What is a near-infrared-absorbing material?

Near-infrared absorbing materials combine high visible light transparency with strong selective absorption against near-infrared light. For example, by applying it to window materials, the energy of near-infrared rays contained in sunlight is efficiently cut while maintaining sufficient brightness, resulting in an effect that greatly suppresses the temperature rise in the room.

Sunlight consists of ultraviolet rays (UVC: ~290 nm, UVB:290 to 320 nm, UVA:320 to 380 nm), visible rays (380 to 780 nm), near infrared rays (780 to 2500 nm), and mid-infrared rays (2500 to 4000 nm). Its energy ratio is 7% for ultraviolet rays, 47% for visible rays, and 46% for near-and mid-infrared rays. Near-infrared rays (hereafter abbreviated as NIR) have a higher radiation intensity at shorter wavelengths, and they penetrate the skin and have a higher heat-generating effect, so they are also called “heat rays.”

Heat absorbing glass or heat reflecting glass is generally used to shield window glass from solar radiation. Heat-absorbing glass is made by NIR-absorption of iron (Fe) components, etc. kneaded into glass, and can be manufactured inexpensively. However, visible light transparency cannot be sufficiently ensured because it has a color tone peculiar to the material. Heat-reflective glass, on the other hand, attempts to reflect solar radiation energy by physically forming metals and metal oxides on the glass surface. However, the reflected wavelengths extend to visible light, which causes glare in appearance and radio interference. Dispersion of transparent conductors such as high-performance sunlight-shielding ITOs and ATOs with high visible light transparency and no radio wave disruption into nano-fine chemicals yields a transparency profile as shown in Fig. 1, and near-IR selective absorption membranes with radio wave transparency.

The shading effect of sunlight is expressed quantitatively in terms of the solar radiation heat acquisition rate (the fraction of net sunlight energy flowing through the glass) or the solar radiation shielding factor normalized by a 3 mm thick clear glass.