The disclosure is directed to methods and apparatus for obtaining a discrete frequency infrared (DFIR) spectroscopic image. The method includes generating, by a laser source, a pulsed laser beam comprising a spectral frequency in a mid-IR region and a pulse repetition rate; transmitting, by a first group of optical components, the pulsed laser beam onto a location of a sample; collecting, by a second group of optical components, an optical signal emitting from the location of the sample onto a detector to obtain a raw electric signal; demodulating, by a demodulator based on the pulse repetition rate, the raw electric signal to obtain an intensity value; and determining, by a control device and based on the intensity value, a pixel value corresponding to the spectral frequency in a DFIR spectroscopic image.