Good post ... just a couple of minor comments to add in two sections.
1. HID lamps give off more heat.
All lamps of the same wattage give off more or less the same amount of heat. A 400W metal halide or HPS lamp gives off the same heat as ten 40W fluorescent tubes or four 100W incandescents. The HID lamps will be hotter because all the heat is being given off in such a small space, but the total heat is the same. A fluorescent tube will gently warm the whole shelf while a HID bulb will fry a few square inches of plant and hardly touch the rest.
Efficiency of the bulb should also be taken into account when choosing lighting for a grow. Since the wattage specification for any bulb is the amount of overall electrical power it uses (only some of which is converted to light), it is true that 400W in any combination of bulbs generates about the same amount of overall heat dissipation. But the light output per watt used varies greatly and you get more bang for your buck (in terms of useful photons for the plant) using bulbs with a higher lumens per watt (L/W) ratio. Incandescent bulbs are typically less than 20 L/W while CFLs average about 70 L/W, MH runs in the 100-110 L/W range, and HPS 140-150 L/W. So MH and HPS with good reflector designs can get more useful light to the plants per dollar spent on electricity than the other types because more of the energy used is converted to useful light output rather than wasted as heat.
4. Plants only use red and blue light, green light is useless.
Plants use light at all wavelenghts from near ultraviolet to near infrared to convert water and carbon dioxide into sugars. They use red light near 650nm most efficiently, then blue light near 620nm, then light in between. Some plants are much less efficient at using green light, some use it almost as well as red and blue. All plants have some capacity to adapt to best utilise the available light.
I think you mean closer 420 nm for the blue light wavelength. 620 nm is orange-red while "blue" is in the 450 nm range.
1. HID lamps give off more heat.
All lamps of the same wattage give off more or less the same amount of heat. A 400W metal halide or HPS lamp gives off the same heat as ten 40W fluorescent tubes or four 100W incandescents. The HID lamps will be hotter because all the heat is being given off in such a small space, but the total heat is the same. A fluorescent tube will gently warm the whole shelf while a HID bulb will fry a few square inches of plant and hardly touch the rest.
Efficiency of the bulb should also be taken into account when choosing lighting for a grow. Since the wattage specification for any bulb is the amount of overall electrical power it uses (only some of which is converted to light), it is true that 400W in any combination of bulbs generates about the same amount of overall heat dissipation. But the light output per watt used varies greatly and you get more bang for your buck (in terms of useful photons for the plant) using bulbs with a higher lumens per watt (L/W) ratio. Incandescent bulbs are typically less than 20 L/W while CFLs average about 70 L/W, MH runs in the 100-110 L/W range, and HPS 140-150 L/W. So MH and HPS with good reflector designs can get more useful light to the plants per dollar spent on electricity than the other types because more of the energy used is converted to useful light output rather than wasted as heat.
4. Plants only use red and blue light, green light is useless.
Plants use light at all wavelenghts from near ultraviolet to near infrared to convert water and carbon dioxide into sugars. They use red light near 650nm most efficiently, then blue light near 620nm, then light in between. Some plants are much less efficient at using green light, some use it almost as well as red and blue. All plants have some capacity to adapt to best utilise the available light.
I think you mean closer 420 nm for the blue light wavelength. 620 nm is orange-red while "blue" is in the 450 nm range.
