The Light Meter app can be used to measure photon energy (lux) in the environment, demonstrating that outdoor light, even on cloudy days, is significantly more intense than typical indoor artificial lighting.

“you can go outside in the morning and you'll see that there's 10,000, 20,000 lux, even though it might seem like it's kinda dim or there's tree cover or cloud cover, you go inside and you shine an artificial light at your phone, press the button on Light Meter and you'll find that it's only 500 or 1,000 lux, and you realize that even though it seems really bright, the artificial light is very condensed, whereas the outside light is scattered in the atmosphere, and so you can think that you're not getting much sunlight but you're actually getting much more outside”