One word:

Google.

Everybody has different opinions on what light is the best, worst, cheapest, most cost-effective. To begin to even suggest something, we would need to know what and how many plants, if he will grow them to maturity or just as seedlings, how much can he afford for electricity. Otherwise, it's like posting that you want to get your cousin something to help him travel - that could be anything from a pair of tennis shoes to a jet plane!

Mike

The most important things we need to know are the light requirements for the plants or type of plants and the coverage area.

Well knowing my cousin they will probably be some cacti or some sort of wierd flowering plant so lots of light will probably be needed. And she would probably put it in a pretty small area.. maybe the size of a small fish tank.. or she might put it over a 2.5 foot long shelf that she has... Im not sure and Im guessing if its a flower she will grow it to maturity there but she will probably grow cacti(if she does cacti) just as seedlings and if it can be conservative energy-wise that would be very nice. Does that help? ...

>> Does that help? ... A tiny bit!!!

Some choices, depending on what she wants:

A simple shop light - inexpensive, will cover 8-10" wide by 4' long area. If kept close will provide enough light for growing.

Some CFL bulbs in reflectors. Lots of versatility. Can stick them in a row or arrange them in a group if she goes with more of a square grow area. You can get 13-45 watt at the local store (equal to 65-150 watt lights).

High Pressure Sodium or Metal Halide lamps - more for a sort of square area. Comes in a variety of power, from 75 to 1000 watts. A good system is a bit pricey but comes with everything you need. Used more for flowering/fruiting than foliage but for about $40 or less you can buy a different bulb (Metal Halide) and use it for foliage. You can get a simple 150 watt fixture for less than $50 plus shipping but you will need to find a way to hang it; I just bought a 400 watt complete system for $150. If your cousin is going to grow cacti to full size, I would suggest a 250-400 watt light.

Mike

Much better than CFL, is the 100w or 150w CMH.
That is, "Ceramic Metal Halide". A ceramic
arc tube gives greater luminosity, spectrum,
and reflectability.

A ceramic arc tube gives greater luminosity, spectrum, and reflectability.

Please, explain "reflectability".

Please, explain "reflectability".

An arc tube is nearly a point-source of light, and casts a smaller shadow than a CFL. This makes it much easier for the reflector to do its job. A CFL, on the other hand, blocks a significant amount of the light it generates due to its size and shape.

CFL's have the advantage of being cheap and simple, though.

An arc tube is nearly a point-source of light, and casts a smaller shadow than a CFL.

If the reflector is sufficiently large, then the CFL can also be practically modeled as a point-source of light. A flat white ceiling and walls of a room or growth chamber might comprise such a reflector.

Also, the outer bulb of HID absorbs reflected light.

A CFL tube emits light in all directions, but since that tube is twisted into a spiral, much of the light is blocked by another part of the tube before it ever has a chance to be reflected. Think of all the light-emitting surface area between coils where the tube faces itself, or the surface area that faces the inside of the spiral. A linear fluorescent is better because the entire surface area of the tube is exposed to either the plants or the reflector with no obstructions. An HID bulb has the same advantage in that there are no obstructions between the arc tube and the reflector (aside from the transparent outer envelope which absorbs only a small portion of light).

CFL's simply trade efficient light use for small size. I still use them in places where I need a small spotlight, but I try to avoid using them whenever possible for the above reason.

much of the light is blocked by another part of the tube

Blocked, schmocked. A large part of that light is reflected from the CFL bulb, part of it is transmitted and part of it is absorbed. The absorbed part (without a reflector) is already accounted for in the lumen or other power output rating of the CFL bulb. Nobody argues that HID aren't more efficient than CFL. I simply meant that for a sufficiently large reflector, a CFL bulb can be modeled as a point source of light, in which case, the additional amount of light absorbed by the bulb, caused by the reflector, is negligible.

Supposedly, manufacturers rate their lumens according to how bright it is a foot from the source, basically in foot-candles. However, in my experiences, the intensity readings I get tend to match Lux, not lumens; a CFL bulb rated at 6900 lumens gives me ~6900 lux a foot away. A HPS rated at 36,000 lumens has a reading of ~70K lux six inches away.

Mike

Well I don't know how lumen ratings are made, but if light losses from the tube shape of CFL's are taken into account, then I guess that's just one of the reasons that HID's have more efficient lumen ratings than CFL's.

As for the "reflectability" of HID's vs CFL's, I think any reflector capable of treating a CFL as a point-source of light would not only be impractically large, but it's size would cause the light to be spread too thin. In your example of the white-walled grow chamber, any chamber large enough to reduce the effect of a CFL's light absorption to that of an HID would require multiple bulbs, thus increasing the absorptive surfaces in the chamber and bringing you back to square one. CFL's use smaller reflectors to achieve a suitable intensity, but they do so at the expense of light loss.

In your example of the white-walled grow chamber, any chamber large enough to reduce the effect of a CFL's light absorption to that of an HID would require multiple bulbs, thus increasing the absorptive surfaces in the chamber and bringing you back to square one. CFL's use smaller reflectors to achieve a suitable intensity, but they do so at the expense of light loss.

One can put V shaped reflectors above fluorescent tubes and cone shaped reflectors above CFL so that substantially less light is reflected back toward the bulbs. Thusly, the vast majority of light can be absorbed by plants.

Reflectors designed to reflect light around the bulb, rather than into it, can be a big help in overcoming the problem we're talking about. But they have limits. They need to be able to accurately focus light in order to direct it around the bulb. Light can be more accurately focused when it is coming from a smaller area, so again the reflector to bulb size ratio comes into play. When a larger diameter bulb like a CFL is used with a reflector that is small enough to concentrate its light at an appropriate intensity, the large bulb size in relation to the the reflector creates significant scattering of light which reduces the accuracy of the reflector.

I'm not saying that CFL's are useless. Just that given the same light output and reflector size, a large bulb like a CFL will never deliver as much reflected light as an HID or linear fluorescent. There's just no way around it. And I believe that's what we were talking about when the term reflectability was brought up.