I need a metal with a finish that has high reflectivity (low emissivity) in the IR (~ 8-15 microns), for a thermal shield in a cryogenically cooled photodiode (PD).
The problem:
- We need the shield around the PD to minimize heat being transferred by the warm vacuum window onto the PD. It is presently made of stainless steel.
- The lens in front of the PD focuses the light from one point of the scence onto the active area of the PD, but visible light from other parts of the scene reach other parts reaches the shield, is reflected/scattered back onto the window and lens and reflected back onto the detector; this produces "haze" when looking at a dark spot in a brightly illuminated scene.
The tried solution:
- We tried replacing the stainless heat shield with black anodized aluminum, but the anodization (oxide) has high IR emissivity and relatively high thickness, so it has relatively high emissivity in the IR; in addition, its porosity creates a virtual leak.
The desired solution:
- A metal coating that has high visible absorption (i.e., black) and high IR reflectivity.
- We would prefer that the metal can be shaped as a can but, if not possible, a flat piece is acceptable.
Thanks
Carlos
czarate@specinst.com

Why has it got to be metal?

Glass is well known as a reflector of IR light whilst transparent to the visible spectrum.

Care to edit the following?
light from one point of the scence onto the active area of the PD, but visible light from other parts of the scene reach other parts reaches the shield

Glass is actually not a mid-IR reflector but a very good absorber; it has very high IR emissivity; a room temperature glass surface emits mid IR almost like a black body; metals have low emissivity so they emit little mid IR heat but, as soon as they are coated with almost anything (e.g. black anodization), they start behaving as a black body too.

im not sure, but your task seems impossible to me.
metals usually dont absorb in the visible or IR, they are good reflectors. when you make it absorbing by surface modification or something, they become good emitters too. so....