Seeking a good explanation regarding the distance function of hps/cfl

Hi!
Whenever CFL and HPS are compared, people always speak of the fact that HPS lights have to be placed a lot further from the plants than CFLs, while CFLs cannot be placed too far away from the plant as it'll render them almost useless.
Can anyone supply a scientific explanation as to why this happens? thanks!

Not very scientific, but in layman's terms, different bulb technologies emit 'usable' light at differing intensities. The higher the light intensity, generally the more heat is produced.

With CFLs, the intensity degrades over a much shorter distance than HPS, and therefore the bulbs must remain within close proximity to the plant (1-3 inches is recommended) for the plant to make use of the available light. They also produce less heat, so the potential for burning the plant is reduced. With HPS bulbs, the intensity is much higher and travels a further distance before degradation, but they emit more heat. When placed too close to the plant, they can burn and bleach it.

Google light output comparison charts.