Anti-reflection coating on the glass. Passes pretty much all the wavelengths of light (that are pertinent to photography) with almost no attenuation.
An IR-cut passes wavelengths shorter than about 700 nm, while a UV-cut passes only wavelengths longer than about 400 nm. You will need both (usually comes in the form of a combined UV-IR cut filter) when you want to get natural color balance from a "color" camera (since they are usually sensitive to near-IR light).
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