The Rotating Mirror Setup
By aiming the camera at a rotating mirror one can get pictures that separate an event by both time and space. The mirror overlays images on the film as each of the flashes go off. It has to be rotating so that the part one wants to see is reflected upon a different part of the film. To achieve this, everything has to be black except for the event that is being captured. This technique adds time resolution to our photographs, thus allowing us to analyze the combustion reaction in reference to time.
The camera is aimed at the rotating mirror so that one can see the event by looking through the camera into the mirror. We referred to the angle where the you can see the event through the camera as the effective mirror angle. Once this angle is set, the rest of the setup is getting the timing right. The mirror has to break a laser which will start the program 'Hydrogen Timer' running on the computer. As the mirror rotates to its effective angle, the explosion is triggered, the reaction begins and flashes go off sequentially. At one point in our experiment we added a constant light source from a stage lamp that would enable us to observe a change in volume during the time between flash discharges. The result is a streak photograph on which we can trace the outline of the top and bottom of the balloon to look for a change in volume. We also combined the constant light source with our flashes to reveal instants of time on top of the streak photograph. An example can be seen on the Analysis page.
For a more detailed explanation on the rotating mirror see M.T. Hinshaw, "Stroboscopic Study of High-Speed Projectiles in Water," Journal of High-School Science Research 2, 1 (1991).
Below are photos of the rotating mirror as used in our experiment.
Here you can see the rotating mirror and camera setup. The mirror rotated at a frequency of either 5 or 10 Hz, depending on the time resolution we desired. The laser and photocell was the triggering mechanism that started the timing program in the computer that would activate as soon as the path of the laser was broken.
When the mirror rotated into the effective angle, it would trigger a series of eight Vivitar 283 flash units to discharge in sequence so the image of the balloon would be distributed across a single frame.
The Apple computer managed a program called Hydrogen Timer that we modified from another timing program. It allowed us to set delays and flash intervals in order to capture the image at the right time. The Apple II was used in this project because of its reliable clockspeed, a crucial factor in our timing calculations. The multiplexer is a device that allowed us to connect multiple flashes to the computer instead of limiting us to four outputs.
All photos on this site are copyrighted by Brian Sweeney
and Tim Collier, 2002.
To inquire about picture use please contact Loren Winters at winters@ncssm.edu