The need for upgrading of existing systems to IR capacity can be solved by add-on IR-sights with unity magnification. This type of sights allows upgrading with a minimum of modification. The principles for unity magnification sights are explained and comparisons are made with other techniques. Critical design principles are emphasized and are exemplified with the upgrading of two Bofors missile sights.
Stefan A. I. Johansson,
Ulf A. Persson,
"Clip-on devices give night capacity to missile systems", Proc. SPIE 1762, Infrared Technology XVIII, (5 January 1993); doi: 10.1117/12.138959; https://doi.org/10.1117/12.138959