Download WINDOWS During the Apollo (manned lunar exploration) space

WINDOWS
During the Apollo (manned lunar exploration) space program, thousands of
pictures were taken from inside the space vehicle of objects and phenomena
on the moon and in outer space. These pictures constituted basic scientific
data. From them, objects were identified, topography was mapped, courses
were charted, phenomena were observed and records were maintained.
The accuracy and validity of the data were, among other things, dependent
on the windows’ ability to transmit images from outside the spacecraft to the
inside for photographic recording without significant distortion. These
windows also had to help to maintain a positive atmosphere inside the
spacecraft. They had to prevent excessive solar radiation from entering and
heating the vehicle, and withstand the intense heat and shock met on reentry
to the earth’s atmosphere.
This example of an optical window demonstrates that windows are precision
optical components that must be selected, specified, and designed prior to
construction and usage. Optically, a window should appear as though it were
not there. Windows most commonly are used in these two classes of
applications:
1. To retain a specific gas pressure or a partial vacuum in a tube or other
enclosure.
2. To prevent dust, moisture or other particles from entering the system.
A window could be deemed as a pane of glass or other transparent material
used to transmit light with negligible distortion and to isolate the
atmospheric and thermal environments on either side.
One common example of windows used in a laser is shown in Figure 4.
Here, the Brewster windows are the ends of a plasma tube for a helium-neon
laser. The intracavity laser beam must pass through each of these Brewster
windows twice during each round trip through the laser cavity. This is a
very low-gain laser. That means that, if each Brewster window caused as
much as one percent loss per pass to the laser beam, the laser would not
operate.
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Fig. 4
Brewster Windows on plasma tube of helium-neon laser
Another application of windows in an electro-optical system is shown in
Figure 5 Here, the window protects the output optics of a laser metal-cutting
instrument from splattered, molten metal. The window is cleaned or
replaced periodically.
Fig. 5
Window used to protect output optics in a laser metal-cutting operation
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The optical quality of a "perfect" window would be such that it would have
no effect on a parallel, plane wavefront of an optical beam passing through
it. Based on this "as if it weren’t there" concept, we can list the C desirable
characteristics of an optical window.
1. Low absorption of transmitted light
2. Low reflection of light incident on the surfaces of the window
3. Minimum distortion of the transmitted beam due to imperfections in the
optical material or the surface finish
Low Absorption
You can achieve low absorption in a window by selecting the window
material properly and by minimizing the window thickness. Light
absorption in an optical material is dependent on wavelength of the light
intensity, absorption characteristics of the particular material at that
wavelength, and thickness of the material. Look at the window model in
Figure 3. The relationship between incident light irradiance, Eo, and
transmitted light, E, is given by:
Equation 1
where:
a = Absorption coefficient of the material at a
particular wavelength.
x = Thickness of the material.
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Fig. 6
Model of an optical window
The ratio of transmitted to incident light is the transmissivity of the window
(or percent transmission if it’s multiplied by 100%). You can rearrange
Equation 1 to calculate percent transmission in the following way:
Equation 2
Percent Transmission =
× 100% =
Low Reflection
If a ray of light of any polarization strikes normal (perpendicular) to an
interface (junction) between two optical materials of dissimilar indices of
refraction (n1 and n2), a fraction of the incident ray will be reflected
according to the relationship:
Equation 3
% Reflection =
This type of reflection sometimes is called "Fresnel reflection." (See Figure
7.)
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Fig. 7
Diagram of Fresnel reflection
For parallel rays of visible light passing through a window, one medium
probably will be air (n1 = 1.0). The other probably will be some other glass
(n2 ~ 1.5). Using Equation 3, you can calculate a reflection of ~ 4%. This
calculation is for one side of the window. When the rays emerge from the
other side, another 4% reflection loss is encountered. In summary, we
realize that a common window pane (example, glass) with air on either side
would reflect approximately 8% of the light incident normal to the surface.
For example, you see a faint image of your face when you look out a
window.
Optical theory shows that, if multiple reflections within a window are taken
into account—as well as the first-surface reflection—the total theoretical
external transmittance is given by:
Equation
4
=
where:
So the total reflectance Rtot is 1 minus Ttot, or
5
Equation
5
=1–
where:
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