Recent events related to the coronavirus pandemic have resulted in a significant increase in the interest in UV-lamp disinfection. Thus, in the article below selected issues regarding UV LED measurements along with practical tips on the specifics of working with LEDs for special applications are discussed.
by Miko Przybyla and Marcin Pelko | GL Optic
Disinfection realized by optical radiation is based on the mechanism of neutralizing viruses and bacteria by damaging their DNA chain. The effectiveness of the “lighting” devices in the process depends on the dose of radiation in a specific wavelength range. In relation to the disinfection purposes, UV lamps have been widely used for several decades now. The main light sources in the existing systems are low- and medium-pressure mercury lamps. This technology requires complex power supply systems to enable the ignition and stabilization of the discharge. Currently, many companies are working on the dissemination of applications for air, water and surface disinfection using UVC radiation obtained from LEDs.
The effective UV LED radiation for air or water disinfection is the UV radiation in the UVC range. When working on new projects using emitting diodes in the UV range for disinfection, mainly between 200 and 280nm, exceptional care should be taken to properly assess the quality of the emitted radiation. This applies not only to the LED radiation sources themselves, but also to entire optical systems as the properties of the materials used in the conventional illuminators do not necessarily work in this unusual application. For this reason, in such demanding applications, the accuracy of measurements of both irradiance and dominant wavelength is particularly important.
It allows precise verification of the optical power and the dominant wavelength. Moreover, it will determine the necessary dose of UV radiation.
Which measuring devices should be used?
With the conventional mercury discharge lamp technology, the spectral emission is constant as it results from the physical properties of the mercury vapors. Therefore, simple measuring devices were used to measure such lamps, mainly radiometers optimized for specific UV ranges. Such device can measure the quantity of radiant power falling on a given surface [W / m2]. However, it does not allow spectrum measurements. In other words, such a meter can measure how much radiation falls on the surface but the source characterization itself cannot be made because the sensitivity of the meter within a certain range is fixed.
It is worth noting that in the case of UV LED systems, there are differences between the production batches of the same LED source (bin to bin and batch to batch). Additionally, the elements used in the design of the lamp and LED operating temperature affect the optical parameters of the entire system. Therefore, in addition to power measurement, it is important to verify wavelength by spectral measurement. For this type of measurement, a calibrated spectroradiometer can be used. It will allow the measurements of irradiance [mW / m2] and will confirm whether there has been a shift in the radiation band or the spectral distribution.
It should be taken into consideration that the spectrometers available on the market, in most cases, are uncalibrated devices designed for comparative measurements of the spectrum itself. Whilst they work well for spectral measurements in scientific research, they cannot be used for radiometric measurements in engineering works, where a need to measure absolute values occurs. Especially in the UVC range, the durability of cheap optical materials and sensors is very limited. For a spectrometer to become a spectroradiometer, calibrated in absolute radiometric units, it must be properly designed, adapted and calibrated for this purpose. Only a device that has been calibrated in terms of wavelength, non-linearity and stray light can be calibrated to measure the spectral irradiance [W / m2 / nm] or the radiant power (total flux) [W]. We must remember that every professional measuring device is delivered with a factory calibration certificate or a certificate from an external calibration laboratory. More about the available solutions for UV measurement from GL Optic here
Typical measurement quantities
Depending on the stage of work on a new project, we may need to verify the parameters of the UV LED source itself, i.e. a single diode or LED module in combination with lenses and other elements.
For single UV LED measurements, a small integrating sphere connected and calibrated with a spectroradiometer can be used. In this way, a single measurement will collect data on radiant power [mW], dominant wavelength and spectral power distribution. This will help to determine the system efficiency and optimize the power parameters.
An optical probe connected to the spectroradiometer, calibrated to measure the irradiance [mW / m2] should be used to measure what part of the radiant flux falls on a given surface from a given distance from the source. In this measurement, there is also the ability to verify the wavelength and complete measurement of the spectral distribution. Based on this data, knowing the necessary amount of energy needed to neutralize viruses, the necessary exposure time can be calculated.
