Hi, because you are already seeing violet light thro' the gap from white light impinging you can forget photons, a scanning electron microscope is your next 'ruler' if you have one handy that will accept a 400mm long 'specimen'. I suppose if you're a dab hand in thin film techniques you might use mechanical methods, feeler gauges, down to 100-200nm. By that I mean depositing a set of films of something like tungsten onto substrates precoated with a soluble layer to allow you to float your 'feeler gauges' free. You'd need to de-stress them first .. somehow .. to avoid them rolling up into tubes. Or, graphene films ought to do, but you'll only be able to investigate between where you are at 400nm & 100-200nm, hardly significant. AND you may loose your sanity trying. Why not just accept them as they are .. while you're in front.