Whats the difference between ....

.... a G36T6L lamp and a G36T5L lamp?

I have a water purification system that specs the G36T6L but various
suppliers have offered the G36T5L lamp as an equivalent. Is this true? I
Googled both P/Ns but couldn't find a single site with specs on both.
Comparing across different sites, the G36T6L is a 39 Watt lamp (425 mA
lamp current) with 13.8 Watts UV output @ 254 nm wavelength. The G36T5L
(from a different web page) is a 39 Watt lamp with 12 Watts UV output
(no wavelength spec'd). Are they equivalent?



Also, the first site (http://www.ultraviolet.com/lamp/sterlra01.htm)
listed the UV output at 120 uW/cm^2 at 1 meter from the lamp.
Calculating the total UV output through a 1 meter radius spherical
surface, I get 15.0 watts, not the 13.8 Watts they published in the next
column. Why the discrepancy?