i guess the biggest advantage to using tubes rather than CFL's is heat, the tubes
spread the heat over a greater area, plus the ballast components are better
shielded so they run cooler than CFL's.

the advantages to CFL's is that you don't need a special fixture as they will
work in any regular light. also, because you have multiple bulbs you can
really customize the spectrum.

here is some info from the old Overgrow site that might be helpful to you in
choosing what light you need to add to your existing set-up.
just keep in mind that this is old info and they've come a long way with CFL
bulbs so, while HID lights will certainly yield more, CFL's now have the power
to produce respectable yields as well.


The color of a light source entails a complicated relationship of different factors, that are important in determining the right light source for your garden. The advantage of Metal Halide lamps is that they provide high-quality, crisp white light in a variety of different color temperatures that meet the needs of many different users.

Correlated Color Temperature(CCT)

The first factor in choosing a color of lamp is to determine whether you need a warm or cool light source. The CCT, expressed in Kelvin degrees, relates to actual thermal temperature. If you've ever seen a piece of metal being heated, you know that as the metal gets hotter, it's color changes. The CCT rating of HID and flourescent light sources indicate how warm or cool the light source is. For instance a lamp with a CCT of 2700 Kelvin is considered warm; with a CCT of 4200 Kelvin is considered neutral; and one of 6000 Kelvin is considered cool.


Spectral Energy Distribution

When you look at a light source, you perceive seeing a single color, but you are actually seeing thousands of hues. The combinations of different wavelengths of light make up the color we see. The relative intensity of the various wavelengths are used to determine a light source's Color Rendering Index(CRI).


Color Rendering Index(CRI)

The CRI is an indication of a lamps ability to show individual colors relative to a standard. This value is determined from a comparison of the lamp's spectral distribution compared to a black body at the same color temperature. Light sources, such as metal halide lamps, are rated with a CCT; however, CCT does not provide any information on the quality of the color. For this, a CRI is also necessary. In general, the higher the CRI rating of a lamp, the better the different colors will show.


5K - 7K Kelvin: Strong Blue Light
Promotes bushy growth. Ideal for rapid growth phase of plants.
Greatly enhances all-around plant growth when used with super
high output, high pressure sodium or 3K warm metal halide lamps.

4.2K - 4200 Kelvin: Cool white Flourescents
Can be used as supplimental blue lighting when used with a 3K
source.

4K - 4000 Kelvin: Neutral Metal Halide
Best single source for plant growth, producing shorter, bushier
growth than 3700 Kelvin and color rendition. Used in general
plant lighting.

3.7K - 3700 Kelvin: Softer Metal Halide(coated)
This coated lamp is used in general plant lighting and for more
rapid growth than 4000 Kelvin produces.

3K - 3200 Kelvin: Warm Metal Halide
Highest photosynthetically active radiation (PAR) value of all HID
lighting for all phases of plant growth. PAR watts account for the
nutritional value of light and are a direct measure of the light
energy available for photosythesis.

2.7K - 2700 Kelvin: High Pressure Sodium Lamps
Redder color mix, used for propagation, blooming, supplemental
greenhouse lighting.