Inexpensive Very High Speed Photography for Mechanics

1. Abstract Methods (continued) Results (continued) High-speed photography is a growing science whose roots go back to Harold Edgerton, who discovered an, until then, improbable use of stroboscopes to aid the capture of high-speed phenomena on film. His discoveries gave way to the development of high-speed cameras, which have come to be very expensive. Since high-speed photography can be a valuable asset in physics classrooms for evaluating high-speed phenomena, it would be extremely helpful to have a low-cost way of allowing students to take high-speed photographs. Loren Winters contributed to this study through the use of the Vivitar 283 flash unit, multiple sensors of his own design, and a regular film camera. In our study, we utilized a Vivitar 285 flash unit, Nikon D100, and The Time Machine, a device created by Brian Mumford. The Time Machine is a relatively inexpensive interface for a camera, flash unit and sensors that allows one to easily take photographs of high-speed events. The average duration of a flash from a flash unit being 1ms, we were able to achieve an average flash duration of 0.6µs. Some of the images we were able to capture were of breaking balloons, fired pellets in mid-flight, and bouncing balls. By: Melissa Chudyk, Steven Dutter, Steven Wilser chudme09@mail.buffalostate.edu, duttsj03@mail.buffalostate.edu, wilssm14@mail.buffalostate.edu Adviser: Dr. Dan L. MacIsaac Department of Physics Buffalo State College Funded by the Early Undergraduate Research Program at Buffalo State College Figure 7 One of the setups frequently utilized was for capturing photographs of shooting pellets. The setup consists of the Time Machine, a Delta air rifle clamped to a table, the ballistic sensor held up using an optics bench and lens holders, the Nikon D100, the Vivitar 285, and a backstop made of plywood, Styrofoam and cardboard (see Figure 19). Figure 17 A photograph of a tennis ball bouncing on a table. One can see the flattening of the ball against the table. The more the flattening of the ball, the greater the speed of the ball will be when it retracts. Figure 18 This photograph was attained by rolling a ping-pong ball off of a curved ramp at approximately 45° below the horizontal, and setting the amount of exposures at 10. One can clearly see the parabolic shape of the ball’s trajectory, and the steady decrease of height as the ball is influenced by gravity (with little sacrifice to horizontal speed except by drag). Introduction Discussion The reason why most cameras cannot capture photographs of high-speed events is that their components cannot keep up with the occurrence. Since many high-speed events occur on the order of microseconds, one needs a camera that can react just as quickly. There are cameras that can do so, but they are very expensive. It is possible, however, to take pictures of high-speed phenomena using cameras that are not designated as high-speed cameras. This can be achieved with The Time Machine1, a device by Brian Mumford that can control the shutter of most cameras, and the flash of any electronic flash unit. The Time Machine can trigger the closing of the camera’s shutter and the firing of a flash unit’s flash with a variety of triggering devices, in order to capture a high-speed event at the precise instant it occurs. Loren Winters is credited for his work involving obtaining high-speed photos using the Vivitar 283 and sensors similar to those that we have used2,3. The types of images we have obtained are: balloons filled with air, flour or water in mid-break; racquetballs, tennis balls and ping-pong balls at the instant of and throughout their bounce; pellets in mid-flight; as well as pellets shooting through water balloons. Having obtained decent photographs of relatively high-speed phenomena, we performed some calculations to determine and confirm the speeds we believed we were achieving. The ballistic sensor measured that the pellet being shot through it was traveling 649 ft/s across the four inches. Using LoggerPro, we took data regarding the amount of time separating the firing of the pellet and the landing of the pellet in the backstop with the use of a Vernier microphone, which detected the distinct sounds of the hammer in the gun striking the pellet, and the pellet hitting the plywood of the backstop. See Figure 19 for the placement of the microphone, and Figure 20 for relative distance from the gun to the backstop. Below are some calculations made on the subject of the pellet’s kinematics. Since common high-speed events can occur on the order of microseconds, and the average external flash unit achieves an average flash duration of 1ms, a quenching circuit was needed to shorten the duration of the flash. The