You can distinguish between dark, dim, light and superbright, but you can’t precisely quantify light with your eyesight. When you need to, do it with Ambient click.
Ambient click converts light intensity into voltage. It carries the Melexis MLX75305 IC. The tiny, shiny speck on top of it is a CMOS optical sensor. The chip also integrates a transimpendance amplifier, and an output transistor.
Ambient light sensors are widely used in portable devices with screens, in order to automatically adjust backlight depending on how bright the surrounding is. If the screen is outdoors in sunlight, brightness is amped up to max, if it’s inside and dark, brightness is lowered to reduce eyestrain and conserve battery.
Also, smart home projects come to mind. Leaving on the light is like leaving an open tap. Automate lighting and have one worry less. And you’ll wake up in the morning easier if the light turns on by itself.
That’s the most obvious application but there’s more to it.
Ambient light sensors are useful wherever useful data can be deduced from the presence or absence of light. You can tell if something, any sort of object, is in the right place based on whether it casts a shadow over the sensor. Say, a piece of paper inside a printer.
There’s also a light sensor click board with a digital output — check out Illumimance click.