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代写英国硕士论文-HAROLD EDGERTON IN WORLD WAR II( The Structure of E(6)

时间:2012-05-23 14:02来源:未知 编辑:留学生作业 点击:
and control the delay between the camera and flash. Thus, the wartime innovations in electronic flash development for reconnaissance were a mere extension in scale of pre-war flash equipment. However,

and control the delay between the camera and
flash.
Thus, the wartime innovations in electronic flash
development for reconnaissance were a mere
extension in scale of pre-war flash equipment.
However, this work was not trivial, and
Edgerton抯 wartime efforts included scaling his
electronic flash components to be able to produce
the power necessary to generate beams of high
enough intensity to illuminate the ground below
and optimization of the overall system design.
SYSTEM DESIGN
While the basic electronic flash
technology remained similar to
that of the smaller pre-war units,
the design of the aerial flash unit
and its layout within the
reconnaissance aircraft were
developed to accommodate the
specific constraints of aerial
photography.
Figure 20 shows a high-level
circuit diagram of one of the
flash systems documented by
Edgerton in November 1943.
Similar to the basic strobe circuit
in Figure 5, the main idea is
charging a large capacitor and
releasing the power suddenly
into the flashtube. As an
addition, in the large aerial flash
system, the 揙N?switch prepares
the light filaments of the bulbs,
warms them up for operation,
and starts the flash bulb cooling
system that is composed of a
37 Edgerton, Electronic Flash, Strobe, 113.
38 Harold Edgerton Papers, Box 77 Folder 5.
Figure 18: A Pre-War Kodatron Flash Unit37
Figure 19: A D-3 Flash Unit38
18
simple air blower. The cooling system is necessary since the sizes of the flash units are large and
exposed to high levels of energy. This preparation is usually done on the ground prior to take off
since it draws very little current.
Once at the desired altitude, the pilot simply
pushes the 揚HOTOGRAPH?button to charge up
the capacitors. This switch connects the battery
to the power inverters where DC current is
changed to AC current. As soon as the voltage
across the capacitor reaches 4,000 volts, a relay
triggers the camera shutter, which in turn
triggers the firing of the flash. While the
揚HOTOGRAPH?button is depressed, this series
of events continues with a delay determined by
the charging speed of the capacitors.
The placement of the flash unit was another
aspect of the system architecture design. When
Edgerton worked with stand-alone stroboscope
units in his pre-war research, this was not an
issue, but now the whole unit had to be
embedded in the plane. Bombers including the
A-20 and the B-24 were modified so that both
the flash unit and the condensers housing the
large energy-storing capacitors could be located
in the aircraft抯 bomb bays. This was the logical
location for these components since the bomb
bays afforded the most space in the airplane for
the bulky condensers and its doors provided a
way to install the flash within the plane while
giving it an opening to the outside world. Given the condensers?placement in the bomb bay, the
camera was placed at either the nose or tail of the plane to provide maximum separation from the
flash unit in order to minimize the loss of contrast caused by placing a flash and camera in close
proximity to one another.
In a letter dated September 10, 1943, Edgerton relates to Colonel Baisley his findings through his
trials with one of the pilots in an A-20, suggesting two possible installations methods:
It seems possible from the available data that the flash unit for the A-20 can be modified
to be used on the bomb shackles in the rear bay beneath the tank since the tank is in the
upper portion of the bomb bay. 40
Figure 21 shows one of Edgerton抯 sketches of the installation.
39 Harold Edgerton, Description and Operating Manual Lamp-Electric Flash Type D-2 Serial No. 106 For Use in an
A-20 Airplane. (MIT, December 1943) Harold Edgerton Papers, Box 77 Folder 4.
40 Harold Edgerton Papers, Box 78 Folder 2.
Figure 20: High-Level Circuit Design for D-2 Flash39
19
The circuit and the layout design considerations were
very technical and experimental in nature. The theory
behind the requirements of the circuit and the size of
the system was derived from basic electronic flash
theory, which determines the parameters of the system.
THEORY FOR FLASH LIGHT SOURCES
The challenge facing Edgerton was the need for
increased light energy output of the flash equipment.
The output of a flash, measured in candlepower, is
integrated over the effective flash duration to determine
the total output, or CPS, measured in candle-powerseconds.
