A temperature monitoring system includes a linear array of six pyroelectric sensors (22) mounted in a block (18). A through hole (20) is provided in the block (18) for each sensor (22). A bore in the block having an axis which intersects the axis of each through hole (20) accommodates a rotary shaft (26). The shaft (26) is provided with a diametrically extending bore (28) for each through hole (20). Thus rotation of the shaft (26) alternately exposes each sensor (18) to the shaft (26) itself and to an object in the line of sight of each sensor(22) as defined by its corresponding throughhole (20). The output signal from each sensor (22) is differentiated and used to provide a temperature measurement. The output signal is corrected for any drift in the temperature of the shaft (26) or block (18). This is effected by periodically bringing a heated calibration head (36) operating at a known temperature into the line of sight of each sensor (22).