High Speed Photography 101
This paper was presented at the XX International Congress on High Speed
Photography and Photonics held in Santa Fe, NM from Oct. 26-Nov. 1, 1996.
Abstract
This paper describes the contents of a unique, introductory, applications
oriented, high speed photography course offered to Imaging and Photographic
Technology majors at the Rochester Institute of Technology. The course covers
the theory and practice of photographic systems designed to permit analysis of
events of very short duration. Included are operational characteristics of
intermittent and rotating prism cameras, rotating mirror and drum cameras,
synchronization systems and timing controls and high speed flash and
stroboscopic systems, and high speed video recording. Students gain basic
experience not only in the use of fundamental equipment but also in proper
planning, set-up and introductory data reduction techniques through a series of
practical experiments.
Course Format
This 10 week course meets for 6 hours per week with two hours dedicated to an
general lecture period where the theory and application of various high speed
cameras, recording schemes and imaging devices are presented. This is followed
by a 4 hour laboratory where specialized or improvised instruments
demonstrating the lecture material are put to practical use. The primary
objective of these lectures and hand-on experiments is for the students to gain
an appreciation for the hardware, the applications and data reduction
procedures associated with basic high speed techniques and to report their
findings in professional quality technical reports.
Topics presented
1. The topic of high speed photography is introduced in terms of basic concepts
associated with the measurement of time. This is accomplished by first
discussing principles of measurements in general and measurements of time in
particular along with and basic calibration concepts. To make the topic
relevant to the student's background, the design and operation of leaf and
focal plane shutters is presented, focal plane shutter distortion, the moving
subject as its own shutter, accounting for magnification and signal-to-noise
considerations are topics covered in the lecture portion of the week. During
the weekly laboratory students calibrate their camera's shutter basing their
work on audio and video standards as the tools of measurement supplemented
with a commercial shutter tester.
2. In the next project the application of calibrated timing devices (the camera
shutters calibrated the previous week) to solving unknown subject rate of
motion problems is presented. The advantages associated with purposeful
introduction of blur in photographs is discussed. Introduction of scales in the
subject and magnification are addressed in the context of making photographs
"magnification" independent. Also, conflics between image size, measurement
accuracy and synchronization of event with available exposure time are raised.
During the laboratory portion the students photograph a car moving at various
rates to check speedometer accuracy and also determine the falling and rotation
rate of a maple or oak seed (propeller seed) with nothing more complex than a
regular (calibrated) camera shutter and the introduction of deliberate blur
into the photographs.
3. Concepts of sharpness and image resolution, control of subject motion induced
image blur are reintroduced in relationship to photographing ballistic
and splash phenomena. In the lecture portion of the project, basic theory behind
electronic flashes and spark gap light sources, their design and operating
parameters, synchronization devices for light, dark or sound activation plus
control of delay are covered.
In the hands-on portion of the project the
students become acquainted with EG&G Microflash spark gap light sources and apply
them in a group setting to photograph the impact of .22 caliber supersonic
bullets on various soft subjects such as vegetables, etc. Individually they use
their own, powered down, electronic flashes to photograph of splashes with dark
activated synchronizers.
4. The theory of animation, the motion picture camera and the video camera are
discussed in lecture, demonstration and laboratory format over a period of two
weeks. Time magnification is presented as it relates to both time-lapse, video
and also high speed motion picture and video photography. Design and operating
characteristics of high speed intermittent and rotating prism cameras,
comparisons between available photographic, videotape and solid-state or
digital instruments, voltage vs. framing rate vs. expended film and elapsed
time charts are presented, use of timing lights and event synchronizers,
establishment of appropriate lighting levels and power requirements plus safety
concerns are covered. Students use high speed cameras such as Fastax, Hycam,
Nova, Photosonics, etc. and make a "qualitative" film, and a high speed
videotape using a Kodak Ektapro 1000, from which they also are required to make
a quantitative analysis of subject behavior.
