Accurate measurement of the leaf temperature is crucial for the determination of stomatal conductance. Measurement of the average temperature of the upper leaves in a plant canopy is important to modeling transpiration rates and irrigation scheduling. Leaf to air temperature gradients are often less than 1 C, which means that leaf temperature must be known to about 0.1 C. This is a challenging task but it can be achieved with carefully calibrated infra-red transducers. Because the sensor output depends on the temperature of the sensor itself, it is necessary to calibrate the sensor over a range of sensor body temperatures. We use a black body cone painted with ultra-flat black paint to achieve an emissivity of about 0.99. The system is automated to independently control sensor body and target temperatures from -10 C to 50 C. A series of polynomials is used to correct for the effect of sensor body temperature. Custom calibration of each sensor allows an accuracy of 0.1C. A relatively wide field of view (50 degrees) is useful to improve signal to noise ratio and to more appropriately determine canopy temperature. The biggest errors in the field are often caused by occlusion of the lens. The effect of leaf emissivity on leaf temperature is poorly understood and will be discussed.