Carbon coated infra red reflectors

Reflectors of infra red radiation are used in infra red (thermal) imager systems to scan and reflect thermal radiation onto detectors. In this invention the highly reflecting surfaces of the reflectors, e.g. mirrors and rotating prisms, are coated with a thin infra red transparent layer of glassy diamond-like carbon. The carbon layer may be formed directly on the surface or an initial thin bonding layer, e.g. of silicon or germanium, may be deposited on the reflecting surface followed by the carbon layer. Both the bonding layer and carbon layer may be deposited by a glow discharge technique.

This invention relates to infra red (IR) reflectors or mirrors. 
Such reflectors are used in thermal imaging systems to sweep and reflect an 
image of a scene onto IR detectors. For maximum sensitivity, and hence 
range of observable scene, in thermal imagers it is essential that all 
reflecting surfaces reflect as much infra red as possible. A thin layer of 
silver when deposited provides a good reflector but unfortunately soon 
tarnishes and its reflective efficiency drops drastically. Another good 
reflective material is aluminium but again this tarnishes through surface 
oxidisation. 
One way of overcoming oxidisation is to coat the aluminium with a 0.15 
.mu.m thick layer of silicon oxide. This has a high reflectivity at near 
normal angles of incidence but a greatly reduced reflectivity at angles of 
incidence greater than about 40.degree., making SiO unsuitable for coating 
reflectors on thermal imagers. 
According to this invention an infra red reflector comprises a substrate 
having a highly reflective surface covered with a thin layer of infra red 
transmitting glassy diamond like carbon. 
The carbon layer may be deposited direct onto the reflector or onto a thin 
layer of infra red transmitting material, such as silicon or germanium, 
deposited on the reflecting surface. 
The reflecting surface may be a layer of reflecting material such as 
silver, aluminium, copper, or gold etc., deposited on a substrate of 
metal, glass or plastics material. Alternatively the reflecting surface 
may be the highly polished surface of an aluminium etc. substrate.

