Optical technique for color imaging of temperature gradients in physiological media: a method to study thermal effects of cw and pulsed lasers

Application of the thermocamera is limited to imaging surface temperatures at an air interface. We describe a technique which enables color imaging of temperature gradients inside optically transparent media. In an optical setup, very small changes in optical density of the media, induced by flow or temperature gradients, are color coded. Depending on the geometry of the temperature distribution, colors in the image can be related to absolute temperatures. The calibration is performed by making use of calculated temperature distributions of specific geometry and by thermocouple measurements. This technique can be applied to study the thermal effects of cw and pulsed lasers during interaction with model and biological tissues. Using fast flash light photography or video imaging, temporal resolution in the microsecond region can be obtained. To study the feasibility of the technique, experiments were performed to image cw Nd:YAG and pulsed Holmium and Excimer laser light interaction with transparent gels and tissues submerged in saline. During the measurements, temperatures were also monitored using thermocouples on selected positions within the field of view. At present, it is still difficult to translate the color images directly to absolute temperature images. The real time color images obtained with this color schlieren technique, however, provide a good understanding of thermo dynamics and thermal relaxation during laser tissue interaction with cw and pulsed lasers.