An optical sensor assembly is provided. The optical sensor assembly includes a circuit board, an optical sensor positioned on the circuit board, and a front cover attached to the circuit board and covering the optical sensor. The front cover includes an optical element configured to guide or condense an incident light of a predetermined wavelength onto the optical sensor. The front cover is made of polypropylene or polyethylene. The predetermined wavelength is in a range from 8 micrometers to 12 micrometers.