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LED-based light source for ultraviolet-visible absorption spectrophotometry

Light-emitting diodes (LEDs) are widely used lighting applications that have several advantages over conventional lamp light sources, including higher mechanical robustness, faster response time, less energy consumption, and longer lifetime. In this study, we investigated the feasibility of LED-based absorption spectrophotometry. To implement an ultraviolet-visible (UV-VIS) absorption spectrophotometer, a light source was prototyped consisting of two types of LEDs: a UV LED (340 nm) and a broadband white LED (about 380-800 nm). A dichroic filter was used to combine the light of the two types of LEDs. Compared with conventional lamp light sources, the area of LED light emission is quite small and the emitting properties are not uniform. Therefore, to make the irradiance areas of two LEDs uniform, we applied an optical diffuser to the ultraviolet LED to widen the light-emitting area and broadly cover the irradiance areas from a white LED. This optical alignment enabled a configuration to withstand the mounting tolerance of approximately ±0.3 mm of LEDs on substrates. Another issue is that temperature must be stabilized because electrical and optical properties of LEDs strongly depend on temperature. Thus, we utilized a Peltier controller to precisely control temperature control of an LED-mounted substrate (less than ±0.01°C) and to achieve the target value of temporal drift within ±0.05% per hour of light intensity in photometric signals. The obtained results suggest that our developed LEDbased light source should be useful for a UV-VIS absorption spectrophotometer.