need for a short flash results from the exposure time of photographs. The light sensor on the Vivitar 285 flash unit (and any other electronic flash unit) detects the amount of light being let off by the flash unit and, according to the amount of light needed for an exposure, will set a time for the flash to be shut off, using a variable resistor; the higher the resistance, the longer it will take for the flash to be shut off. Since we are trying to take pictures of high-speed events, it is necessary for the length of the flash to be as short as possible without forfeiting too much light intensity (as flash duration decreases, light intensity decreases). It is possible to control the flash duration of the Vivitar 285 manually, by removing the light detector, and placing a resistor in two of the vacant holes4 (see Figure 2). Figures 8-10 are three tests using different resistances in the Vivitar 285. Methods *-kΩ = kilo-ohms; a measure of resistance in semi-conductors. Figure 1 is a picture of The Time Machine, and Figure 2 is a picture of our basic setup we used for preparing and taking the pictures. Since The Time Machine includes many different modes and configurations, we were able to customize the parameters of each configuration and mode according to the type of picture we were trying to take. See Figures 4-6 for the configuration and modes we put into use. Figure 8 This photograph was taken with the resistor in the Vivitar 285 having the value of 1.0kΩ.* The image is mostly obscured by darkness, due to the flash intensity being too low. The image is rather crisp, however. Figure 9 This photograph was taken with the resistor in the Vivitar 285 having the value of 4.7kΩ.* The image is illuminated properly, and the image is sharp. Inexpensive Very High Speed Photography for Mechanics Figure 10 This photograph was taken with the resistor in the Vivitar 285 having the value of 10kΩ.* The image is obscured by glare, and what can be seen of the image is rather blurry. Results After trying different adjustments of distances, flash angles and intensities (amongst other modifications), we were able to acquire several good photographs of high-speed phenomena. Having determined the appropriate settings for each event, only slight modifications were needed to touch up the photos. The first pictures we were able to attain were of balloons being broken open. The microphone sensor used with the Time Machine on flash configuration and bulb-flash mode gave satisfying results, and only needed modifications in flash intensity to achieve a crisp picture (see Figures 9 and 15). The same settings were used for breaking balloons filled with flour (Figure 16) and breaking water balloons (Figure 13) with superior results. Figure 12 is a series of different instances of dropping a water balloon, with different set delays of the flash to be triggered to take the picture after the microphone sensor sensed the balloon hitting the table. Figure 18 is a multiple-exposure shot of a ping-pong ball having been slid off of a ramp; flash duration and object drop angle and height were all that were needed to achieve a decent 10-exposure photo of a bouncing ping-pong ball. Going with the theme of dropping objects, we dropped a tennis ball to observe the flattening of the ball as it hits the table, which can be examined in Figure 17. The ballistic sensor was used to produce photographs of shot pellets in mid-flight (Figure 14), with slight changes in flash intensity being the only modification. Figure 11, a photograph of a pellet being shot through a water balloon, was produced using the ballistic sensor and the light modifications used when breaking water balloons. Figure 1 The Time Machine, a device by Brian Mumford, allows a camera, flash and sensor to work in unison to capture photographs of high-speed phenomena. Figure 2 After removing the light detector from the Vivitar 285, one may place a resistor in the bottom and bottom-left holes to create a circuit that will quench the duration of the unit’s flash (the lower the resistance, the lower the flash intensity, and the shorter the flash duration). Figure 3 The pellets used for our high-speed endeavors were GAMO Match pellets (.177 cal). Figure 19 This is the setup of the microphone used to obtain data concerning the amount of time between the sound of the pellet being shot and hitting the backstop, which was used to determine average velocity and verify theoretical calculations. The way the Time Machine, Nikon D100 and Vivitar 285 were coordinated was by keeping the Time Machine configured to trigger the flash, and manually opening the shutter of the camera (the shutter speed was set to 3 seconds) while we executed the event, which was triggered by a sensor. The pictures were taken