radiums will look completely different depending on what ballast you run them on.
they have a 20k or higher look to them when run on a true electronic ballast (like icecap).
if you run them on an HQI ballast, it will overdrive the bulb (kinda of like how a magnetic ballast overdrives), and it will look more like a 14k. but it will severely decrease the life of the bulb.
for instance, a 250w electronic ballast when measured on a amp meter, registers right around 250w. a magnetic 250w ballast actually measures around 270-280w.
a 250w HQI ballast runs closer to around 330w.
so the higher in wattage you go, the more "umph" you are putting through that bulb, which burns off that blue pigment they put inside the bulb, and makes it look more like 14k. and if the wattage you are running on the ballast is higher than that rated for the bulb, you are going to reduce the life of the bulb.
for an HQI ballast, i wouldnt run a radium bulb any longer than 6months. at 6months (on 8hrs a day), its gonna look much less brighter, and closer to 10k by then with a HUGE drop off in PAR.
so the huge downside to these bulbs is the cost ($90), and you have to replace them every 6months.
so unless you have deep pockets and dont remind replacing expensive bulbs twice a year, if you want a 20k look you are better off to stick with XM 20k's or something else that is cheaper and will last longer.
the reason why i use a 250w radium on an HQI ballast, is b/c it has the highest PAR possible in a blue bulb in that wattage. its way higher than an XM20k, thus i can get better color and more growth out of it. and since i only have a single light over a cube tank, i only need to replace 1 bulb so its not that bad of an expense every 6 months.
