There is no reason a well appointed UVB lighting system, installed in an appropriate manner, within the correct environment/enclosure, should result in 'eye issues' and certainly not blindness, no matter how long the photo-period is.
Poorly chosen bulbs, inappropriately installed and of incorrect strength/wavelength are the only things which can result in such retinal issues and will do so over time whether exposure periods be short or long.
What is of great danger however, is purchasing any UVB bulb of indeterminate strength and then installing those bulbs wherever they will 'fit' and presuming that will be OK. One must always be able to either read, or accurately predict the UVI the animal will be receiving from the bulb at its closest point for example and that light must reach the animal in a natural manner, being able to look directly at any bulb while in their natural basking position will almost certainly result in long term eye damage for example and such issues, as you have found, will not be treatable by conventional means.
UVB must always be provided to the appropriate level and within an appropriate enclosure, with areas of both partial and complete shade aswell as areas of upper index basking. Thereby allowing each animal to determine the level of exposure it needs, we as keepers, can never hope to do this as effectively as they can. This the main reason we are starting to see the move away from guessing at doses of synthetic supplements and toward safe UVB and mineral provision in our more forward thinking keepers.
On the topic of natural length UVB timing, on the contrary, having an appropriate length photo-period provided by a UVB lighting system is as close to 'natural' as we can hope to provide in captivity, but it must be done so in the correct manner.
We, as keepers, can do nothing but wildly guess as to what each animals UVB requirement at any given time, which is why it is imperative that we allow them to self regulate, taking them out in the sun for a random period of time each week, while undoubtedly beneficial, is far from measurable and certainly should not be relied upon as the animals sole chance of access to UVB.
- Elizabeth
Multivitamin - Arcadia Earth Pro A
Calcium - Arcadia Earth Pro Calcium-Mg
I do indeed use a UVB Meter - Solarmeter, the model I use is 6.2 (currently)
As for UVB provision re Leopard Geckos, it is my view that all but the truly nocturnal species (very few of which exists, let alone are kept in captivity) benefit greatly from appropriate provision of UVB. As highlighted above however, that must be done in the correct way.
Currently I am not aware of a solution which would be suitable for the common 'tub' system used by many.
As for minimum dimensions, one can source quality light units nowadays which
safely project low index UVB light at a strength of about 3-4% around 1 foot from the face of the bulb, so the absolute minimum I would suggest in terms of height would around 30cm approx.