in complete darkness. The two sensors we employed during our study were the microphone sensor and the ballistic sensor. The microphone sensor is a microphone that responds to loud, sharp sounds (the sensitivity can be adjusted on The Time Machine). The ballistic sensor is comprised of two photogate sensors spaced four inches apart, and two infrared LEDs directly across from the photogate sensors, all in a tube, and connected to The Time Machine. When a projectile (a pellet in this case; see Figure 3) crosses the sensors, The Time Machine calculates the amount of time it took for the projectile to complete the four inches, then calculates and displays the velocity of the projectile. Using this measurement, The Time Machine can fire the flash of the electronic flash unit (or close the shutter of the camera, depending on the set configuration) at a distance set by the user (in 4 inch increments). Figure 20 Relative distance from the barrel of the gun to the backstop; used to calculate average velocity and time. In order to calculate the flash durations that we were achieving (after having used different valued quenching circuits in absence of the light detector in the Vivitar 285), we obtained a photo-detector and constructed a circuit meant to run the photo-detector. We then connected the circuit to an oscilloscope, and collected data while flashing the Vivitar 285 into the phototransistor (see Figure 21). The most common resistances we used were 4.7kΩ (for balloons) and 628Ω (for pellets), so we collected data for several executions using these resistances. We were able to obtain consistent data, and determined that: using the 4.7kΩ resistor, the flash duration was about 60ns (or 0.6µs); and, using the 628Ω resistor, the flash duration was less than 10ns. Figure 13 A photograph of a breaking water balloon. This was taken with a 4.7kΩ resistor in the Vivitar 285 (dubbed “Fuzzy Peach”). Figure 21 The setup for determining the duration of the flash using the quenching circuit values that was used most often for our pictures. The Time Machine - Used Modes The Time Machine - Used Configuration: Flash A website containing the information here as well as other projects done during our study has been under construction, and can be viewed for more information.5 Figure 4 The configuration used for all of the photographs thus far has been the Flashconfiguration. References Figure 11 A photograph of a pellet being shot through a water balloon. This shot is particularly interesting because of the turbulence in the water of the balloon, which closely imitates the shock waves that would be characteristic of a projectile traveling at a speed below the speed of sound5 (198m/s). Figure 14 A pellet in mid-flight5. This shot was taken with the ballistic sensor’s multiplier set at 4 inches. See Figure 3 for more details. 1Mumford, B. The Time Machine. Retrieved June 2, 2008, from The Time Machine Camera Controller/ Intervalometer Web site: http://www.bmumford.com/photo/camctlr.html 2Winters, L.M. (1990 May). High-speed flash photography with sound triggers. The Journal of the Acoustical Society of America , 87, Retrieved August 7, 2008, from http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JASMAN0000870000S1000S33000001&idtype=cvips&gifs=yes 3Winters, L.M> (1991 September). High-speed photography with computer control. The Physics Teacher, 29, Retrieved August 7, 2008, from http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PHTEAH000029000006000356000001&idtype=cvips&gifs=yes 4Document by Loren Winters sent by Brian Mumford (have yet to obtain proper reference). 5Nennstiel, R. (2005, March 13). How do bullets fly?. Retrieved May 26, 2008, from How do bullets fly? Web site: http://www.nennstiel-ruprecht.de/bullfly/index.htm 6http://physicsed.buffalostate.edu/EURP08 Settings per Type of Event Figure 12 Photographs of water balloons being bounced off of a table. These are all different events using the same water balloon, with the Time Machine set to trigger the flash after an a set delay after the triggered event (the balloon hitting the table). Top to bottom: 0.01s delay, 0.02s delay, 0.03s delay, 0.04s delay, and 0.05s delay. Figure 5 These are particular modes on the Time Machine that were put into use for the photographs that were taken. Figure 15 This is a picture of a balloon popping with a 4.7kΩ resistor in the flash, the flash angle at 45°, with a whiteboard directly overhead, reflecting the flash. Figure 16 A shot of a balloon filled with flour being broken open. This shot was taken using the Vivitar’s yellow filter in the light detector. Figure 6 After establishing the proper settings for the Time Machine that would be utilized, each selected mode was assigned for the type events they would be most fit for.