Multispectral imaging, or photography using wavelengths other than ordinary visible light, has various applications ranging from Earth observation to forgery detection in art. For example, titanium white and lead white, two pigments used in different historical times, look identical in visible light but have distinct signatures in the UV range. Similarly, IR imaging can reveal the inner layers of a painting if the pigments used are transparent to IR.
The equipment for such niche use is naturally quite expensive, so [Sean Billups] decide to turn an old model of smartphone into a portable multispectral camera, which can help him analyze works of art without breaking the bank. It uses the smartphone camera with a filter wheel that allows it to capture different spectral ranges. [Sean] chose to use a Google Pixel 3a, mainly because it’s available at a lower cost, but also because it has a good image sensor and camera software. However, modifying the camera to allow IR and UV imaging proved a bit difficult.
Image sensors are naturally sensitive to infrared and UV, so cameras typically include a filter to block out all but visible light. To remove this filter from the Pixel’s camera [Sean] had to heat up the camera module to soften the adhesive, carefully remove the lens, then stick a piece of plastic over the filter and peel it off once the glue has set. Perfecting this process took a bit of trial and error, but once he managed to make a clear separation between camera and filter, it was just a matter of reattaching the lens, assemble the phone and mount the filter wheel on its back.
The 3D printed filter wheel has slots for four different filters, which can enable a variety of IR, UV and polarized light imaging modes. In the video embedded below [Sean] shows how the IR reflectography mode can help reveal underdrawing in an oil painting. The system is designed to be expandable, and [Sean] has already considered adding features such as IR and UV LEDs, magnifying lenses, and even additional sensors such as spectrometers.
We’ve already seen a handful of multispectral imaging projects; this drone-mounted system was a candidate for the Hackaday Prize 2015, while this project contains an excellent introduction to UV imaging.