The original infrared instruments were of the dispersive type. These instruments separated the
individual frequencies of energy emitted from the infrared source. This was accomplished by the use
of a prism or grating. An infrared prism works exactly the same as a visible prism which separates
visible light into its colors (frequencies). A grating is a more modern dispersive element which better
separates the frequencies of infrared energy. The detector measures the amount of energy at each
frequency which has passed through the sample. This results in a spectrum which is a plot of
intensity vs. frequency.