The visible light spectrographs are used to identify atoms from elements. The infrared regime is used when you want to detect molecules. This offers different capabilities and can identify chemicals and compounds with their infrared spectral signature.

Fourier Transform Infrared Spectrograph (FTIR) Fourier Transform Infrared Spectrograph (FTIR) Infrared spectrums of minerals. Infrared spectrums of minerals.

Infrared can also be detected passively with the proper type of system. Here I am using a liquid nitrogen cooled thermal camera. This is part of a research project where I am trying to detect spectral signatures of fossils buried in the ground.