Infrared Thermometer: Pistol Grip with a Trigger-Swung Mirror That Zeroes on the Cavity (Michael, US 4,005,605, 1977)

Make a housing with a pistol handle, in three pieces for easy assembly. Fix a cone-shaped nose to its front, cut off at the tip to leave a round opening about one tenth of the cavity's inside diameter, so little stray radiation gets in or out. Paint the inside of the nose and cavity flat black. An infrared-transparent sheet may close the nose to keep out dirt and draughts. Not given: sizes, the materials.

Hold the thermistor bolometer on the housing's axis with a spider. Mount a concave mirror in a gimbal ring on two trunnions in bearings in the housing wall, with the detector at its focus when it faces the nose. Put a bead thermistor inside the cone. A leaf spring holds the ring tilted so the mirror faces the thermistor. Not given: the mirror's focal length, the metals.

Pivot a trigger in the handle. A finger on it pushes the gimbal ring against the spring, so the mirror turns to face the nose opening, and at the same moment opens a small switch beside the trigger. Fit the off-on switch and two control knobs on the housing, and the meter in its back. Ours: a microswitch and a moving-coil meter.

Wire the bolometer's active and passive elements as two arms of a Wheatstone bridge, with two resistors and a balancing potentiometer as the others; batteries in the handle feed it through the off-on switch. The bridge feeds a differential preamplifier with a gain potentiometer. Through the trigger switch its output charges a capacitor on the + input of a FET-input op-amp whose input bias current is under 1 pA; match the divider resistors to the op-amp's resistors within 0.1 %. Feed that op-amp's output into a summing amplifier together with an ambient correction from the bead thermistor (a fixed resistor across it, and a potentiometer to set the starting reading), and drive the meter from the sum. To use, switch on and balance the bridge; with the trigger released the mirror faces the cavity and the capacitor stores that reading. Aim at the target and pull the trigger: the meter shows the target minus the stored cavity reading, plus the ambient correction, so detector and preamplifier drift cancel. Not given: the component values.