Which measure is primarily related to evaluating light sources' brightness?

The primary measure related to evaluating the brightness of light sources is luminous efficacy. This term refers to the ratio of the luminous flux (measured in lumens) to the power consumed (measured in watts). Essentially, luminous efficacy tells us how effectively a light source converts electrical energy into visible light. A higher luminous efficacy indicates that a light source produces more brightness per unit of power, making it a critical factor in assessing the efficiency and brightness of various lighting technologies.

In contrast, radiant intensity focuses on the power emitted by a light source in a particular direction per unit solid angle, which does not directly reflect perceived brightness. Irradiance relates to the power of electromagnetic radiation received per unit area, which is more relevant to how much light falls on a surface rather than the light source's brightness itself. Illuminance measures how much luminous flux falls onto a surface area, closely linked to the amount of light that illuminates a specific area but again not a direct measure of the light source's brightness.