A good point is that, when developing a new UV disinfection system we can measure the signal level from a specific distance. Then if we do the math we can calculate how much radiation will be “delivered” from a typical working distance. Usually, germicidal lamps provide a very strong signal because they can be placed in quite a large distance from the surfaces. This is why disinfection lamps still use strong power discharge lamps. UV LEDs are suitable mainly for the air and water disinfection. However, there is a growing increase in the use of UV LED sources to replace conventional lamps. This, in my opinion, will take a long time before we are ready to replace existing mercury lamps.
The angle matters too!
Continuing with the physics of UV radiation, we cannot forget that the irradiance / the radiant power falling on a surface / changes with the angle cosine. It means that the signal level measured flat on a surface which is parallel to the source is the highest. When we start changing the surface’s angle, we will measure changes in the signal level. This is known as the cosine law ( Lambert’s Cosine Law) which we sometimes forget about when working on the lamp design. It is important to understand that LED is a discreet but flat emitting surface and the emission from the source is changing with the angle. Gas discharge lamps which are built with the use of glass tubes have much different shapes and sizes comparing to UV LED. We cannot change the physics even if we work with semiconductor sources. Please note that the available UV LEDs are usually delivered with optics which are modifying the Lambertian distribution to more directional. This also lowers the efficacy of the source before it is built in the system. The picture below shows the simplified radiant intensity angle dependence. Full signal level at normal to the surface and dropping to 9% if the direction of the radiant distribution is “almost” parallel to the surface.
What about the wavelengths?
Depending on the UV range you are working with, you need to consider different critical parameters. Usually for UV curing application, it is the radiation intensity that maters the most. The deeper in the UV, the more difficult is to verify the performance of the lamps. This is because the calibration of the measuring devices gets more challenging. But above all, the precise dominant wavelength characterization of the source becomes crucial. Killing viruses with UV radiation is possible only when a specific radiation is penetrating the molecules. A small shift of the wavelength determines the effectiveness of the process. This is why simple radiometers with a broadband UV response are not good enough for such demanding applications. This is where a spectroradiometer calibrated for the UV range measurement should be used. Only by the measurement of Spectral Power Distribution, precise and repeatable UV measurements can be done for performance testing of UV germicidal lamps or UV disinfection sources.
Last but not least – the safety consideration!
UV radiation is harmful for all living organisms. It kills viruses, bacteria and other biomaterials and so it can damage our skin and eyes. Please remember that UV lamps cannot be used for bare skin disinfection!!! CIE released important publications on UV radiation for disinfection purposes HERE.
Also when working with UV sources, special care must be taken to provide skin and eyes safety by protection glasses and clothes. Read more about the assessment of photobiological safety of lamps.
The irradiance measurement is a very practical method to determine the actual performance of the UV system. But there are other ways to characterise and measure UV lamps and sources. Stay tuned for the upcoming new posts on the Radiant Power measurements and Radiant Intensity Distribution measuring systems.
If you have specific questions regarding the UV measuring instruments you may contact GL Optic