The required total output in nighttime aerial
photography is determined by two factors:
?The altitude at which the plane is flying, measured in feet. This determines the
distance between the flash and the subject.
?The lens aperture used in the camera, which determines how much light the lens, lets
in. This value, measured in f-stops, is the focal length of the lens divided by the lensopening
diameter. The aperture value becomes larger as the diameter decreases, and
less light is allowed to reach the film. This affects the flying altitudes where larger
aperture values call for being closer to the ground in order to capture enough light to
expose the film.
The combined effect of these two elements is called the Guide Factor (Equation 1, below). The
Guide Factor is proportional to the amount of light energy needed to be output by the flash
adjusted by the effect of the reflector (Equation 2). This relationship is due to the inverse square
brightness law (Figure 22), which states that the area covered by a light source increase as the
square of distance from the light source. As a result, the intensity of light per unit area decreases
as a square of distance. The light energy output (CPS) is directly related to the electrical energy
(EElec) provided to the flash bulb and the bulb抯 efficiency in transforming the electrical energy to
light energy (Equation 3).
41 Harold Edgerton Papers, Box 78 Folder 2.
Figure 21: Placement of a Flash Unit in a
Plane41
Figure 22: The Inverse Square Brightness Law42
20
Edgerton used the above calculations to determine the circuit requirements that would provide
the electrical energy for the bulb.
Edgerton also had to consider timing of the energy build-up and discharge to ensure short flash
duration at high speeds in order to avoid blurring of images.43 These calculations are determined
by changes to the capacitors and resistors that determine the time constant of a circuit.
TECHNICAL ISSUES
Despite the advantages of flash equipment for aerial
photography, electronic-flash systems were not without
drawbacks. The weight of the system, the lack of shadows in
pictures taken with electronic flash, and possible fogging of the
film were the main disadvantages in Edgerton抯 technology.
As explained in the section Theory for Flash Light Sources, for
a plane operating at higher altitudes, the intensity of the flash
required illuminating the target increases quadratically as
altitude increases. Since the power of an electric flash is
directly proportional to the capacitance of the circuit, operation
at higher altitudes requires the number of condenser units
(Figure 23) containing the capacitors to also increase
quadratically. Because the weight of the condensers dominates
the equipment, Edgerton noted 搕he weight of the flash
equipment increases as the square of the altitude at which
photographs are made.?5 Edgerton抯 initial calculations showed that the equipment required for
the operation of the flash at altitudes suitable for reconnaissance flights would weigh several
tons. However, while he initially saw this as a limiting constraint,
Goddard was completely unimpressed by the very considerable weight of the proposed
nocturnal photoflash equipment. He knew that the new airplanes, then on the drawing
board, would have no trouble carrying such heavy equipment.46
43 Harold Edgerton, 揟heory and Application of Electronic Flash.?Harold Edgerton Papers, Box 107 Folder 10.
44 Harold Edgerton Papers, Box 136.
45 Harold Edgerton, 揘ight Aerial Photography?Technology Review March 1947: 273-278.
46 Edgerton, Electronic Flash, Strobe, 289.
Figure 23: Condenser Banks44
EElec = ?Capacitance ?Voltage2
1) Guide Factor = Distance ?Aperture
2) Distance ?Aperture ?CPS ?Reflector Factor
3) CPS ?EElec ?Bulb Efficiency
21
Another issue in the use of electronic flash units in nighttime aerial photography involved the
amount of separation in distance between the flash bulb and the camera itself. A light source
placed too closely to the camera would result in 揵ack-scattered light from the source caus[ing]
fog on the film, which reduces contrast.?7 Similarly, a light source placed in close proximity to
the camera resulted in a picture without shadows. Shadows were useful in determining the
height of objects. Flash bombs were superior to the electronic flash in this regard, as the
separation they provided after being dropped from the plane was enough to produce distinctly
shadowed objects. The inability of the electronic flash units to produce shadows in the
photographs was unavoidable due to the self-contained nature of the equipment and the limited
dimensions of the airplanes. Table 1 summarizes the advantages and disadvantages of the flash


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