People do use reflectors, it just depends on their growing system/style.
If for example you're doing a SCROG or SOG canopy and the lights will be hung above the canopy people use reflectors. If not, a percentage of the light will go straight up if the bulb is mounted horizontally. If vertical, it'll shoot out the sides and not down.
Think about it. Inside the bulb there is an arc tube that emits light from it's length. The light comes off the sides/circumference of the arc tube. Not shooting out the end of it.
OK so..here's some reflector "theory". When you move a reflector away from the source, the light energy off the reflector diminishes by the square of the distance you move the reflector. So, say you went from 2" to 4" from the bulb. The intensity would decrease by a factor of 4 because you doubled the distance of the reflector from the source.
Another tidbit...in photography they'll tell you that reflected light does not diminish. That's why you see lighthouse, theater film projector, train headlights...aimed at the reflector and not straight out.
Also..if you are taking a photo of the full moon, you set your camera to the same settings as you would if you were taking a shot on a sunny day. That's because the difference in distance from the sun between the earth and moon is negligable. When taking a photo of a full moon, it is being lit by the sun and the sun's light is being reflected off the moon to earth. If there were no other forces like crap between us and the moon like dust and stuff..., the full moon would be as bright from here as it is on the moon's surface. Theoretically.
So...if you're hanging lights above a canopy, it's best to mount the bulbs horizontally and with a reflector that's as close to the bulb as possible. That way. a percentage of light is coming off the arc tube and shooting down. The rest hits a reflector and then is directed down.
If you mount a bulb vertically with a big parabolic type reflector, ALL the light shoots off the sides. All the light has to be reflected downwards.....not always a bad thing as discussed earlier....but...if the reflector is huge and far from the source, well, that isn't very efficient.
Some of us grow a relatively small number of plants and grow them big into trees. We hang the bulbs in amongst the plants vertically without any reflectors. The light shoots off the arc tube in 360degrees and all the plants around the bulb get the light. None is reflected and none is shooting towards the ceiling or above the plants' "heads". VERY efficient.
Hope that helps.
