OSpRad: an open-source, low-cost, high-sensitivity spectroradiometer

Spectroradiometery is a vital tool in a wide range of biological, physical, astronomical and medical fields, yet its cost and accessibility are frequent barriers to use. Research into the effects of artificial light at night (ALAN) further compounds these difficulties with requirements for sensitivity to extremely low light levels across the ultraviolet to human-visible spectrum. Here I present a open-source spectroradiometry (OSpRad) system that meets the design challenges of typical ALAN research. The system utilises an affordable miniature spectrometer chip: the Hamamatsu C12880MA, and combines it with an automated shutter and cosine-corrector, microprocessor controller, and graphical user interface “app” that can be used with smartphones or desktop computers. The system is designed to be user-friendly, adaptable, and suitable for automation/data-logging. All code and 3D printed parts are made available open-source. I constructed 5 units and tested their linearity, spectral sensitivity, cosine-corrector performance, and low-light performance. There were modest unit-specific differences in spectral sensitivity, implying calibration is required for maximal accuracy. However, depending on the application, it may be acceptable to use a default calibration template together with careful experimental design considerations to mitigate unit-specific differences. All other performance characteristics were highly consistent. The OSpRad system was able to measure spectral irradiance down to around 0.005 lx, and spectral radiance down to 0.001 cd.m-2, meaning it would be able to measure night-time lighting under the vast majority of real-world conditions. The OSpRad system’s low cost and high sensitivity make it well suited to a range of spectrometry tasks in general, and ALAN research in particular. The project is hosted on GitHub here: https://github.com/troscianko/OSpRad