Grow Lights: is an electric lamp designed to promote plant growth by emitting an electromagnetic spectrum appropriate for photosynthesis. The emitted light spectrum is similar to that from the sun, allowing indoor growth with outdoor conditions. Natural daylight has a high color temperature (approx. 6000 K) and appears bluish.
Metal Halide: high-intensity discharge (HID) family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained. These are typically used for replicating a summers day at high noon sun. This is great for the vegetative stage. Of course this is just my opinion.
High Pressure Sodium Lights: is a gas discharge lamp which uses sodium in an excited state to produce light. These types are better used during the flowering stage and finishing of fruiting. As if you want to mimic a fall or end of summer day light spectrum
These are all good sources of artificial sun. These lights can help you manipulate your crop and extend the fruiting time. The length of day also has an impact on the flowering process and its hormones. The length of the day lets the plant know that summer is starting or ending. Therefor letting the plant know if it should flower or not. Depending on what you are growing you can need anywhere from four hours all the way up to 14 hour days. If your plant is one that would need 14 hours of sunlight for one function and then 11 hours for the other function, these light will be great in assisting you in achieving what it is that you want to accomplish. One more point, it takes three days for the plant to develop a schedule.
I would suggest reading up on some of the lights and what the do for you. I also like to keep them in my garden so that it will keep them nice and warm also.
There are a number of grow lights out there. I would suggest doing your homework so that you can achieve what it is that you are looking for.
For more information check out wikipedia.org/wiki/Grow_light
This site is pretty informative.
Plantstay try us your plants will love you for it.