I have my cacti indoors in a western window, I live in a northern country so all the natural sunlight is very importent to use.
I found that much of the light is offcause getting in to the room, and heat it up. I have a light meter and just startet to mesure the light on the backside of the plants.
The sun produces just about 75,800 lux in the afternoon.
I mesured what would be the backside of a plant, now the meter is showing about 4,230 lux.
Now I have lowerd the "shade" or in Danish "persiene" down behind the plants and use it as a reflector. It is really improves the light on the backside of the plants. Now the meters is showing 31,400 lux.
Another examble: without the reflection from the back 3,500 lox.
With the reflection: 25,800 lux.
Euphorbia without backside reflection.
Euphorbia with backside reflection.So it clearly shows, that with a very simple thing, you can improve the natural lightning over your plants a lot
Hope you find this info helpfull
Henrik
It keeps the heat from the room as well 
