HID lamps at 2 feet away do little more than waste energy.
Think about it. First of all there is the light provided. Most HPS bulbs are 2700K color temperature with the specific wavelength peak about 30nm off of ideal. Essentially, 10% of the energy provided by the bulb is photosynthetic active radiation. Add in the fact that the bulb is a distance of "3 units" away when at 2 feet and you'll see that the plant is getting 10%-12% of the 400w due to the law of squares. Finally consider that a small seedling might be 2"x2" large, for a total area of 4 square inches, but the light is spread across an area of say 576 square inches (a 2 foot x 2 foot space with reflective walls). So the plant takes up 0.7% of the total area, and cannot get to many of the photons that are lighting other parts of the room.
Now add that all up, you are getting 0.7%, of 10%, of 10% of 400w. Or approximately 0.028 watts of energy is being actively absorbed and used by the seedling, and that's on a good day with full bore photosynthetic action. In addition to all this, of the wavelengths being absorbed the majority is red. Red light triggers flowering hormones and increases the stretch between internodes.
Now think about all that power versus a 27w Day Light CFL bulb in the 6500K color temperature kept at within 2". You are at a factor of about "0.8 units" from the bulb, so you're getting more than the advertised lumens that close. Lumens are a poor way of conglomerating all the effects of light but simply put: more is better. So you're sitting at 100% of the wattage, with more than 80% of the energy available for photosynthetic absorption in the highly reactive blue part of the spectrum that encourages leaf growth and decreases internode distances. Then consider with a clamp light reflector you can direct your lighting to an area no larger than 9 square inches, increasing the amount of light the plant is able to absorb to close to 50%.
So... you make the choice...
0.7% of 10% of 10% of 400w for 0.028 watts to the plant
or
50% of 80% of 100% of 27w for 10.8 watts to the plant
An increase of 385 times the energy to the plant with less than 7% of the energy used.
Talk about a no brainer. An HPS light that provides so little energy AND causes my plants to stretch from both the distance and the wavelength or a lower wattage CFL bulb (purchased for less than $10) that forces plants to be bushy and stocky due to close proximity and wavelength. I mean, there's a smart way, and there's the other way.
Now, that isn't to say that a CMH bulb wouldn't be better, or maybe a metal halide or an HPS in the appropriate spectrum (2100K to 2300K). There are variables to consider, but light is physics, physics is math, and you can't argue with numbers. It's science. Scotchy, scotch scotch!
