A photometer having a plurality of input fibers to its optical entrance, at least one of which is for transmission of calibration light and at least one of which is for transmission of sample light. The exit ends of these fibers are aligned into a linear array, thereby producing an effective entrance slit for the optical entrance of the photometer. The fiber(s) for calibration light are positioned at the center of the linear array to avoid miscalibration due to photometer astigmatism.