What is a pyrometer designed to measure?

A pyrometer is an instrument specifically designed to measure high temperatures, particularly those that exceed the capabilities of traditional thermometers, such as mercury thermometers. This makes it especially useful in industrial settings where extremely high temperatures are common, like in metalworking or furnaces. Pyrometers can operate based on different principles, like infrared radiation or thermocouples, and are capable of providing accurate readings of temperature from a distance, which is helpful in environments that are too hot for other temperature sensing devices.

The other options pertain to measurements outside the scope of a pyrometer's function. Humidity levels are typically measured with hygrometers, vapor pressure is a concept related to the behavior of gases and liquids, and light intensity is measured with photometers or light meters. Hence, the specific design and application of a pyrometer make it ideal for measuring temperatures above the range of mercury, confirming that this answer is accurate.