UVB Tube brightness 18" vs 48"

[Psi]

So my dad had a 48" dual bulb fixture lying around and now that I have the two cages next to each other at the same height I was planning to use it so I can use 1 tube instead of 2.

My question is I picked up the 48" reptisun 5.0 yesterday and it doesn't seem nearly as bright as the 18" tube I'm using right now.

I'm guessing this is due to the fixture. The fixture is designed for 2x40w tubes (I only have the 1 40w t12 bulb in).

I'm likely going to hit a hardware store and grab another cheap 48" 40w tube for lighting, but I would figure that it would be brighter with 1 tube instead of 2 or am I forgetting my electronics calculations

Let me know if I'm just going crazy

 

[pssh]

Pick up a plant light. Those are nice and then you can keep hibiscus easier.

I'm not good with electronics and all that, but plant lights are pretty nice...