Spectral Properties of UV LEDs and UV LED Systems – Characteristics, Measurements, and Effects
Peter Sperfeld | Physikalisch-Technische Bundesanstalt
The spectral characteristics of LEDs are important parameters for determining electrical efficiency, efficacy and cost-effectiveness of LED-based systems. Quantities such as peak wavelength, centroid wavelength, irradiance, total radiant power can be determined from measurements of the spectral (and spatial) distribution of optical radiation emitted by LEDs. The more accurate these measurements are, the better LED systems can be planned, and efficient use can be achieved for a wide variety of applications. However, especially for UV LEDs in the short wavelength spectral range, spectral measurements are particularly challenging because many parameters can influence an accurate measurement. Measurements with low measurement uncertainty in the UV often require a lot of equipment and a reasonable amount of competence and experience in the field of spectroradiometry.
In this tutorial we will discuss the minimum requirements for a proper spectral measurement of UV LEDs and the deviations that can result from erroneous measurements and measurement conditions. It will be explained how key radiometric characteristics of LEDs can be determined from a spectral measurement and what accuracy may be expected. The development of UV LEDs with high radiant power, increasing stability and lifetime, as well as increased efficiency has progressed considerably in recent years.
However, the spectral characteristics of UV LEDs of one type still depend on important parameters such as working temperature, electrical power and aging. Also, the variation in production of one LED type can lead to significant variations in spectral properties. Therefore, the characteristic spectral quantities of an LED system are usually not the sum of the individual LEDs used. The dependence and variation of the radiometric characteristics of UV LEDs of one type will be considered in this lecture, and the possible effects on the design of LED systems will be addressed. Furthermore, it will be discussed how the radiometric characteristics of different LED systems can be compared with each other, for example, in order to be able to define rules for the interchangeability and equivalence of systems.
Especially in the field of UV disinfection, the effectiveness of the lamps used plays an important role. It depends on the microbicidal effective irradiance, which can be calculated in knowledge of a microbicidal action function and the spectral irradiance. In addition, for UV LEDs an efficacy factor can be determined, which is strongly dependent on the spectral location of the LEDs.
In the presentation, the calculation of the microbicidal irradiance and the efficacy factor for LEDs will be explained and compared with the values for existing UV disinfection systems. Finally, it will be discussed how radiometric parameters of UV LEDs and UV LED systems could be included in future standards.