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Computer Video Analysis: An Inexpensive Alternative

Video Analysis is becoming an important tool for studying motion in physics laboratories. Using one of several available computer programs, students make position versus time measurements on short movies which have been obtained by digitizing videotape or laserdisc recordings. They graph their data immediately or store them for later analysis using graphing or spreadsheet programs.

The process can be made more appealing if the students are able to design the experiments and produce their own movies for analysis. Unfortunately, the usual procedure for producing digitized movies is rather complex and expensive, requiring a video camera to record the motion, a computer equipped with a digitizing board, and a VCR with ability to play the tape one frame at a time, since most computers and digitizers cannot grab frames in real time at the rates needed for accurate analysis.

The QuickCam provides a faster and cheaper alternative. This is a device which plugs directly into a serial port on the Macintosh computer and produces digitized QuickTime movies without the intermediate step of recording to tape. The movies are black and white (16 gray levels), and up to 320 x 240 pixels in size. Software for making movies and still pictures is included with the QuickCam; no other hardware is needed.

Frame rate and timing accuracy can be improved by keeping the camera's brightness setting as low as possible, making it important to use bright lights and good contrasting backgrounds for the objects being studied. A second method of improving the frame capture speed is to crop the picture as much as possible. If the motion is one dimensional, you can crop to the region of interest. The objects being studied should move fairly slowly--oscillations with periods of about 2 s work well, for example. Appropriate experiments, in which the motion is one dimensional, include slowly falling objects such as coffee filters, a mass on a long spring, a simple pendulum, or collisions of cars on an air track.

I have written a HyperCard stack which allows the students to observe the experiment in a "live video" window, crop the window to the region of interest, record the movie, and analyze it. In addition to cropping, they are able to control brightness, frame rate, and the length of time for the movies (typically about 5 seconds). They make position versus time measurements by clicking on the moving object in successive frames of the movie. These numbers are then copied into a graphing program, such as Vernier's Graphical Analysis, for further analysis. The experiments are easy to set up and quick to perform, typically taking no more than an hour to complete.

Results for three experiments using the QuickCam and HyperCard stack are given here. They show that, with appropriate cropping, frame rate, and brightness settings, the timing intervals are quite uniform. The smoothness of the resulting graphs indicates that the timing information is sufficiently accurate for good quantitative results. This is true even for experiments performed on a "minimum" system: a Macintosh Performa 400 (equivalent to a Macintosh LCII) running with only 4 MB of memory.

The cropped QuickTime movies may be analyzed using VideoPoint (available from PASCO Scientific) or VideoGraph (available from Physics Academic Software) to take advantage of some of the special features provided by these video analysis tools. The QuickMovie software included with the QuickCam camera may also be used to produce movies with appropriate brightness, timing, and cropping settings.

This method of video analysis, because it is limited to observation of objects moving slowly in one dimension, excludes two important types of experiments: projectile motion, and the physical motion of the students themselves. Its value is in the ability to add video analysis capability to computers at a fraction of the cost of other systems, and to provide the means for planning, running and analyzing an experiment in a short time.

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