Heated Germanium Window Assembly

[jmcaero]

I'm trying to figure out how to specify a direct heated Germanium sensor window for rugged LWIR sensor. I can't use a thin-film/kapton heater placed around the perimeter of the clear aperture. I've read it is possible to heat the Ge window directly but there is a dearth of data online about this solution. Only one supplier claims the capability, but no data posted online about it.

Ideally, I would like to have bus bars at the top and bottom on the inside surface of the rectangular window and contacts for a thermistor. But, since the Ge is the "element", does the electrical connector need to be made on the edge of the window?

And direction to a supplier or example of a heated Ge window will be gratefully accepted.


JMCAero

 

[IRstuff]

Direct heating of Ge windows can be done, depending on the resistivity of the Ge, and the intent of the heating. You can think of the Ge as a flat resistor material, and by running current through it, you can cause it to heat up. BUT, bear in mind that uniformity of the heating is difficult to achieve. Hypothetically, you can have multiple connections to power sources with different potentials, depending on their lateral location relative to a centerline across the window. This would allow you to run slightly less power in the middle, to minimize the bowing that would otherwise occur.

The second issue is how much power you need to apply. For a typical anti-icing application in moderate airspeeds, you need something on the order of 2.5 W/in^2 across the window to get effective results. No matter how uniformly you apply the power, there will still be a slight bow, so anything that requires precision boresighting would need to operated with the heater turned off.

Several alternatives to direct heating exist. Battelle and several other companies make a photolithographic metal heater grid that's deposited on the surface. This allows the use of non-conductive windows. Ditto, Indium Tin Oxide (ITO), which is a surface resistive element. Both of these will tend ot reduce transmission through the assembly.

Note that Ge will lose its higher wavelength transmission when heated.

TTFN

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