Photography using multispectral imaging, or wavelengths other than ordinary visible light, has a variety of applications, from observing the earth to detecting fraud in the industry. For example, titanium white and lead white, two pigments used in different historical eras, 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 the IR.
Tools for using such a niche are naturally quite expensive, so [Sean Billups] Decided to transform an old model smartphone into a handheld multispectral camera, which could help him analyze the workings of the industry without breaking the bank. It uses the smartphone’s camera with a filter wheel attachment that enables it to capture different spectral ranges. [Sean] Chosen to use a Google Pixel 3a, mainly because it is cheaper, but also has a better image sensor and camera software. Changing the camera to enable IR and UV imaging turned out to be somewhat of a challenge.
Image sensors are naturally sensitive to IR and UV, so cameras usually include a filter to block anything other than visible light. To remove this filter from Pixel’s camera [Sean] The camera module needs to be heated to soften the glue, the lens has to be carefully removed, then a piece of plastic glue has to be put on the filter and it has to be taken out when the glue is set. This process took some testing and error to perfect, but once he was able to implement a clear separation between the camera and the filter it was simply to reconnect the lens, assemble the phone, and mount it behind the filter wheel.
The 3D-printed filter wheel has slots for four different filters, which can enable different types of IR, UV and polarized-light imaging modes. In the video embedded below [Sean] Shows how IR reflection mode can help to express underdowns in an oil painting. The system is designed to be stretched, and [Sean] Already looking at adding features like IR and UV LEDs, additional sensors like magnifying lenses and even spectrometers.
We’ve seen a few multispectral imaging projects before; This drone-mounted system was a contender for the 2015 Hackade Award, while the project contains an excellent primer for UV imaging.