MIR lens back focal length measurement system by Newton's equation

This paper presents a new method of measuring the back focal length of middle infrared (MIR) lens that uses MIR pulse collimated beam, focimeter and thermopile sensor. An MIR illuminated target is at the first focal point of a collimating lens and MIR signal detected largely through an IR test lens, collected lens and thermopile sensor. The IR test lens would be located between the collimating lens and collected lens. The MIR target is moved until MIR signal detects largely again. The target displacement is a measure of power of the lens by Newton's equation. The experiment results state that the measurement accuracy of the back focal length of MIR lens is up to 5% at 5∼25mm range EFL. The advantages of this testing system are low costs, fast measurement speed, high precision, less complicated system and replacements of light source and detector of different IR spectrum for measuring BFL of lens.