Lumens is the unit of luminous flux. It's energy per time. So what is written on the product is luminous flux. Is it true that if we want to see the real lighting effect, we need to calculate intensity? The unit for intensity is candelas, which origins from "equivalent lighting effect with how many number of candles". So I think that intensity usually reflects our perception about how light it is.
In lecture Professor Ng talked about a candela being equivalent to a standard candle. Do all candles generate roughly the same amount of power regardless of size and composition? That seems really cool if true and I would wonder why that's the case
so that could be 815 lumens/(4pi/20 degree)?
I don't think it 20 degrees would be the solid angle, you'll have to integrate the phi from 70 degrees to 90 degrees to find that.