Infrared blocking filters

Q: What are some common naturally occurring materials that absorb infrared sunlight radiation from 1.1-2.5 microns?

Research into infrared-absorbing materials, both inorganic and organic, is making significant advances. These materials are useful in fabricating light-emitting diodes, fiber optics, and liquid crystal displays. From Wang et al. (2004):

"The radical anions of stacked aromatic imides, fused phorphyrin arrays, polythiophenes, sandwich-type lanthanide bisphthalocyanines, semiquinones, and mixed- valence dinuclear metal complexes are a few known examples of NIR-absorbing organic materials."

Getting away from naturally occurring materials, acrylic sheets absorb some infrared in the that range. Their optical light transmission is 92% and they have a sharp drop in transmission at 1.6 microns, though their transmission rises to 60% for 1.7- and 2-micron light. Filters made of a combination of acrylic and plastic are often used where lightweight optics are required.

Infrared blocking filters are available for standard SLR cameras, such as this one from Hoya. (Note that this is not a product endorsement.)

Reference: Zhi Yuan Wang et al. 2004. Near-infrared absorbing organic materials. Pure Appl. Chem., Vol. 76, Nos. 7–8, pp. 1435–1443.

-Barry Walker and Sally Dodson-Robinson