Hinds Instruments has provided photoelastic modulators (PEMs) to the photonics industry for over 40 years. In the past Bob Wang, et al. from Hinds have made two presentations,<sup>1,2</sup> at SPIE meetings, <strong>Basic Optical Properties of the Photoelastic Modulator Part I: Useful Aperture and Acceptance Angle and Part II: Residual Birefringence in the Optical Element.</strong> This presentation, Part III in the series, addresses the characteristics of the PEM that vary with temperature, highlighting effects on frequency and signal drift due to retardation changes. In addition, Part III will introduce a new PEM with advanced thermal control (PEM-ATC™). Data is presented that shows the increased stability of the PEM frequency and retardation signal using this new device.