I used to crank the laptop brightness right down as Cx2 said, but I've not noticed a physical brightness controll on any modern machines for a while now, and I think te only way to do it would be muck about with display settings but I've never worried enough to go and look.
To be honest I personally never care if people in public read my screen, ---- I do afterall use my laptop on trains, on stations or in the street quite a bit, but I would like a way of turning the screen completely off sinse it would save a lot of batteries, and the screen on a laptop is not good enough for me to do anything graphical on anyway, ---- well not on a 14 inch like this one, maybe on a 17 inch.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)