Wearable Inverse Light-Emitting Diode Sensor for Measuring Light Intensity at Specific Wavelengths in Light Therapy

Optical measurement is conducted by employing components such as a spectrometer or photodiode (PD) to detect light intensity. However, the size of spectrometer is large because of its complex optical path and component. Moreover, a PD usually detects the total light intensity instead of the intensity of light at a specific wavelength. Therefore, a novel inverse light-emitting diode (LED) sensor was developed to measure light intensity at specific wavelengths because of the reverse effect of LEDs. In the experiments conducted in this paper, red, green, and blue LEDs were used in combination as the inverse LED sensor and light source. Moreover, commercial equipment such as a PD, an illuminometer, a power meter, and Actiwatch 2 were simultaneously employed to detect the light intensities. The results revealed that the intensity ratio evaluated using the inverse LED sensor was proportional to that of the light source. This implies that the correlation between the incident light and measured intensities from the proposed inverse LED sensor was stronger than commercial equipment. The relationship between intensity ratio, irradiance distance, and illumination angle was also determined. The least difference between the measured and ideal evaluated intensity ratio was 2.4%. The proposed inverse LED sensor has following advantages: low cost, small size, high-wavelength selectivity, and high-intensity correlation. In a future study, light intensity can be measured using wearable devices comprising the proposed inverse LED sensor, and the presented relationship can be evaluated to investigate the effect between the light intensity and physiological mechanisms or diseases.