A sensor is paired with an emitter, such as in a postage metering machine, and the sensor is calibrated for ambient light conditions. Ambient light is measured with a sensor while the emitter is unpowered. If the measured ambient light is less than a current trip level (voltage level at the sensor), then a current level of light is measured with the sensor while the emitter is powered. If the measured current level of light exceeds the current trip level, then the signal output from the sensor indicates that an object such as an envelope awaiting postage to be printed thereon is present at the sensor. The current trip level may be determined by measuring light at the sensor when the emitter is on and adding some voltage margin to it, but if the sensor is blocked during this calibration a default threshold is used as the current trip level.