Design of a Smartphone Platform Compact Optical System Operational Both in Visible and Near Infrared Spectral Regime

We report here the design of a low-cost and field portable smartphone-based compact optical system, which is suitable for applications in the visible (VIS)-near infrared (NIR) spectroscopic region. Using the universal serial bus-on the go protocol, the battery of the smartphone is used to power an external light-emitting diode, and its ambient light sensor (ALS) is used as a photo-detector. Using simple optical arrangements, a collimated light beam of a specific wavelength is allowed to pass through the sample solution, and the transmitted modulated light signal from the sample is received by the ALS of the smartphone. A custom built application "PhotoSense" has been developed, which configures the smartphone to measure intensity variation of the modulated signal into a readable form from the ALS sensor. Further using the same application, the field sensing data can be saved as a text file in the phone memory, and by using the communication facility, it can be shared anywhere in the world. The performance of the designed optical device has been evaluated for the VIS-NIR wavelength region of the incident light by measuring iron(II) and phosphate concentrations in water medium. The sensitivity of the device in measuring iron(II) and phosphate concentrations is found to be in terms of absorbance unit (AU) as 0.189 and 0.274 AU/(mg.L-1), respectively. The limit of detection of the proposed device is found to be 0.053 and 0.069 mg.L-1 for both the analytes.