Difference between revisions of "Shutter"
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Latest revision as of 22:54, 11 May 2013
For recording holograms, some form of shutter is always necessary: switching on and off the laser instead is not feasible, as a laser will always take some time until it comes into thermal equilibrium, and before that there will be a lot of frequency drifts and Mode Hops that will ruin the hologram. The settling time of a laser can be minutes up to one hour or so.
The simplest method is to manually remove a black cardboard out of the beam path, but there is a great danger of induced vibrations on the holography table. This source of potential failure of a hologram is one of the easiest to exclude, by using an electronic shutter. Such a shutter can be used remotely controlled, or even fully automated if allows a programmable exposure time going off after some delay. This allows the holographer to be outside of the room during the shot, which is an important factor for cutting down disturbing vibrations and air currents. Using an electronically timed shutter is also convenient for achieving reproducible exposure times.
There are mainly two ingredients: first, the actual shutter and then, the controller.
The actual shutter is in most cases a mechanical device. If it is to be present on the same table as the holography setup, it is important to avoid any kind of vibrations. One method to build such a thing is to glue a small piece of mirror or aluminum foil to the needle of a mechanical voltmeter, and to drill a hole through the scale of the voltmeter to provide a path for the beam. The advantage is that the motion is very gentle and hardly induces any vibration.
For higher power lasers, somewhat more massive shutters are better suitable, and these need to be mechanically isolated from the holography setup (eg by putting them on an extra table together with the laser). Often Ilex (now Melles Griot) electronic shutters are available as surplus for photographers.
Another method to build fast-acting shutters from broken hard disks is described here.
Many shutter controllers made for photographic dark room use are suitable and are easily available on ebay. Just make sure that the programmable exposure time lies in the range that is useful for holography (ie, one second to one or several minutes depending on your setup). Having the possibility of an extra delay of 10 minutes or so before shooting allows a fully automated setup without direct user intervention. All this can conveniently be achieved also by a simple computer interface.