PHYS 351
Augustana College
Winter 2010-2011
Observation of Very Fast Events and Changes in Velocity Using High-speed Photography
Derick Peterson, Mitchell Anliker, Steve Ash, Augustana College, Rock Island, Illinois, USA
INTRODUCTION
High-speed Photo
In this experiment we examine two different methods of high-speed photography in
observing the motion of some event. The first method we studied involved using a high
exposure time on a digital camera together with high-speed flashes triggered by a microphone
signal. When the aperture on the camera is open during the course of some unilluminated
event, any flashes of light would result in a single frame of the event exposed on the resulting
picture. This is called a high-speed photo. Thus, if two flashes occurred the image would show
two superimposed images at two different points in time. Because the time of each flash is
dependent upon activation by a microphone signal, in this way we can take advantage of the
relatively high velocity of sound to observe two points in an event that occurs more quickly
than the human eye can normally see, as long as the event is associated with an audible sound.
The high-speed can observe very quick events because the flashes only light for an extremely
short period. This produces a highly resolved (short time, so less blurry) image of each point in
an event. Such an example is the tearing of a popping balloon, as we examined in our
experiment. If we are able to determine the time of each flash and the change in some quantity
(such as position) of the relevant event, we can thus calculate the rate of change of this
quantity. We can only use this method if the rate of motion of the changing quantity in an
event (e.g. the tearing of a balloon) is constant. If the rate of motion were not constant or linear
then this method would not be sufficient to properly analyze the event, and a different method
would be used.
Strobe Light
If the event of interest features some quantity whose rate of change is not constant,
more event frames, and data points, are needed. Using the same principle of multiple
exposures on a picture, we can produce a many-frame photo of some event by using a strobe
light to provide a constant rate of exposure. The strobe light is limited to producing a constant
rate of frames, just as the high-speed flash method is limited to producing only two frames.
Using this method, we can observe the changing rate of some event at a constant rate. By
adjusting the speed of the strobe light we can obtain the time between frames, and if we can
somehow measure the position of some event at each frame we can find the velocity between
PHYS 351
Augustana College
Winter 2010-2011
each successive frame as we did with the high-speed flash. With these many values of velocity
at each point we can thus find the change in velocity over time, thus calculating acceleration.
This method would be used to observe some event which progressed at a changing rate. The
limiting factor in this is the ‘speed’ of the flash of the strobe light. In contrast to the high-speed
flash method, the strobe light does not have a short illumination time, so the resolution is
decreased. Thus the strobe light is better suited for observation of relatively slow events which
occur at changing rates.
Procedure and Conclusions
1. High-speed photo
High-speed photography was used to analyze the motion of the tear in a popping balloon.
Our experiment found that a high-speed photo is a capable tool for observing the motion of an
event that progresses much quicker than the human eye can track, such as the tearing of a
popped balloon. The primary difficulty lies in finding the correct setup to capture two points of
interest in the high-speed event. A high enough data acquisition rate is also needed to properly
resolve the two points and an appropriate scale is needed in the photo in order to track the
change in the progressing event. The experimental apparatus is shown below in Figure 1.
Procedure
1. A well-inflated balloon was placed on a table against a black backdrop
2. A meter stick was placed horizontally directly behind the balloon
3. Two high-speed flashes were placed at different positions, so that they did not directly
face the balloon
PHYS 351
Augustana College
Winter 2010-2011
4. Each flash was connected to a microphone via a boom box and a silicon controlled
rectifier (SCR) (see Figure 2. below)
5. The microphones were taped onto the table directly in front on the balloon, in a line,
13.0cm apart
6. A photocell on each flash was set to quench the flash when a certain amount of
reflected light is received
7. The volume on the boom box was used to adjust the sensitivity of the microphones
8. Science Workshop was set to record light intensity over time, with a sampling rate of
10000Hz. The light sensor was set to a gain of 1.
9. The aperture time on the digital camera was set to 3 seconds, and a picture was taken
10. While the balloon was held at one end, a pin was used to pop the balloon at the other,
roughly one second after the camera button was pushed
11. The resulting photo was used along with the light intensity times to calculate change in
position of the rip from point A to point B.
