In this experiment, near infrared (NIR) spectroscopy is used to detect blood glucose non – invasively in earlobe since it is thin. Beer Lambert’s law is applied for calculating amount of light absorbed by glucose and water. Additionally, light intensity ratio declares the absorbance:
Then, intensive light is converted into electric specifically in voltage (V) by using Indium Gallium Arsenide (InGaAs) photodiode detector. When light achieves to photodiode, it emits a current which will transform into voltage. Ip represents the current from the photodiode and Rf is the feedback capacitor which minimize the noise gain and keep SNR big. Additionally, op-amp also known as transimpedance amplifier (TIA) circuit which connects with photodiode to decrease high frequency noise. Photodiode voltage directly proportional to intensive light of near infrared which relates to concentration of blood glucose4.
The reason why the absorbance is converted to voltage is solutions transmit more than 90% or less than 10% incident light will deviate from Beer’s law and give unreliable results. Voltage is logarithmically related to the absorbance: A = logVo – logV5.