Quesion about chalcogenide optics in FLIR One

[Ben321]

First a bit of background about what I already know. I read that the FLIR One uses this type of material instead of germanium. For one thing, it's cheaper, but also I've read that it protects the vanadium oxide LWIR sensor from damage if pointed at the sun. I remember reading that, but I don't remember where. Also I remember reading (but I don't remember where) that it isn't LWIR from the sun that usually can damage a vanadium oxide sensor (as most of this is absorbed by the atmosphere), but rather is the MWIR (middle-wave infrared) that much more easilly passes through the atmosphere that can damage a vanadium oxide sensor, and that while germanium passes both LWIR and MWIR, the chalcogenide optics pass only LWIR, and therefore prevent damage that would normally instantly occur even if you only accidentally briefly pointed a traditional thermal imager at the sun.

Here's the question. Is this true? Is it really safe to try to image the sun with a thermal imager like the FLIR One that is protected with chalcogenide optics? I'd love to see what the sun looks like in LWIR, but I don't want to fry the FLIR One that I just bought.

[encryptededdy]

I was under the impression that it's a silicon lens.

http://image-sensors-world.blogspot.co.nz/2014/12/systemplus-publishes-flir-lepton.html

Digital Optics' WLO brings an important part of the cost reduction. The silicon lenses are made at the wafer level with lithography and etching processes.

With that said FLIR does indeed market all of their microbolometers as "Sun-safe".

I think after you point the sensor at the sun you may see a warm spot, however this should be gone as soon as the shutter actuates / FFC occurs.

[mhosier]

I don't have any specialist knowledge of the Flir One, but I did once accidentally point an old FLIR InfraCam at the sun while attaching it to a tripod.  It did gain a spot on the image, but this disappeared after switching it off and then on again.  I have always been very careful not to do the same thing since, as I considered myself extremely lucky that it showed no lasting effects. 

I'd have to trawl through some archives to find the image, but as I recall it was simply over-range in the area whee the sun was, so I don't think you would get any meaningful image anyway, especially as the Flir One temp range is only up to 100C.  Not sure if anyone with a professional camera featuring a high-temp filter has ever tried this?  My current camera is an older model (PM695), but doesn't have the high temp filter enabled, as it would be of little use in my line of work.

[Bill W]

Chalcogenides are also wide band transmission:
http://eom.umicore.com/en/infrared-optics/blanks/GASIR-for-infrared-optics.pdf
What will make a difference is coatings for both lens elements and the sensor window, wide band coatings would be unusual especially as it also requires effort to focus the 3-5um the same as the 8-14um.

Silicon is not the best 8-14 material, but smaller thinner lenses and windows will be viable.
http://www.optotl.com/cms-files/Image/Si-8-14-AR.gif

'Sun-safe' usually means no lasting damage, clearly heating up a few pixels a lot will take a while to dissipate.  Some earlier bolometers could suffer irreversible changes, although these would only show up as gain change viewing a hot bland surface, the shuttering taking care of any long term offset changes.

regards
Bill

www.fire-tics.co.uk

[Bill W]

Looked a bit more at the OP's point regarding 3-5 vs 8-14µm.

The main point is that doing some sums on Planck curves shows that there is far more energy-flux at lower wavelengths when you are dealing with a 5500°C black body.  The relative energy between 3.5 and 5.5µm is five times that over the 8 - 14µm band.  Both those bands have reasonable atmospheric transmission.

Bill

www.fire-tics.co.uk