I have used led panels to backlight a stone slab but there is a shadow in the middle. How do I fix this?
It looks like an object is located in the cavity behind the stone. It's not directly against the back of the stone (otherwise that area would be completely dark), but it's positioned a short distance away.
(This could be related to the ventilation slots above the stone slab.)
You could try changing the LED's mounting position, for example, moving it further forward towards the stone. However, this could result in the edges being significantly brighter than the center, and it's uncertain whether this would achieve even illumination.
The best results would be achieved with an LED panel behind the stone slab.
With luck you'll find one in the right size.
Alternatively, you could build your own panel using special frosted LED Plexiglass and aluminum rails for side lighting (similar to the profiles used for glass display case shelves).
Smooth the cut edges of the plexiglass with a utility knife and then polish to a high gloss; the light will then refract on the matte surface, making it appear to glow.
This is similar to how it's done with LED monitors and LED panels.
The brightness might decrease slightly towards the center, but it would be very even and without harsh transitions.