SunMaster - Cool Deluxe Metal Halide are the overall market leader in grow lamps in the UK. The Sunmaster halide has 7200 kelvins. Blue light is measured in kelvins as opposed to lumens (which is the measure of light commonly used for sodium and dual spectrum lamps), the higher the kelvin value the more plant usable light the lamp gives out. Higher kelvin ratings result in healthier plants, shorter internodal length and increased yields later in life.
The 7200K versions are also the lamp of choice for adding extra blue spectrum to a lighting setup that contains mostly sodium lamps - adding approx one halide lamp to every two sodium lamps dramatically increases overall plant health, and is generally considered by most experienced growers as the optimum lighting mix. Highly recommended - these are by far the best metal halide lamps on the market and won't break the bank.
The Sunmaster range of lamps has proven to be excellent value for money providing reliability and quality lamps time and time again. Horticultural lamp light output will decrease with use.