SDU-5/E Distress Marker Strobe Light + Flash guard / Blue filter FG1C and Infrared lens cover
This famous strobe light was used by the U.S. military since late 1960s. It was originally issued with various aircrew survival vests and life preservers. It was waterproof and produced an intermittent flash of light visible for beyond five miles. Accessories include a carrying pouch, flash guard / blue filter (FG1C), and various types of lens covers. It was later replaced by the MS-2000 strobe and other modern strobes.
Flash guard / Blue filter (FG1C) attaches at the front of the strobe. It converts the color of the flash from white to blue. This will make it clear to friendly forces that it is not a muzzle flash and that they should not shoot back. The flash guard also allows the light output to be concentrated and aimed at the target rescuers or observers, thus preventing the enemy from seeing the exact position of the operators. The Flash guard can be stowed upside-down over SDU-5/E when not in use.
Infrared lens cover converts visible white flash into infrared flash which is only visible by night vision equipment. It is designed for signaling at night when enemy troops are nearby.
Special Force operators commonly attach the strobe at the top of their helmets for easy signaling and rescue facilitation.