Answer from Fran Baines on Facebook Group Reptile Lightning. This explains everything plus some more.
Hi. These grow to be big lizards, up to 30cm long, so you're going to need a much larger terrarium for him. But if he is still small, and you add some more climbing branches (ideally, living plants; a small weeping fig (Ficus benjamina) for example) that terrarium might be ok for a short while.
As for UVB, these are diurnal arboreal geckos, and they are partial sun baskers (between Ferguson Zones 2 and 3); they bask in small patches of sunlight between the leaves, in the rainforests where they live. So you need to add some "artificial sunlight" to the top of the vivarium, with plenty of dappled leafy shade below.
For UVB and basking warmth, you have a problem because both lamps should definitely be above the mesh screen, but you'll find it difficult to fit them both on that little strip of mesh. What a pity that you can't remove the glass and make an all-mesh top... A UV Index of about 3.0 - 4.0 right under the mesh will be safe; it does not want to be any stronger than that because the gecko is arboreal so will climb right up to the mesh, even walking across it upside down.
My first choice for the UVB would have been the Arcadia ProT5 Shade Dweller 7%UVB but it is 30cm long; although it would sit comfortably to one side of the mesh, there would be no space to fit the basking lamp, which must be in a dome reflector to hold it safely over the mesh.
So have a look at a ZooMed Mini Combo Deep Dome Reflector like this:
https://www.reptiles.swelluk.com/zoo-med-mini-combo-deep-dome-lamp-fixture
and in one of the lampholders, you could put a low-wattage basking bulb (the one you have already might be OK, but measure the temperature it reaches on the mesh and on the nearest branches below, and control it with a dimming thermostat to a safe 35C surface temperature, air temperature elsewhere in the vivarium 23-25C.
In the other lampholder you can put a UVB compact fluorescent lamp. Because of the small size of the dome, you are restricted to the smallest size of lamp, too (or it sticks out below the dome, and so the dome will not rest on the mesh.) The ZooMed Reptisun 5.0 13watt compact fluorescent lamp will be safe to use, even at close range. In a dome, resting on mesh, with this lamp I measured UVI 5.9 right beneath the mesh.
I would advise against using the only other small 13watt lamp which will fit into this dome, the ExoTerra 100. This has a much stronger output, giving a UVI of nearly UVI 10 right beneath the mesh, which is much too high.
But when you get a bigger vivarium for him, I would definitely advise changing to one with a full mesh top and use a linear tube, something like an Arcadia ProT5 60cm, 24watt fixture with the 6%UVB T5-HO tube.
You could still use the basking lamp, and put a daylight (non-UVB) compact lamp in the second lampholder to increase the visible light, which would be good for the gecko and any plants as well.
