Pharmacam.

I am trying to keep my cool and not post something that will get me tossed out by your comment.

21k is 21000 kelvins this is the color of the light produced from the bulb which is in the red spectrum. The proffesion I do involves dealing with all different types of lighting and electrical designs and trouble shooting. As for wasting your time I spent a good hour or more looking thru catalogs and on line to find these bulbs in order to give accurate information.
When Wheaty said duel spectrum it made it sound like this bulb was designed for both stages of growth. I want to make sure that was not the case before I tell you and others not to buy this bulb thinking it can be used for both stages of growth and get the same results as using a MH bulb because you will not. These bulbs are designed with some added blue light to help with photosynthisis during flowering and are probably your best choice for the flowering cycle. As for which of the 2 is better I would go with the Sylvania Grolux due to brand name and reputation as a quality lighting provider.
Jerry Garcia 2007 Reviewed by Jerry Garcia 2007 on . which bulb is superior which of these bulbs are better Grolux 400w Dual Spectrum Sunmaster 400w Son T Plus Rating: 5