Spectral Responses of Light-Emitting Diodes as a Photodiode and Their Applications in Optical Measurements

Spectral responses of four light‐emitting diodes (LEDs) were investigated because their uses as a photodiode are growing these days. An LED showed reasonable sensitivity to shorter wavelengths which the LED emits. Their spectral response ranges are same or relatively wider than their emission range. In addition, LEDs were used as detectors to analyze test dyes with absorbance measurement technique or with fluorescence measurement technique. Absolute noise level of 0.0001 absorbance unit was observed with the blue LED (nominal λmax = 455 nm) photodiode. Phosphate amount in Kumho River (located in Gyeongsan, South Korea) was determined using IR LED (nominal λmax = 805 nm) as a photosensor. A green LED (nominal λmax = 520 nm) was used to excite rhodamine B (excitation λmax = 560 nm, emission λmax = 575 nm) and red LEDs (nominal λmax = 660 nm) were used to detect fluorescence light emitted from the fluorescence dye. The LOD of rhodamine B was found to be 26 nM with four red LEDs as a photodiode and without using optical filters.