Thermography: https://en.wikipedia.org/wiki/Thermography
IDK what the standard error is for medical temperature estimation with an e.g. FLIR ONE thermal imaging camera for an Android/iOS device. https://www.flir.com/applications/home-outdoor/
I'd imagine that sanitization would be crucial for any clinical setting.
(Edit) "Prediction of brain tissue temperature using near-infrared spectroscopy" (2017) Neurophotonics https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5469395/
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