Hackaday Prize 2022: Multispectral Smartphone Camera Reveals Paintings’ Inner Secrets

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Multispectral imaging, or photography using wavelengths other than ordinary visible light, has a variety of uses, from earth observation to counterfeit detection in the arts. For example, two pigments used in different historical times, titanium white and lead white, look the same in visible light, but have distinct characteristics in the UV range. Similarly, if the pigment used is transparent to IR, IR imaging can reveal the inner layer of the painting.

Equipment for such niche use is of course quite expensive, so [Sean Billups] Decided to convert an older model smartphone to a handheld multispectral camera. This allows him to analyze the work of art without spending money. He combines a smartphone camera with a filter wheel attachment to enable capture of different spectral ranges. [Sean] I chose to use the Google Pixel 3a not only because it’s cheaply available, but also because it has excellent image sensors and camera software. However, changing the camera to enable IR and UV imaging turned out to be a bit difficult.

Because image sensors are naturally sensitive to IR and UV, cameras usually include filters that block anything other than visible light. To remove this filter from your Pixel camera [Sean] Heat the camera module to soften the glue, carefully remove the lens, glue a piece of plastic to the filter, and pull it out when the glue has set. It took a bit of trial and error to complete this process, but once the camera and filter were clearly separated, all I had to do was reattach the lens, assemble the phone, and attach the filter wheel to the back.

The 3D printed filter wheel has slots for four different filters, allowing you to enable different IR, UV, and polarization imaging modes.With the video embedded below [Sean] Shows how IR reflection mode can help reveal underdraws in oil paintings. The system is designed to be extensible and [Sean] We are already considering adding features such as IR and UV LEDs, magnifying glasses, and even additional sensors such as spectrometers.

So far, we’ve seen several multispectral imaging projects. The drone-equipped system was a contestant at the 2015 Hackaday Awards, but the project includes an excellent introduction to UV imaging.

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