Most of the light sources used in indoor cannabis cultivation require special reflectors that ensure all of the generated light makes way to your plant.
Reflectors are required whenever High Intensity Discharge lamps are used, such as HPS or MH, but also CFL (the large energy saving bulbs).
The only situation when you might succesfully cultivate your cannabis plants without an additional reflector is while using LED light systems, which usually have inbuild reflectors.
The reflectors used in indoor gardening come in a variety of types and multiple sizes.
Starting from the budget range, one can find a relatively cheap reflector that will do its job, with the reflection factor of approx. 80%, which will still yield a decent crop of marijuana.
If you are looking for a more professional hardware, look for 95% efficiency, which will save you lots of energy that otherwise gets wasted.
Best results, and thus highest crops, are achieved with 97% reflectors that allow some degree of regulation - these come usually in the 'wing' form, with two separate parts that could be adjusted depending on the area you wish to cover.
An additional piece of equipment that you might consider is a spreader, that sometimes come as a standard in the more advanced wing reflectors.
This bit of reflective brass is attached directly below the light bulb, thus allowing the light to be spread even more equally, as well as doubling as a heat barrier.