Light meters are the standard tool used in filmmaking to measure the amount of light falling on a subject, which helps to determine exposure. Pretty basic stuff—most who have ever worked with one or watched a quick tutorial know the basic principle of how it works. However, if you're itching to learn a little bit more about the physics behind light measurement, Matt Workman of Cinematography Database offers up a great breakdown in the following video that will help you understand some of the more basic, albeit confusing, concepts in light measurement. In it, he explains the three main ways cinematographers measure the intensity of a light, basic terminology, as well as the differences between luminance and illuminance. Check it out below:

If a lot of this stuff went right over your head, don't feel bad. These concepts are some of the more difficult to understand regardless of your experience level.

To be honest, you may not need to know the physics behind the measurements of light when you first start out, but as you grow as a cinematographer, you might find the information useful, especially when deciding on which lighting units to use, as well as how to shape the light they emit. Being able to read a light's specifications and have a general idea of what to expect when you fire it on will prove to be extremely useful when planning a shoot, as well as working with them on set.

Keep an eye out for Workman's next video in the series, which will be a much deeper dive into the concepts he talked about.

Source: Cinematography Database