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Pixel pitch vs spectral response... Myth busted?

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sam1275:
Hi everyone, I've heard a saying that the sensor's pixel size cannot be smaller than the spectral it works on, but with the Flir Boson, this myth seems to be busted, look at the spec of it:
http://www.flir.com/cores/content/?id=74595
It have 12um pitch and spectral up to 13.5.
Is just me too stupid and used to believe that myth?

Fraser:
It is possible to have a pixel size smaller than the spectral wavelength, but you are then into the world of pixel combining. Smaller pixels are less sensitive so often require better optical blocks. The desire to reduce pixel size can be driven by a need to increase resolution whilst either maintaining or reducing sensor die real estate. It is sometimes used to reduce production costs, as seen with the FLIR ONE G2 and SEEK Thermal cameras. 12um pixels have been used in full LWIR spectrum cameras for some time.

I personally prefer nice large pixel size and optical block. My PM695 has a relatively large microbolometer die and nice large optics :)

A word of warning though. Specifications can be a total minefield as you really need to see a spectral plot of the microbolometer in order to see how sensitivity is compromised in some areas of the response in order to meet the design criteria.

Fraser

sam1275:
Thank you Fraser.
I also think bigger pixel and bigger optics are better.
I used to believe the wavelength cannot be bigger than the pixel, and I just realized that's a total bullshit...
However you said pixel combining(binning?), that's not a must for smaller-than-wavelength sensor, just a method to make picture better in cost of lower resolution, right?

Fraser:
Sam,

Bill.W is very knowledgeable on microbolometers so maybe he will share some of his extensive knowledge here on the topic of smaller than wavelength pixels.

It should also be remembered that pixels do not have to be round or square. Pixel shapes can help with improving die pixel density and even wavelength sensitivity. There are compromises involved though.

IMHO, smaller than wavelength pixels are not great for performance so I would look very closely at the real world performance of a camera that used such technology.  FLIR want use sub 12um pixels in the next generation of LEPTON but how well it will perform is not known to me. The lens becomes a very important factor in any such <12um pixel design.

There is the potential for development of MWIR and SWIR sensors that are able to use smaller pixels than a LWIR camera without the pixels being smaller than the wavelength. The problem is the cooling of the sensor. There is a SWIR TAU that has overcome this challenge to some extent. Sadly the cost and power consumption of SWIR and MWIR cameras will likely always be far higher than a LWIR microbolometer though. Not what FLIR are looking for when designing for the lower priced consumer market.

Military customers have bigger budgets and needs however. The miniaturisation of thermal cameras will continue and we can expect to see thermal cameras mounted on the tiny Recon drones that foot soldiers use on the battlefield.

The LEPTON is already a superb example of thermal camera miniaturisation. FLIR just need to work out how to cram 320 X 240 pixels into the same die dimensions ! That would mean approx 5um pixels !

Fraser

Bill W:
There's a few factors to consider here.

Small pixels are inherently less sensitive as there is a finite 'waste' from separation and support structure that hit fill factor quite badly.  You can win a lot back with better low noise circuitry and better vacuum, but that conflicts with a  desire for simpler packaging. 

Once the pixel is down to wavelength size you are more designing an antenna structure not a simple black body 'thermistor on a stick' as before.

There is another problem that hits anyone trying to design a camera around these sensors, which is the resolution limit of the lens.

Smaller apertures (f/1.4 not f/1) have so far helped system optical quality as this reduces lens aberrations.  Once you are dealing with pixels around the wavelength you then meet diffraction coming the other way (Google/Wiki 'Airy discs' etc for more)  where f/2 is better than f/8.

What we may find is either a floor around 10um or the use of smaller pixels to up the pixel count and to use image processing to fix things.  The very cheapest cameras may even get bigger pixel pitches depending on where the costs lie for each manufacturers' business model.

Bill

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