Marking laser position with glow in the dark material

laseroptics

This is a weird set up but I'm trying to figure out at what angle of deflection a pendulum has based on using a laser point reflecting off an attached mirror. I've worked out the math but I'll need to know the height from the laser origin the beam is deflected at in order to figure out all the relevant angles.

This beam cannot be directly observed, however it is possible to observe after a few seconds so I need a way to 'preserve' the last position of the laser pointer for a few seconds.

Here is my current idea:

Take a glow in the dark paint and shine a UV light on it to 'charge' it and make it glow. Then use a 5mW red laser pointer in order to cause the point where the laser hits to lose it's charge so it looks like a dark dot surrounded by glowing green material.

Is there any alternative ways to doing what I'm trying to do? I know that by using a 405nm blue laser it will instead cause the point to glow so I can just make one specific point glow – but I do not have a blue laser pointer on hand so that's why I'm doing it in reverse with this red one.

Best Answer

The usual way to detect an IR beam is to use an "laser viewing card" or "IR sensor card".

These are sheets of paper or plastic coated with a phosphor material that can be "charged" by multiple IR photons (I don't know if this works through a 2-photon process or by two charging steps through an intermediate state), and then emit a visible photon. They're sold by the major optical components distributors like ThorLabs, Newport, etc. The size of the sensor region is typically between 25 and 75 mm square. If you're working in an organization with some IR lasers already in use, look around and there should already be a few of these available.

I'm not sure if these will maintain a glow for "a few seconds", but if you observe them with a sensitive camera in a dark room, they may work.