In a very frequently photographed place use lighting for less than 1/60th of a second every few seconds to expose or make appear an object. Only cameras will see the object this way. In one out of every 600 vacation photos in a particular spot: ET!

also useful for haunted houses.
-- Voice, Oct 31 2010

More like 1/100th of a second. http://www.psych.ny...entation_intent.pdf
[mouseposture, Nov 01 2010]

But how are you going to mount a laser apparatus in the middle of the sky without it being visible?
-- DIYMatt, Oct 31 2010


I didn't realise the sky had a middle.
-- nineteenthly, Oct 31 2010


It’s in between the top of the sky and the bottom of the sky, I presume.
-- pocmloc, Oct 31 2010


There's often clouds in the way.
-- daseva, Oct 31 2010


You could, instead, put it 'way up in the middle of the air.
-- mouseposture, Oct 31 2010


Why will people not see things if they're only illuminated for less than 1/60th second? Flashes from flashguns are much shorter than that and they're visible.
-- hippo, Nov 01 2010


There's a lot written about tachistoscopic presentation of subliminal visual images, but mostly in the psychoanalytic and marketing literature; not strong on technical detail. But I found a <link> that gives an actual figure for how short the stimulus presentation needs to be for people not to see it: seems to be about 10 msec.

Also, the trick probably only works if the image has about the same average brightness as what it replaces (c.f. [hippo]'s comment on flashguns) so you'd have to project *continuously* and tachistoscopically *replace* the image, rather than merely projecting tachistoscopically.
-- mouseposture, Nov 01 2010



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