There is provided a method and apparatus to monitor hardness using laser infrared photothermal radiometry. The emphasis is on the ability of this invention to monitor in a non-contact and non-destructive manner the hardness case depth of industrially heat treated steels. The present invention provides a method and apparatus comprising signal generation and analysis as well as instrumental hardware configurations based on the physical principle of photothermal radiometry. The method comprises (a) irradiating the sample surface with an excitation source of suitable emission wavelength and intensity. (b) producing periodic frequency pulses of the laser beam by means of a modulator in the appropriate frequency range (but not confined to) 0.1-100000 Hz. (c) passing the emitted beam through appropriate optics for beam conditioning (d) splitting the incident beam in two beams using a beam-splitter (e) focusing one of the beams onto a photodiode connected to a lock-in amplifier referenced to the modulator frequency to monitor laser power fluctuations (f) directing the second beam normal or at an angle to the surface of the sample (g) directing the generated infrared emissions from the sample surface to an infrared detector connected to another lock-in amplifier also referenced to the modulator frequency (h) recording the signals obtained from both lock-in amplifiers by a computer (i) processing the obtained signals through proprietary software and pre-calibration curves.