Justification of procedure
1. The black backdrop was used to eliminate any objects behind the balloon, thus resulting
in a much clearer picture of the balloon, which could be more easily analyzed. The
balloon was well-inflated to a relatively constant size (as well as could be estimated with
our limited equipment) to reduce any inconsistencies between trials, in order to more
easily arrange an optimal setup.
2. The meter stick was placed as close to the balloon as possible without touching it. This
was to give the best estimate of position of the tear in each frame. Ideally the setup
would have a meter stick directly along the line of the tear, but the randomness of
tearing makes this difficult and we estimated that any loss of accuracy in our setup due
to meter stick position was insignificant. The meter stick itself was black with white
markings, so as to more clearly show up in the photos.
PHYS 351
Augustana College
Winter 2010-2011
3. The flashes were not pointed directly at the balloon because we found that doing so
would result in large glares that would obscure the image and hinder analysis. We found
that the variable illumination of the flashes was problematic, because it resulted in two
differently illuminated frames. Often parts of the image would be much less visible in
one frame versus another, such as parts of the meter stick being overexposed, which
prevented us from taking measurements. Other times the tear on the balloon would not
be entirely visible, preventing us from taking data. Ideally the ‘flashes’ would be highintensity and widely spread. This would allow for uniformly exposed frames, with
enough light to illuminate the whole picture.
4. The boom box was used to relay the microphone signal to the SCR, and then to the
flash. When the SCR received a signal from the microphone, it shorted the electrodes of
the flash unit, triggering the flash. In this way the pop sound from the balloon translated
to a flash whenever the sound waves hit each of the microphones.
5. Various positions of the microphones were tried in order to produce a suitable period of
separation between each frame of the picture. The actual position relative to the
balloon was not numerically important, as the separation translated into time
separation in the light intensity peaks on science workshop. Even so, we chose to
position the microphones in a line direct from the balloon in order to simplify
adjustments. We finally settled on 13.0cm apart as a suitable distance for resolving two
different points in the tear of a balloon.
6. The photocell was pre-set and required no modification by us, but it served to make the
flash ‘quick’ – to have the period of illumination be very short so as to achieve the high
resolution required to properly distinguish two frames of the quickly-progressing
balloon tear.
7. The boom box also served to adjust the sensitivity of the microphones, which enabled
us to talk and move at normal noise levels without setting off the flashes, while leaving
enough sensitivity that the balloon pop would still set off each balloon.
8. Due to the high rate of motion of the balloon tear, many settings were tested on Science
Workshop in order to resolve the short illumination of each flash. The sampling rate was
eventually raised from 1000Hz to 10000Hz, which was able to resolve a distinct spike in
intensity at each flash. Uncertainty in these measurements were taken to be ½ of that,
or .05 ms. However, the limitations of either the computer or the program itself would
cause the memory to fill up after a very short period of time (2-3 seconds), so the
recording had to be started almost immediately before the balloon was popped in order
to have enough time and memory to get to the flashes. The gain on the light sensor had
to be set to 1, or the peaks from the flashes would get cut off. This part of the
experiment could be improved by using a more sophisticated data acquisition program
and a more powerful computer.
PHYS 351
Augustana College
Winter 2010-2011
9. The aperture on the digital camera was set to 3 seconds, so that there was enough time
to easily start recording data and pop the balloon in that period of time
10. The balloon was held in place so that it could be easily popped. The time lapse of a
second between pushing the camera button and popping the balloon was used because
it was found that the camera did not actually open the aperture until a little less than a
second after the button was pushed
11. We took many photos and went through many balloons in order to get a good doubleexposed picture of the tearing balloon. The change in position was acquired from the
meter stick, with uncertainty of +/- .25cm. The velocity of the tear was computed using
the distance equation, v = d/t
2. Strobe light
A strobe light was used to analyze the change in motion of a cart rolling down a ramp. Our
experiment found that using a strobe light is a capable tool to capture the movement of some
object with a changing velocity, such as a cart rolling down a ramp. The primary difficulty in this
setup lies in being able to properly identify the position of the object at each time. A good
solution is to use a small highly-reflective marker such as a line made from neon paint. This
would allow for easy comparison between frames, limit uncertainty from semi-visible points,
and allow the frames along the edges to be better resolved, increasing the amount of data that
can be taken. Additionally, as in the high-speed photography setup, a strong, wide-spread light
source would increase visibility and improve conditions for taking data. The experimental
apparatus is shown below in Figure 3.
