Re: Spectral Design
I actually got interested in the light power of Spectral Design LEDs several weeks ago. I contacted Kurt and asked him if he knew the PAR value of any of his LED hoods, and he said that he didn't. Would that I could find or rent an affordable PAR reader!
I tested the lux level of various heights in my 24*18*24 Exo Terra tank, too, which uses a 22*12" no-spotlight Spectral Designs hood (which is also an extra inch tall, in order to accommodate the MistKing nozzles). The readings ranged from something like 10,000-12,000 lux at ground level (which for me was about at the upper limit of the "lip" of the tank that holds substrate in. I forget the proper name for it), ~20,000 lux at about the highest that plants are likely to grow while still looking attractive, and ~25,000 lux when the light sensor was pressed up against the glass top, as close as it could possibly get to the LEDs. So, not super-high.
My impression of Spectral Design lights is that they aren't a money-efficient grow light if all you want is a grow light. It's for hobbyists who want an attractively built light source that produces a nice light color, low-heat, and no noise, and that can be easily modified to imitate the gradual increase and decrease of sunlight. Of course, not having the stronger spotlight-included Spectral Designs hood of the same size, I don't know if they aren't much more powerful. But that gets expensive
Of course, it could be something else than the lighting. I'm not a plant guru, but yellowing doesn't typically sound like light scorching.