IR stands for infrared. An infrared light, which is not visible to the naked eye, is often used to supplement the ability of night vision optics when the available ambient light is insufficient to provide a usable image. Due to the widespread use of night vision technology in the military, it has become commonplace for military specification (milspec) to require the use of materials treated to minimize the reflection of infrared light.
Man-made fabrics that are not IR treated can reflect infrared light in a way that is different from natural objects and vegetation. This can cause the fabric to stand out from the background, even if the fabric is camouflaged to blend in during the day. Even a fabric with the best camo pattern to fit the background without IR treatment can cause the fabric to have a reflective glow when viewed through IR illuminated night vision.
The following two photos were taken of the same four rifle slings. The sling on the left is a non IR treated sling from a popular sling maker. The three slings to its right are RifleCraft slings, from left to right, a coyote tan RS1, a coyote tan RS2, and an A-Tacs FG RS2, all of which are IR treated. Both photos were taken within minutes of each other. The first photo shows the slings under the light of visible flash photography.
The second photo was taken using a PVS-14 night vision monocular. I used a hand held IR flashlight to illuminate the slings.
Obviously the untreated sling on the left is brighter. What is also interesting is that the A-Tacs FG camouflage sling, which blends in better with the colors of the trees around it, is barely visible in the night vision photo. It just goes to show that proper choice of color or camo pattern still matters at night.
RifleCraft offers several solid colors and camo patterns. All RifleCraft sling webbing is IR treated.