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| THIN
FILM COATINGS |
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| Thin
film coatings play a very important part in any optical system. An
uncoated
glass surface will reflect or transmit a percentage of the light
depending on its refractive index and the angle of incidence. |
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| Thin
film coatings change the
amount of light reflected or transmitted through the optics. These are
very thin layers (of the order of Angstroms)
of materials coated on to the substrate, hence the term "Thin
Films". These can be single layers or multiple layers coated
on top of one another to achieve the required reflection /
transmission. Oxides, Fluorides, metals are used for this purpose.
The evaporation has to be carried out in special coating machines
developed for this purpose and under high vacuum. |
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whole process of coating is
quite complicated and requires continual development. The coatings are
designed using special software and
then stabilized on the coating machine to get performance as per
the design. |
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| Astro
Optics has a two "box coater"
units from Leybold,
Germany,
equipped with turbo molecular pump, e-beam evaporator, thermal
evaporator, radiation heaters and Ion source. Starting with coating
designs and processes purchased from Leybold in 1988, we have learnt
and developed all coatings in-house over the years. We design our
coatings
using "Essential
Macleod" from Thin
Film Center, USA. With years of experience
in this field, we have a vast range of coatings, mainly developed
as per customer requirements. All the coatings are checked for
performance on our Perkin Elmer spectrophotometers. |
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are able to do coatings in the wavelength range 190-2500nm. Our
coatings can withstand adhesion
/ abrasion tests as per MIL 675C / MIL
13830B. The cleaning of the optics prior to coating
is done in Class 10,000 clean room area and the final checking
and putting the pieces in the coating jigs is done on class 100
clean room tables. Each piece is individually inspected for
defects prior to coating. |
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| TYPES
OF THIN FILM CoatingS: |
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Anti - reflection coatings |
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All
glass surfaces have some reflection depending on their refractive
index. N-BK7 glass which is most widely used, has a refractive index of
about 1.51 in the visible region of light, and reflects an average of
4.25% of the light incident on it. Materials with higher index reflect
a greater percentage of light. Anti-reflection coatings (or ARC's) are
used to reduce the
reflection from the surface of the optics and thus increase the
transmission. These coatings can be of single layer or multi-layer
depending on the material to be coated and application. |
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Click here
for detailed description. |
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Reflective
coatings |
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Some applications
require the substrate to be coated so that large amount of the incident
light or a
specific amount is reflected. These are high reflection or partial
reflection
coatings. These can be either metal coated or dielectric coated. |
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Click here for detailed description. |
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Beam splitter coatings |
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Beam splitters are used to split the incident
beam in two parts. These are used at an oblique incidence, usually
45°. These can be designed
for either of the two polarization states or for the mean polarization.
The beam is split in required R / T ratio. |
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Neutral
Density filter coatings |
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Coated
Neutral density filters are spectrally more flat than absorbing
type filters. They have front surface coating of Inconel or Nichrome.
Available in different optical densities and can be grouped together to
get diffrent densities. |
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Dichroic
filter coatings |
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These
coatings
seperate two wavelength regions, reflecting one and transmitting the
other. Can be used for normal incidence or 45° incidence. |
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