Why does nickel reflect infrared, but gold does not? I've seen devices that shine infrared at a metal surface and measure the reflectance. They're mainly used for looking at thin film coatings. For example, you could have a metal part made of nickel, and thinly coated in gold. The gold thin film will not reflect the infrared, but the nickel will. This allows you to see how clean your surface is, because if there are other contaminants on the metal part, the reflectance will be much lower.