Figure 3. Experimental setup for strobe light
PHYS 351
Augustana College
Winter 2010-2011
Procedure
1.
2.
3.
4.
5.
A track was placed at an incline on a table against a black backdrop
The strobe light was positioned directly above the camera
The strobe light was set to 1024 flashes/minute
A low-friction cart was placed on the top of the track
The aperture time on the digital camera was set to 3 seconds, and a picture was taken,
and the cart was released
6. The final resulting photo was used to observe the change in position of the cart over
time, and using that data and the known period between frames the velocity at each
point was calculated and used to obtain the average acceleration of the cart
Justification of procedure
1. Once again, the backdrop was used to eliminate any objects in the background of the
picture to ease analysis of the photo. We initially just tried rolling the cart along the
surface of the table, and observing its motion. However, we soon realized that the cart
did not change its velocity very much during the course of its motion, so we chose to
create a setup which would provide the cart with a force for acceleration: the inclined
track. The height, length, or angle of the track is not of relevant importance for our
experiment, as the only thing we needed to observe was the change in position of the
cart in its setup.
2. The strobe light proved to be even more problematic than the high-speed flashes in
terms of providing sufficient spread-out illumination. Because there was only one light
source, the photos invariably lacked sufficient illumination on all ends. This was an even
greater issue than in the popping of a balloon, because we were trying to observe the
motion of the cart across the entire table, so we needed to be able to see the cart and
the meter stick at every point along its motion. In order to better track the motion of
the cart through the many frames, we eventually placed a blue piece of tape on the
front end of the cart to provide a more noticeable reference point.
3. We tried various different settings on the strobe light in order to get sufficient data for
analysis. Originally we thought we were constrained to lower frequencies (300-400Hz)
that resulted in 4-5 cart images on the photo, in order to get separation between the
cart images so that we could easily observe their position. At higher frequencies the
images of the cart would overlap, and it became difficult to determine where the front
of the cart actually was. This issue was solved using the blue tape, as it provided a
definite line which we could identify more easily in each frame. Using this method we
were able to obtain many data points, which allowed us to produce a more accurate
PHYS 351
Augustana College
Winter 2010-2011
graph and resulting acceleration. Ideally, to observe the behavior of some moving object
using this method, it would be best to use a highly-reflective marker, such as neon paint,
so that the position of the object can be clearly identified.
4. In selecting items to observe their motion, we settled on a low-friction cart, since it
would introduce less uncertainty in the propagation of motion of the object. Any bumps
or irregularities in surface contact could add an additional drag force, which would slow
down the cart and introduce systematic error. That being said, the cart was not entirely
ideal as there was still friction on the wheels and it would occasionally scrape up against
the side of the track. This could be a source of error in our experiment. Ideally, you
would observe the motion of a near-frictionless system, but the whole purpose of this
experiment is to observe changes in velocity in a system so changes due to friction could
be part of the observation. For example, if you were to observe a rolling ball down a
smooth slope, and suddenly the slope turned grassy this would cause a change in
velocity that could be observed and analyzed.
5. We kept the aperture time at 3 seconds only because this was a sufficient amount of
time to observe the cart rolling down the length of the track. This could change as
needed if we needed to observe a more slowly progressing event.
6. The photos of the moving cart initially proved difficult to examine, as stated in 3 above,
but was improved by using the tape. The lack of sufficient illumination prevented us
from making out the outer points well enough, so this limited the amount of data we
could obtain.
References:
1. An Introduction to Error Analysis, John R. Taylor, 2nd Ed. (1997).