Measurement of temperature using a thermocouple or any other device requires a certain length of time. This time interval is of the order of magnitude of the thermal time constant of the measuring device. Here we describe an electronic circuit which makes the temperature measurement ``instantaneous.'' This is accomplished by combining the temperature at any instant with the rate of change of temperature. Thus the only limiting factor becomes the electronic time constant of the circuits involved. We have used our device in absolute measurement of optical power and have found that the typical time necessary for a temperature measurement can easily be 10–3× the thermal time constant of the measuring device.