5. The photographic strip-chart recorder or streak camera is discussed as a
potentially useful yet highly underrated analytical tool for precision
measurements of elapsed time, simultaneity, velocity and acceleration,
frequency, luminance, etc. Streak photography and velocity recording cameras
are covered with particular reference to rotating drum and rotating mirror high
speed streak cameras and advantages/disadvantages of all camera types are
covered. Capping shutters, synchronization characteristics, "always alert",
multi-exposure and color systems are mentioned. In the laboratory, students use
a simple rotating drum camera to determine the velocity of small gauge rifle
bullets of two different calibers. These are fired in succession from a single
gun in close vicinity to markers in subject space located a known distance
apart.
6. The potential of using a mechanical stroboscope or repeating flash as an
industrial visualization or measurement tool is introduced in a general lecture
on such techniques. The pioneering work of "Doc" Edgerton is an integral part
of this lecture including a demonstration of a "piddler" slowed down with
electronic as well as mechanical stroboscopes. Advantages and disadvantages of
the stroboscope as a motion analysis tool are covered and applications of
multiple images on stationary as well as moving film are described and
illustrated. The "problem" of the _dark_ interval between flashes is given
particular emphasis.
The application portion of this technique is demonstrated in an experiment
based on moving-film stroboscopy (students use their own cameras, rewinding the
film with the shutters locked open while the action takes place) where the
subject is a sewing machine operated at various (unkown to students) rates and
to the needle of which a small incandescent lamp is attached to positively
track the motion of the needle over time.
7. Requirements, design, construction and operating principles behind various
ultra high speed cameras is presented by way of a lecture. Included are
rotating drum, rotating mirror framing cameras, image dissection techniques and
application of image tubes for recording at 1 frame per microsecond and
faster. Students work with a rotating drum Dynafax high speed camera to record
the passage of a supersonic bullet across a 12 inch diameter Schlieren field
and from the record they determine the velocity of the bullet.
8. Finally, expanding on the streak camera, strip photography and line-scan
electronic imaging devices are introduced as a high speed imaging solution
especially as applied photofinish and synchroballistic photography. The
students apply this technique to determine the velocity of self-built model
rockets launched in a laboratory-simulated, photo-instrumented, ballistic range
using a rotating drum strip camera equipped with a capping shutter installed in
the "range" so that it opens just as the rocket's image is about to reach the
camera's slit and which closes shortly after it passes the slit so as to
prevent rewrite that would be caused by the image of the exhaust overwriting
the image of the rocket recorded in a previous rotation of the drum.
Grades and examinations
There is a midterm exam intended to familiarize students with the course
subject matter and the style of questions that are associated with the course.
It is simply given as an exercise. In addition, there is the opportunity for
students to design and carry out one independent project expanding on any topic
included in the course or to investigate high speed topics not covered.
Finally, the course grade is determined on the basis of the completion in a
satisfactory manner of class experiments substantiated by the submission of
written project reports. The final written exam, covering all the topics of
the course, is offered to all students but is not required to pass the course
if the reports are of substantial quality. However, the highest course grade
can _only_ be earned by opting to take the final exam and turning in an above
average performance on this exam.
Conclusion
Students in the Imaging and Photographic Technology program of study at the
Rochester Institute of Technology complete an introductory course in high speed
photography and photonics which provides them with a general overview of
various systems, techniques and instruments associated with the field. It is
recognized that not many will become practicing professionals but the
experiences gained in solving problems associated with high speed events,
handling simple to sophisticated imaging devices, writing complete technical
reports, presenting their findings and experiences to an audience, etc. add a
dimension to their studies that serves them well in most any technical imaging
field they eventually enter.
Bibliography
Davidhazy, Andrew; Tech Directions Magazine, "There is more to a blur than meets
the eye!", Vol. 52, No. 6, January 1993, pp. 13-15.
Davidhazy, Andrew; 20th International Congress High Speed Photography and
Photonics, SPIE Proceedings, "Some stroboscopic spinoffs in Photographic
Technology", 1992, Vol. 1801, pp. 672-676.
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