Choosing a Lamp:
Which lamp is best for your growing needs? 

METAL HALIDE LAMPS - This type of light is blue-orientated in the spectrum. It is the best type of light to be used as a primary light source (if no or little natural sunlight is available). This type of lamp promotes plant growth. 

HIGH PRESSURE SODIUM LAMPS - These lamps are red/orange in the spectrum. They are the best lamps available for secondary or supplementary lighting (used in conjunction with natural sunlight). This type of light promotes flowering / budding in plants. It is ideal for Greenhouses / Commercial growing applications. 

CONVERSION LAMPS - 
A). Sodium Lamps which run on halide ballasts -more common. 
B). Halide Lamps which run on sodium ballasts. This type of lamp allows you to tailor the light source to the growth stage of the plant (halide blue light for growth and sodium red light for 
flowering/budding) by merely by changing lamps.