Not sure why the original poster chooses not to mount under the tank.
However, I choose
not to mount the UVB tube inside the cage, because of the height. My tank is 36x18x16". My Beardie is very active and constantly jumping at the top of the tank (when he was in his 30x12x12 tank I'd constantly find him with his claws hooked over the top edge). To me, the odds are too high that he could accidentally come in contact with the bulb causing it to shatter (which could possibly impale him with glass shards) or he could get burned by coming in brief contact with the bulb.
Yes, I could engineer a safety barrier of wire framing the light and remove all climbing areas from the tank to make a bulb mounted in the tank safer, but doing that does not eliminate the chance of injury happening. My beardie loves to climb and bask. It would be a significant diminished quality of life to remove all climbing limbs and basking rocks from his home. If signs of MBD do appear I will re-think this.
However, even then my first response would be to cut the screen, on the top of the cage, under my ZooMed 36" fixture and Reptisun 36" 10.0v bulb. And again the fact that I am using a hood with a mounted reflector so that the rays from the top and side of the bulb are bounced off the reflector angling them down into the tank increases the amount available in the tank (the photos of all the fixtures mounted sideways on the top back of the tank causes me concern for the human eyes that are being exposed to them).
The UK site
http://www.uvguide.co.uk/index.htm provides some interesting reading on this subject. However, I gained much more scientific data from reading all the source articles quoted. :study: For example, one of the pages ends with a warning about 'excessive uvb.' However, the source material only references 'excessive uvb' being recorded in mammals NOT reptiles (different skin structure plus shedding). Plus there is the issue of the different voltage in the UK. Meaning the voltage (power) excites the electrons in the tube. Given the different voltage here in the US a change in readings is to be assumed (thus you might notice the ZooMed packaging being used here in the US is actually the UK packaging specifying a different wattage than is imprinted on the actual bulb).
The source material on the UK site has a reference to the filtering of a screen top. I believe the source material quotes 30% filtering with variances arising from the mesh of the screen. Thus a top with fine mesh window screen filters more than the wide mesh of a reptile screen top. I have not been able to find any vet journals or dissertations reporting the 50% that gets thrown out so much.
I have also found ZooMed is quite good at responding to questions by email. I'm trying to get them to find a physics major at Cal Poly to test the bulbs in the lab (I did my Physics degree at UCSB but also toured Cal Poly before making my decision where to go). I'm also contacting former peers to see if I can find someone working in a lab with equipment. I'd love to do my own experiments, but at last I am a poor slave who can't afford to buy all the different bulbs and don't trust the readings from the cheaper handheld UVB meters.
I guess, at this point, for me I have to trust the biological influences my Beardie has. If he needs light it is there, spanning the width of his cage. If he needs less he has his hides to get out of the light.
