Many of the criteria for sunglasses worn when piloting an aircraft are similar to those for land vehicles. Protection against UV radiation is more important, as its intensity increases with altitude. Polarised glasses are undesirable as aircraft windscreens are often polarised, intentionally or unintentionally, showing Moiré patterns on looking through the windscreen; and some LCD displays used by instruments emit polarised light, and can dim or disappear when the pilot turns to look at them