The distribution characteristics of LED sources significantly differ from conventional discharge lamps. Therefore, another very useful measurement system for verifying UV LED products is a goniometer connected to a spectroradiometer. Such a measuring setup enables the spatial measurement of radiant intensity distribution (RID). Based on such measurements a radiometric spatial distribution diagram can be created. These data allow precise verification and characterization of UV LED based emitter.
How to perform the measurements properly?
Recommendations for UV LED measurements are very similar to those for measuring LED modules and lamps for general lighting purposes. However, due to the nature of the source, attention should be paid to the issues of safety and risks to the skin and eye with UV radiation. It should be remembered that this radiation is harmful to all living organisms. During the measurement and operation of such lamps, the operator must not be exposed to direct radiation and should use safety glasses and clothing.
Technical data sheets of light-emitting diodes provided by manufacturers contain photometric data. In the case of diodes, radiometric data is determined for a junction temperature (T junction) equal to the ambient temperature, usually 25°C. Information about the radiant flux value and wavelength values in the catalog card correspond to the spectral distribution (spectrum) properties of the LED for the “cold” junction (at 25°C).
In order to verify the parameters of the diodes against the specification after application to the PCB, or to verify the mathematical model of the LED module after the PCB is made, it is necessary to measure the system while maintaining the junction temperature provided by the manufacturer in the specification (usually 25 ° C).
One of the methods applied can be the forced cooling of the PCB. It can be achieved through the use of a complex cooling system with a Peltier cell and knowledge of the thermal resistance between the board and the junction. The kits available for mounting and temperature stabilization of the LED module allow a constant temperature of the PCB during the measurements to be maintained and simulations of the module operation in various temperature conditions. This helps to improve the repeatability of measurements and allows the user to measure the operational parameters of the LED module at a temperature close to the operating temperature in the luminaire.
However, this method has its limitations because when different types of diodes are used on the module, it is impossible to obtain the same temperature of all junctions on one PCB due to different thermal resistances.
An alternative method is the measurement made with a short DC current pulse with a value at which the diode parameters are provided in the technical specification (in the production of LED). The essence of such a method is to measure optical parameters before the connector warms up. In practice, it is usually assumed that such a measurement should be made within 20-30ms. This can be done precisely with a spectroradiometer equipped with a hardware trigger (trigger) synchronized with the current source enabling exposures for measuring times <30ms. The measured spectral distribution allows the determination of radiometric parameters, enabling direct reference to the specification of a single LED. Diode manufacturers also use pulse measurement during “binning”, which makes it easier to refer to the specifications of individual components.
Another version of LED modules’ measurement at a junction temperature equal to the ambient temperature is the so-called Train pulse method, i.e. continuous supply with current pulses at a frequency of 100 Hz and pulse width below 0.5 ms. This solution allows the registration of several cycle periods at larger measurement times (helpful when measuring LED sources with large flux values) and the conversion of the obtained value by a factor corresponding to frequency and duty cycle for the train signal. With properly selected pulse parameters, the result is identical to the value obtained by measuring a single pulse with the length of 30 ms.
Similarly, to the visible range LEDs and UV LEDs are sensitive to power parameters. Therefore, stabilized power supplies with appropriate resolutions should be used for the measurements. Additionally, in the case of pulse measurements, it is necessary to select the power supply system so that the module for testing can be precisely and quickly supplied.
In the case of UV LEDs, the optical parameters’ dependence on the power supply conditions and operating temperature significantly influences their effectiveness in specialized applications in disinfection or other medical lamps. Small wavelength shifts are decisive in the effectiveness of specific impacts in the ultraviolet range (disinfection, treatment of skin diseases). Whilst, the stability of the system’s work over time is crucial for an effective fight against biological pollution.
When deciding to buy equipment for optical, electrical and temperature measurements, attention must be paid to the fact that their price directly reflects the quality of components and sub-assemblies used to manufacture these devices. Among other substantial issues, there are the calibration and adjustments of the measuring equipment. Especially in the field of disinfection realized by UV LED solutions the quality of the radiating component has a direct impact on our safety.
If you have specific questions regarding the UV measuring instruments you may contact GL Optic