Piston temperature plays a major role in determining details of fuel spray vaporization, fuel film deposition and the resulting combustion in direct-injection engines. Due to different heat transfer properties that occur in optical and all-metal engines, it becomes an inevitable requirement to verify the piston temperatures in both engine configurations before carrying out optical engine studies. A novel Spot Infrared-based Temperature (SIR-T) technique was developed to measure the piston window temperature in an optical engine. Chromium spots of 200 nm thickness were vacuum-arc deposited at different locations on a sapphire window. An infrared (IR) camera was used to record the intensity of radiation emitted by the deposited spots. From a set of calibration experiments, a relation was established between the IR camera measurements of these spots and the surface temperature measured by a thermocouple. Transmissivity of the chromium spot was investigated by using different background media. The deviations between the thermocouple readings and SIR-T measured temperatures were noted to be within 10°C for the working range of 75°C to 180°C. The technique was demonstrated to measure the optical piston temperature during engine operation at 1500 rpm and 2000 rpm. A piston warm-up strategy was implemented for optical engine studies to attain metal engine steady state piston temperatures. The effect of piston warm-up on in-cylinder soot formation was studied using high-speed imaging.