Optical linewidths are influenced by temperature and pressure. Outdoors, humidity can be a factor as well. When I did photoacoustic spectroscopy, we used two

Optical linewidths are influenced by temperature and pressure. Outdoors, humidity can be a factor as well. When I did photoacoustic spectroscopy, we used two or more wavelengths in the measurement; one on resonance, one off resonance to help with background discrimination. I am not sure whether these ultracompact instruments have that capability. Additionally, contaminants on any surface the light strikes will generate spurious signals. Lock-in (synchronous) detection at the optical source modulation frequency can help get rid of a lot of noise but if the noise appears at that frequency, it will corrupt the measurement. This classic paper from 1986 covers a lot of what can be done with the technique and how to do it correctly. Reviews of Modern Physics is behind a paywall but there's a copy on the web at the URL below. Applications of photoacoustic sensing techniques Andrew C. Tam Rev. Mod. Phys. 58, 381 – Published 1 April 1986 http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf http://users.df.uba.ar/dkunik/photothermal/review/tam86.pdf