A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph. They are also used in cinematography and scenic design and lighting design todetermine the optimum light levels.
Photographers working with controlled lighting and cinematographers use handheld light meters to precisely measure the light falling on various parts of their subjects. There are also some multimeters that can do this (I have MS8209). Many modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that.
I recently watched video Lightmeter Apps, Do They Work that tells that there are light meter apps for smart phones.
The video takes a look two free light meter applications for Android phone.
I also tested those app and they seemed to work on my smart phone (Samsung S4 mini9. They gave lught levels at around the same range as my multimeter with light meter (I have MS8209). They might not be exact scinetific instruments, but seem to do the jot. I expect that the results could vary dependng on the phoney they are use (I don’t know how accurarely smart phone cameras and/or other lught sensors are calibrated).
Some more apps are shown in Smartphone Photography Apps video.