As seen in FIG. 1 an infra red reflector comprises a substrate 1 of glass 
which has a surface 2 cleaned and polished. Onto this cleaned surface 2 a 
thin layer of nickel chrome is evaporated followed by a layer 3 of 
aluminium in a well known manner typically to a thickness of 0.3 .mu.m. A 
thin layer 4, e.g. 0.1 .mu.m thick, of silicon or germanium is deposited 
onto the aluminium layer 3 to provide a good bond to the aluminium 3 and 
to a thin layer 5 of diamond like carbon having a thickness of 0.15 to 0.5 
.mu.m. 
Alternatively the carbon layer 5 may be deposited direct onto the aluminium 
layer 3. 
Both the layer 5 and 4 (if used) are substantially transparent to infra red 
radiation. The layer should be as thin as possible consistent with 
adequate physical protection. For example the layer 5 may be less than 0.5 
.mu.m providing no pin holes exist. However, to resist abrasion the carbon 
layer 5 may be thicker although too thick a layer will result in a high 
infra red absorption. 
FIG. 2 shows, schematically, a known thermal imager system comprising a 
telescope 10 having germanium lens 1 focussing the thermal image of a 
scene onto a rotating multi-faced drum 11 which causes a horizontal scan. 
From the drum 11 the IR is reflected onto a concave strip mirror 15 and 
onto a flapping mirror 12 which provides a vertical scan. From the 
flapping mirror 12 infra red is reflected through a lens 17 onto a 
detector array 13 whose output forms a signal which is applied in an 
amplifier 16 and then used to display the thermal image of the scene onto 
the screen of a television monitor 14. 
From FIG. 2 it is clear that the drum must be highly reflecting, at all 
angles of incidence. The tables 1-4 below give details of reflectivity for 
various surfaces at different wavelengths. In the tables: 
Rs is co-efficient of reflectivity perpendicular to the plane of incidence; 
R.sub.p is co-efficient of reflectivity parallel to the plane of incidence; 
0.degree., 45.degree., 60.degree. are angles of incidence. 
TABLE 1 
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Reflector aluminium (untarnished) 
AL 
Wavelength 0 45.degree. 60.degree. 
(.mu.m) R.sub.s,p 
R.sub.s 
R.sub.p 
R.sub.s 
R.sub.p 
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3 96.8 97.8 95.6 98.4 93.8 
3.5 97.0 97.9 95.8 98.5 94.2 
4.0 97.5 98.2 96.5 98.7 95.1 
4.5 97.4 98.2 96.4 98.7 94.9 
5.0 97.5 98.2 96.5 98.7 95.1 
8.0 97.9 98.5 97.1 99.0 95.9 
8.6 97.9 98.5 97.1 99.0 95.9 
9.0 97.9 98.5 97.1 99.0 95.9 
9.6 98.0 98.6 97.2 99.0 96.1 
10.0 98.0 98.6 97.2 99.0 96.1 
11.0 98.1 98.7 97.3 99.0 96.2 
12.0 98.2 98.7 97.4 99.1 96.4 
13.0 98.3 98.8 97.6 99.1 96.6 
14.0 98.3 98.8 97.7 99.2 96.7 
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TABLE 2 
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Reflector aluminium coated with SiO to a thickness of 0.15 .mu.m 
AL + SiO(t = 0.15 .mu.m) 
Wavelength 0 45.degree. 60.degree. 
(.mu.m) R.sub.s,p 
R.sub.p 
R.sub.p 
R.sub.s 
R.sub.p 
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8.0 97.9 98.5 97.0 98.9 95.9 
8.6 97.8 98.4 77.2 98.9 59.0 
9.0 97.7 98.4 78.0 98.8 61.0 
9.6 97.5 98.2 88.8 98.7 80.1 
10.0 97.2 98.0 91.9 98.6 86.2 
11.0 97.7 98.4 96.0 98.8 93.9 
12.0 98.1 98.6 96.6 99.0 94.8 
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TABLE 3 
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Reflector aluminium coated with carbon to a thickness of 0.5 .mu.m. 
AL + C(t = 0.5.mu.) 
Wavelength 0 45.degree. 60.degree. 
(.mu.m) R.sub.s,p 
R.sub.s 
R.sub.p 
R.sub.s 
R.sub.p 
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8.0 93.1 94.9 91.7 96.3 90.7 
8.6 94.1 95.7 92.5 96.9 91.3 
9.0 94.6 96.1 93.0 97.2 91.6 
9.6 95.3 96.6 93.6 97.5 92.1 
10.0 95.6 96.8 93.9 97.7 92.3 
11.0 96.2 97.3 94.6 98.1 92.9 
12.0 96.7 97.6 95.0 98.3 93.4 
13.0 97.1 97.9 95.5 98.5 93.8 
14.0 97.4 98.1 95.8 98.7 94.1 
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TABLE 4 
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Reflector aluminium coated with carbon to a thickness of 0.15 .mu.m 
AL + C(t = 0.15 .mu.m) 
Wavelength 0 45.degree. 60.degree. 
(.mu.m) R.sub.s,p 
R.sub.s 
R.sub.p 
R.sub.s 
R.sub.p 
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3 93.9 95.6 92.3 96.8 91.0 
3.5 95.1 96.5 93.6 97.5 92.0 
4.0 96.3 97.4 94.9 98.1 93.4 
4.5 96.5 97.5 95.1 98.2 93.5 
5.0 96.3 97.7 95.4 98.4 93.8 
8.0 97.7 98.4 96.6 98.8 95.1 
8.6 97.7 98.4 96.6 98.9 95.2 
9.0 97.8 98.4 96.7 98.9 95.2 
9.6 97.9 98.5 96.8 98.9 95.4 
10.0 97.9 98.5 96.8 98.9 95.4 
11.0 98.0 98.6 97.0 99.0 95.6 
12.0 98.1 98.6 97.1 99.0 95.8 
13.0 98.2 98.7 97.2 99.1 96.0 
14.0 98.3 98.8 97.3 99.1 96.2 
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Apparatus for coating the aluminium layer 3 is shown in FIG. 3. The 
specimen to be coated (i.e. the substrate 1 and layer 3) is placed in an 
air tight metal container 21 which is then evacuated by a vacuum pump 22 
to about 10.sup.-5 Torr. A gas such as methane, or propane is continuously 
bled into the container 21 through an inlet pipe 27 and the pressure 
maintained at about 10 Torr by throttling the pump 22 and the gas inlet 
pipe 27. Metal parts of the container 21 are connected to earth 23 whilst 
the specimen 20 is connected 24 through an insulated connector 25 to a 
-700 volts D.C. supply. As a result ionisation of the gas takes place with 
a consequential dissociation of hydrogen and carbon atoms. Carbon atoms 
impinge onto the heated specimen 20 to form a layer having a time 
dependent thickness. For example a 0.5 .mu.m thick layer is formed in 
about 25 minutes. 
As an alternative to D.C glow discharge RF flow discharge may be used. In 
this technique the D.C. supply of FIG. 3 is replaced by an RF supply at 13 
MHz frequency supplied through a capacitor 26 developing a cathode 
potential of about -700 volts. Operating conditions are as for D.C. 
operation. 
Carbon deposited as above forms an abrasion resistant, chemically durable 
layer similar to that found in diamonds and is substantially transparent 
to infra red radiation, i.e. the wavelengths between about 3 to 14 .mu.m. 
The soft form of carbon such as graphite is not suitable. 
Deposition of hard carbon and its properties is described in for example 
Thin Solid Films 58 (1979) 101-105; 107-116; and 117-120 together with 
references (papers read at the Fourth International Congress on Thin 
Films, Loughborough, Great Britain, Sept. 11-15 1978). 
To deposit the layer 4 of silicon or germanium onto the layer 3 the 
apparatus of FIG. 3 is used in a manner similar to that for depositing 
carbon. However, the gas used is silane (SiH.sub.4) for depositing 
silicon, and germane (GeH.sub.4) for depositing germanium. 
A layer of about 1000 A is deposited. Alternatively the layers of germanium 
and silicon may be deposited using conventional techniques, such as RF 
sputtering, in a separate process.