Horizontally oriented lamps are more efficient for several reasons and should be seriously considered as the grower shops for a good light solution.
In a horizontal lamp arrangement, the arc tube that produces the output is oriented with its full length exposed to the reflective hood insert and plants. This provides excellent efficiency due to a proper direct and indirect lighting arrangement.
In contrast, a vertically placed lamp/arc tube has the poles (which don't provide any light) oriented such that they are facing the plants and the hood, bad deal.... calls for a parabolic type hood to deliver the light to the plants below. This is very inefficient due to losses incurred by the light first having to travel the distance to the inner surface of the parabolic reflector, and second, by relying on the reflective properties of the reflector's coating to distribute the light downward effectively which wont happen. There WILL be losses. Efficiency takes a big hit with vertically oriented lamps in parabolic reflectors.
Regarding horizontally oriented reflector efficiencies - the reflector should be small (for reasons already stated) and should be well designed with a gull wing insert which uses a quality baked-on white enamel designed to resist discoloration from the adverse affects of heat over time. A much better solution is to choose a corrosion resistant specular aluminium insert. The use of quality specular material is the way to go regarding getting the most from your investment.
Contributed by: Medman
When using bulbs above 600watts and growing plants taller than 1meter commercial professionals make use of broad side lighting. The lamps are positioned hanging straight down between the plants without a reflector.
PLEASE NOTE: When using any lamp in any orientation it is always important to prevent heat damage by ensuring there is ample spacing between the lamp and adjacent plant material.
Photo contributed by: Gypsy Nirvana