(1)  NEAR INFRARED

This region extends from 0.75 to 2.5 micro meter, with wave- numbers 12,500- 4000 c/m. In this region majority of the absorption bands are due to overtones of hydrogen stretching vibrations.

(2)  MIDDLE OR FUNDAMENTAL INFRARED

This region extends from 2.5 to 16 micro meter with wave numbers 4000- 625 c/m. Molecular vibrations and rotations take place in this region.

(3)  FAR INFRARED

This region lies in a range from 16 to 1000 micro meter  with wave numbers 625 — 10 c/m. The skeletal vibrations and rotations take place in this region.