High-Speed Photography
High-Speed Photography
Background
Many physical events occur too fast to be discerned by the unaided human eye or photographed without blurring. However, many of these events can be captured by camera, and the photographs analyzed. Some rapid processes can be analyzed by looking at frame-by-frame changes in a video. Extremely rapid processes must be captured by using a very short time exposure. This latter technique is called high-speed photography.
The "high-speed" in high-speed photography does not refer to the shutter speed; in many cases, even the fastest shutter speed would still produce a hopelessly blurred picture. Instead, one leaves the shutter open and uses flash units to illuminate the scene (and make an exposure) for a sufficiently short time while something interesting is occurring. One therefore needs a way to trigger the flash units at the instant of the event. For instance, a microphone placed to pick up the sound produced by a popping balloon can trigger a flash just after the balloon pops. Moreover, if one places two microphones at different distances from the popping balloon to trigger two different flash units at different times, the film will be double-exposed and the picture will show two moments during the destruction of the balloon. From this double exposure, one could then calculate the speed of the rip in the popped balloon. Note that, in this case, one must somehow determine the time elapsed between the two flashes.
Procedure
Part I — Video Mode
In this part of the experiment, you will be determining the rate of change of some quantity(for example position, angle, volume, etc) for some process that changes too rapidly to measure directly, but not so quickly that it would cause blurring of the camera exposure. Set the mode selector on the digital camera to video mode. The camera will begin recording 15 frames per second (fps) when you press the exposure button, and continue until you press the button again (or until memory capacity is reached). Record a process of your choice for a short time (1-3 seconds). The process you choose should change a small, but measurable amount between frames of the video. Be sure your video contains any measuring apparatus you may need to make measurements on the video frames. For example, you may need to have a meter stick in the frame to determine the distance an object moves.
Analysis
Review the video to see if it meets your requirements. If not, record the reasons, make changes to the setup, delete the video, and try again. Once you have an appropriate video, download it to the computer. Open the video in QuickTime.
QuickTime Player has onscreen controls similar to those found on CD players and DVD players. Use the controls to play or pause the movie, to move forward or backward, and to jump to the beginning or end of a movie. To go to a specific point in the movie, drag the playback head (the small black triangle) along the timeline. To step through frame-by-frame, first click the small black triangle and then press the Right and Left Arrow keys on your keyboard.
Step through the frames and measure the quantity of interest, Q, in each of at least 10 frames. Find the average rate of change between frames: Q/t. Note that t is 1/15 s. Plot these nine values of Q/t as a function of time. Does this rate appear to be approximately constant? If so, find the average value. If not, fit the data to some smooth curve (either with curve-fitting in Origin, or by hand). From the graph, determine the average slope d2Q/dt2 (if Q is the position, this is the average acceleration.)
Part II — High Speed Photos
Figure1 shows the general setup of the apparatus. The rotating disk is black with a white radial line. This disk will help you determine the time elapsed between the two exposures. A meter stick is set up next to the location of the balloon for the purpose of determining scale in the pictures. It is important that each picture include the balloon, rotating wheel, and meter stick. A black cloth serves as a backdrop to eliminate reflections of the flashes.
Set up the triggering circuit that is depicted schematically in Figure2. The signal from the microphone is amplified by the tape recorder; each microphone will use a separate channel. The output from the tape recorder (which normally goes to the speakers) is input to the silicon controlled rectifier (SCR), which acts as a switch on the flash unit. When sound reaches the microphone, current flows out of the tape recorder and the rectifier will basically short the electrodes of the flash unit, thereby triggering a flash.
Examine one of the flash units. To achieve the fastest possible flash and thus minimize blurring in the picture, you will use a component of the flash unit that measures the light reflected back to the unit and quenches the flash when it has received a set total amount of reflected light. The reflected light is measured by a photocell mounted on the front of the unit. Turn the knob so that you see a yellow square. This places a fairly transparent filter in front of the photocell, ensuring the greatest sensitivity to reflected light and thus resulting in the shortest flash.
Plug one flash unit and mike into the left input and output.Plug the other ones into the right input and output.
Now test the triggering circuits. The tape recorder must be in RECORD mode. If you clap your hands next to a microphone, the corresponding flash unit should flash. You can control the sensitivity of the triggering circuit by adjusting the VOLUME and BALANCE controls on the tape recorder. The main source of noise will be the rotating disk. Plug in the motor and adjust the sensitivity of the triggering circuit so that the noise from the motor does not produce flashes, but a popping balloon does.
Before you take pictures, you should watch a few balloons pop with the room lights out and the flash-triggering circuits turned on. Try varying the distance between the microphones and the balloon; useful distances will probably be between 1cm and 15cm. Also try using different sizes of balloons, and try sticking the balloons with the pin in different places. You will probably achieve the best results with the large balloons. You might also want to try various positions of the flash units. Finally, observe the rotating wheel while you pop a balloon.
Once you have a general feel for the experiment, take some pictures. The digital camera should be set up so that it does not flash. You should also set the exposure time long enough that you have time to pop the balloon after opening the shutter.
Inflate a balloon, and position the microphones and the flash units. Place the camera on the tripod, adjust the zoom so that the balloon and rotating wheel fill the field of view, and be sure the balloon and meter stick are in focus. Record relevant information, such as the distances between the microphones and the balloon, the length of the balloon, andthe focus setting. Plug in the timing wheel and allow it to ramp up to speed (you will want to unplug it when not in use, as it is noisy). Turn out the lights, open the shutter and pop the balloon. There should be two flashes. Allow the camera to end the exposure, then turn on the lights and observe your photograph. You may need to change to review mode. Decide whether this frame is worth saving. If not, record what needs improving, then delete the photo from the camera. Do not delete the information from your lab write-up. A useful criterion for good frames would be whether you can see two interesting exposures.
Try different microphone positions being sure to record the distances between the microphones and the balloonwhenever you change positions.You should take at least two pictures with each arrangement you use.
When you have several good frames, download your photos to the computer and save them. You should print any frames that you analyze to include with your write-up.
Period of the Rotating Wheel
Turn off the room lights and start the rotating wheel. As before, allow it to ramp up to speed. Illuminate the wheel with the strobe. Vary the frequency of the strobe flashes. The strobe frequency reads in flashes per minute.
Question: Under what circumstances will the wheel appear to have three lines? Two lines? One line?
Using the answer to this question, determine the rotational frequency of the rotating disk. Determine the angular velocity of the wheel in units of rad/s.
Analysis
What experimental conditions gave you the best pictures? Discuss the positions of the microphones as well as the sizes of balloons used.
Using the meter stick in the picture to set up a distance scale, and the angle of rotation of the wheel to determine the elapsed time, find the speed of the rip in each popping balloon. Estimate the uncertainty in your calculation and discuss the sources of error in this experiment.
Question: Which balloons ripped fastest, or were all speeds similar? Why might that be the case?
High-Speed Photography