Using new foreground layer to allow hidden tokens over the whole map

I'm putting together a simple wargame for my players which requires hidden units and am using the trick of duplicating the map to the foreground layer and setting it 'as darkness'. Then on the lighting layer I mark out hills etc. which hide areas & affect movement.  I then have dynamic lighting on (no light sources or daylight mode) and give the player tokens night vision.  This is working well, the only issue is that the edges of the night vision revealed area is very jagged (see screenshot) Night vision effect is 'none'.  Any suggestions for smoothing it out, or is it a limitation I need to live with?

Go it! it was the "lighting texture resolution" setting on the graphics options, changed it from 'auto' to 'max' and things are much better, only system effect was about another 5% GPU utilization.