Photography is all about light. It’s about capturing light, manipulating light, and interpreting light. Today’s digital cameras use advanced algorithms and sensors to properly measure light and then record what you’re seeing with your eyes. But cameras weren’t always so smart and sophisticated, and with more and more people exploring film photography with vintage cameras, light meters are once again becoming required tools for those serious about photography. A light meter measures the light in an environment and helps you set your camera for the proper exposure on your image. This is doubly important in film photography, where it’s much harder to correct a poorly exposed image (too dark or too light) after the fact.

That’s where the Lumu comes in. There are two types of light meters in photography. The $149 Lumu, which plugs into your smartphone’s headphone jack, can measure incidental light, the light that falls right on your subject or what you want to be exposed properly in your photo. But a camera is only capable of measuring the light reflected back into it, another step removed from the light directly on your subject. Exposing an image using incidental light is generally considered to be a more accurate method, which is why you’ll still see professional portrait photographers and cinematographers with light meters dangling around their necks.