The Shape of Light Squared

by Don Webb

The National Ignition Facility (NIF) is the world’s largest optical instrument, comprising 192 37-cm square beams [...]. The Facility houses a massive [...] target chamber within which the beams converge onto [a] target for deuterium/tritium nuclear fusion.
— from the abstract of “National Ignition Facility system alignment” in Applied Optics

I think I shall never see
A flashlight squarer than a tree.
More’s the pity: square won’t roll
When I need a light and my hands are foll.
But now I wonder inventorly
What shape the flashlight’s beam would be.

To Challenge 431...