St0ney

Metal Halide
This is a bluish light that most closely matches daylight.
It is the most versatile of the two lights. It is perfect for growing plants that are in a vegetative state of leafy growth such as lettuce and spinach. Also note that seedlings are initially in a vegetative state of growth even if they will bloom later in life.
It is also the light of choice if you have a mixed garden with lettuce and spinach growing nest to chilies and strawberries.
Sodium conversion bulbs are available that deliver High Pressure Sodium light from a Metal Halide system.

Bulb Comparison

Standard: This the economical choice. Its spectrum is blue and is good for vegetative growth. High Output: This has the same blue spectrum as the standard bulb but emits roughly 10% more light, and is excellent for vegetative growth.
Clean Ace: This has the most blue of all the bulbs. Also good for reef aquariums. Best for vegetative growth.
Agrosun: This is also a high output bulb and has 30% more red. It is the bulb for a mixed garden containing both vegetative growth and blooming plants.

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"A man's ethical behavior should be based effectually on sympathy, education, and social ties; no religious basis is necessary. Man would indeed be in a poor way if he had to be restrained by fear of punishment and hope of reward after death." -- Albert Einstein