Stroboscopic repetitive flash photography provides a convenient method for recording data in dynamics experiments. Xenon flash tubes operate by charging a capacitor to high voltage, then quickly discharging this stored energy through the flash tube. The tube converts the energy to an intense burst of light of short duration (on the order of 1 microsecond).
Stroboscopic (strobe) systems generally provide calibrated, variable flash rates. Electronic logic circuitry derives the strobe trigger pulses from the 60 Hz power line frequency. Continuously variable rate strobe systems may be calibrated against the line frequency. In either case, the flash rate is as accurate as the line frequency--better than 0.1 percent anywhere in the United States.
Two or more light intensity settings may be provided. High intensity operation, and high flash rates, both shorten the lifetime of the Xenon flash tubes, therefore use low intensity and low flash rates whenever they are adequate, and operate the strobe system only when actually taking data. The smaller tubes, such as those used in the Ealing By-The-Lens system, quickly overheat and become inoperative until cooled. Ealing supplies the following data: At 10 Hz, after 1 minute operation, 2-4 minutes cooling is required; after 2 minutes, 6-8 minutes cooling; 3 minutes, 8-10 minutes cooling. Operation above 20 Hz is not recommended; accidental operation at higher rates may require several hours of cooling time and will substantially shorten the lifetime of the tube.
The larger Xenon lamps are less temperamental. But observe the manufacturer's precautions. The Ealing strobe slaves have two intensity settings, the high intensity setting must not be used at rates above 10 Hz. The General Radio strobe, a much more expensive unit, has accurate variable rate over a very wide range of frequency, and automatically switches to low intensity on the higher rate ranges.
Multiflash (or repetitive flash) photography uses strobe illumination and a camera whose shutter may be left open long enough for many flashes to record on the film. The "time" or "bulb" shutter setting of the camera is used. Since the background is illuminated by every flash, but the moving body is recorded by a single flash at a given location, there is a problem of providing enough light to record the moving body without overexposing the background. The body is marked with light-colored paint (or Scotchlite retro-reflective tape) and the background should be as black and non- reflective as possible. Note that black bodies may still be reflective if they have smooth surfaces. The surface of the air tables has this property, so the strobe light must be positioned where it will not produce "mirror" reflection into the camera lens. Aim the strobe light at the table from a low angle; sometimes it is helpful to move the flash lamp during the exposure to "follow" a moving puck.
The Ealing By-the-Lens strobe system employs a flash lamp small enough to be located on the camera directly beside the camera lens. The moving bodies are"marked" with Scotchlite tape, similar to the material used on reflective highway signs, license plates, and movie screens. Its surface has many glass beads which reflect light back approximately in the direction of the light source. Thus when the source and camera lens are near each other, Schotchlite marked objects will appear much brighter than other objects. This system allows the use of smaller camera apertures. About thirty times as many flashes may be recorded on a single picture without degrading contrast, as compared to ordinary strobe illumination. The by-the-lens flash lamps may also be used without the Scotchlite, for oblique illumination, just as an ordinary flash lamp.
Keep the room lights off, and close the window shades.
Several strobe lamps may be connected to a single power source. But when several are operating at the same time the flashing may be unreliable; the lamps may occasionally misfire. Cooperate with other groups to ensure that only one flash unit is operating from a given source at any time.
CAMERA PLACEMENT AND OPERATION
The air track or air table must lie perpendicular to the axis of the camera lens if the photographs are to be easily and accurately analyzed. Also, the moving objects must lie in the plane of best focus of the lens. Frosted glass or plastic screens are available; they may be placed in the camera film plane with the shutter held open to observe and evaluate the image before taking photographs. Use the largest camera aperture setting when doing this. On the model 320, ASA75 gives the largest aperture. On the model 160, use setting EV- 10, 11, or 12. For other cameras, set to the smallest f-number value.
"Time" exposures are obtained with the Polaroid model 320 by affixing a small piece of black tape over the electric eye to disable the automatic exposure system. The camera battery must remain in the camera, for it powers the shutter. The shutter will then remain open as long as the red shutter button is held down. With the older Polaroid model 160 cameras, a small toggle switch below the lens must be flipped to the "B" position before each exposure (it flips itself back to the "I" setting every time a picture is taken.)
If the above instructions have been followed, you should obtain photographs with clearly defined (though perhaps overlapping) images of the moving bodies. We will describe data measurement techniques for air table pucks, these instructions may be easily adapted to air track gliders.
Photograph an object of known length, (perhaps a ruler or meter stick) lying in the plane of the top (marked) surface of the pucks. This photo will establish the relationship between lengths on the photo and lengths in the experimental plane. Self-stick rulers may be applied at the edges of the table, to aid in calibration.
Locate the center of each puck image and mark it with a pin-prick on the photo surface. The spacing between images may then be measured by use of a metric transparent ruler, or vernier calipers. An alternative method: Use a pin to make holes in the photo at the center of each puck image. Mount the perforated photo in a frame so that it may be placed in a lantern slide projector and projected on a wall, or directly on a piece of graph paper. Then make pencil marks on the graph paper at the locations of the bright images of the holes. Further analysis is then done on the graph paper.
SUMMARY OF EXPOSURE SETTINGS, CAMERA APERTURE SETTINGS
Using 3000 ASA speed films. Ealing Slaves, By-the-Lens, Low setting with retro-re flective tape. Polariod model 320 camera........ 75 3000 Polariod model 160 camera........ EV-13 EV-17 Other cameras.................... f: 3.5 f:11
A two-position slide switch above the lens sets the aperture on the model 320. A rotating wheel above the lens sets the aperture on the model 160.
© 1994, 2004 by Donald E. Simanek.