Can I use an light meter to get monitor brightness?

Started 2 months ago | Discussions thread
MrBrightSide Contributing Member • Posts: 868
Using your camera meter for incident light

You are correct. Incident meter readings are the gold standard for setting exposure, everything else is kludgey and imprecise.

Can you put a translucent dome over the lens and point it at your light source to get a reading? I think that may have been an actual product at some point in the past. Or  perhaps ExpoDisc can tell you how to use their product for measuring incident light.

Which brings us to the very misunderstood Kodak 18 Percent Gray Card. You can get a reading that's pretty close to an actual incident meter if you use a the gray card according to the instructions for angling the card toward the light source and then adding additional exposure compensation (it's between 1/2 stop under and 1 1/2 over depending on the subject).

It's a system that film people used for decades. I've used it in the field a few times and it's a perfectly reasonable way to measure the amount of light falling on the subject. I also measured it against my handheld light meter and it was within a third of a stop.

You can download the instructions at

One danger: there are people who will warn you not to use an 18 percent gray card and recite meandering stories about a conspiracy between Ansel Adams and Kodak. They'll also claim they have special insider information that cameras really measure middle gray at 12 percent .

These people don't understand how the gray card technique works; they're usually also the same people who try to tell you that ISO doesn't matter.

Doug Haag wrote:

This topic is not unrelated to a question I had.

The monitor is a light source. The light coming from the monitor is not reflected light.

Incident light meters measure the intensity of light falling on a subject or location that has been emitted by a light source (the sun, studio lights, etc.) On the other hand, the camera meter is designed to measure reflected light (measuring how much of the light a surface receives from a light source is reflected rather than absorbed). Even if the light source is identical, a black surface will absorb more and reflect less while a white surface will absorb less and reflect more. So the camera measurements of such reflective surfaces would be different even if the light source remains constant.

My question is whether a camera's reflective meter could be adapted to work as an incident meter. Perhaps it would involve some lens covering that works like the Expodisc to measure white balance. But while Googling everything I can think of, I have found no reference to turning a camera's reflective meter into an incident meter.

Because the monitor is a light source requiring an incident meter to measure the intensity of light that is being received at your viewing position, I am guessing that your idea is problematic.

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