Cargille Sub-Lux Gels are profoundly black gels with refractive indices close to many common glasses. The purpose of each gel is to minimize ghost images caused by secondary reflections off of a lens surface. When the gel is near the refractive index of a glass element, light going through the glass goes into the gel and is trapped by absorption in sub-micron particles of black pigment.
The gels are very stable and will not harden, dry out, or evaporate. They are available in 4 ounce bottles in three refractive indices: nD = 1.46, 1.52, and 1.54 at 25 °C: