Photothermal deflection spectroscopy (PDS) is a sensitive technique that measures optical absorption by detecting the deflection of a laser beam caused by thermal changes in the sample. This method is applicable to solids, liquids, gases, and thin films. The theory of PDS involves calculating the temperature distribution in the sample and solving for the optical beam propagation through an inhomogeneous medium. The predictions of the theory are experimentally verified, and the implications for imaging and microscopy are discussed. The sensitivity and versatility of PDS are compared with thermal lensing and photoacoustic spectroscopy. The theory of PDS is developed for both continuous-wave (cw) and pulsed cases of transverse and collinear PDS. The experimental considerations include the setup for transverse and collinear PDS, as well as pulsed PDS. The experimental results are compared with theoretical predictions, and noise and background analysis are described. The findings are compared with related techniques, and the implications for imaging and microscopy are presented. The theory of PDS involves solving the temperature distribution equations for different regions of the sample and calculating the optical beam propagation through an inhomogeneous medium. The results show that the temperature distribution can be decomposed into distributions of the form $ J_0(\delta r)e^{-\beta_i z} $. These distributions act independently and have an effective thermal length given by $ l_i = 1/\text{Re}(\beta_i) $. The optical beam propagation is calculated by considering the effect of the temperature distribution on the probe beam. The index of refraction is a function of temperature and pressure, and the propagation of the Gaussian probe beam through the spatially varying index of refraction is given. The change in the complex beam parameter, q, is calculated, and the effect of the curvature of the index of refraction is equivalent to an astigmatic lens of focal length, $ F_i $, in the $ S_i $ direction. The solution for beam deflection is derived for collinear and transverse PDS. For collinear PDS, the beam is deflected by the temperature gradient in all three regions. For transverse PDS, the probe beam propagates completely within Region 0. The results show that the temperature distribution extends significantly beyond the beam profile for low chopping frequencies. The experimental results show that PDS is sensitive to small absorptions in thin films, solids, liquids, and gases. The sensitivity and versatility of PDS are compared with thermal lensing and photoacoustic spectroscopy. The results demonstrate that PDS is as sensitive as thermal lensing and more versatile and easier to use. The implications for imaging and microscopy are discussed, showing that PDS can provide information similar to photoacoustic imaging.Photothermal deflection spectroscopy (PDS) is a sensitive technique that measures optical absorption by detecting the deflection of a laser beam caused by thermal changes in the sample. This method is applicable to solids, liquids, gases, and thin films. The theory of PDS involves calculating the temperature distribution in the sample and solving for the optical beam propagation through an inhomogeneous medium. The predictions of the theory are experimentally verified, and the implications for imaging and microscopy are discussed. The sensitivity and versatility of PDS are compared with thermal lensing and photoacoustic spectroscopy. The theory of PDS is developed for both continuous-wave (cw) and pulsed cases of transverse and collinear PDS. The experimental considerations include the setup for transverse and collinear PDS, as well as pulsed PDS. The experimental results are compared with theoretical predictions, and noise and background analysis are described. The findings are compared with related techniques, and the implications for imaging and microscopy are presented. The theory of PDS involves solving the temperature distribution equations for different regions of the sample and calculating the optical beam propagation through an inhomogeneous medium. The results show that the temperature distribution can be decomposed into distributions of the form $ J_0(\delta r)e^{-\beta_i z} $. These distributions act independently and have an effective thermal length given by $ l_i = 1/\text{Re}(\beta_i) $. The optical beam propagation is calculated by considering the effect of the temperature distribution on the probe beam. The index of refraction is a function of temperature and pressure, and the propagation of the Gaussian probe beam through the spatially varying index of refraction is given. The change in the complex beam parameter, q, is calculated, and the effect of the curvature of the index of refraction is equivalent to an astigmatic lens of focal length, $ F_i $, in the $ S_i $ direction. The solution for beam deflection is derived for collinear and transverse PDS. For collinear PDS, the beam is deflected by the temperature gradient in all three regions. For transverse PDS, the probe beam propagates completely within Region 0. The results show that the temperature distribution extends significantly beyond the beam profile for low chopping frequencies. The experimental results show that PDS is sensitive to small absorptions in thin films, solids, liquids, and gases. The sensitivity and versatility of PDS are compared with thermal lensing and photoacoustic spectroscopy. The results demonstrate that PDS is as sensitive as thermal lensing and more versatile and easier to use. The implications for imaging and microscopy are discussed, showing that PDS can provide information similar to photoacoustic imaging.