What is the metric unit of measure used to express light intensity?

The metric unit used to express light intensity is the candela. The candela is one of the seven base units in the International System of Units (SI) and specifically measures luminous intensity, which is the amount of light emitted in a particular direction. This unit quantifies how much light is produced by a source, which is important in fields such as photography, lighting design, and various scientific applications where precise measurements of light are necessary.

In contrast, other options represent different physical quantities. The joule is a unit of energy, the watt is a unit of power (which is energy per time), and lux is a unit of illuminance, measuring how much light is spread over a certain area. While lux relates to light intensity concerning a surface area, the candela specifically defines the intensity of light from a source, making it the proper unit for this context.