Illuminance (lux) is a measure of the amount of light falling on a surface, not the amount of light being emitted. Any reflected light will count towards the measured illuminance.

Sunlight is just incredibly bright. There's a reason why film crews often need to use huge metal halide lamps powered by a generator truck - it takes an immense amount of power to compete with daylight.

https://en.wikipedia.org/wiki/Illuminance

https://www.arri.com/en/lighting/daylight/arri-daylight-18-1...

You can just use a light meter to measure directly.

Normal indoor lighting is a few hundred lux and with the right setup, I was able to increase it to 1000 lux at where my head is generally physically located.