phased frensel lens optics for thermal?

[coppercone2]

Do they exist ?

[bap2703]

Do you mean zone plates or just regular fresnel?

[Ultrapurple]

A while ago I did some experiments with plastic fresnel lenses designed for thermal infrared.

They weren't very successful. See this thread for details.

[Ultrapurple]

This is about the best result I could get with a plastic fresnel lens, even with a hot soldering iron on its stand as the target:



Not what you'd call good focus.

There was scope for improvement - read the whole thread I mentioned above - but I don't think plastic fresnel lenses are going to replace good old Ge or ZnSe anytime soon.

[Bill W]

not a variant of these 'Quaver' lenses ?

http://www.ee.bgu.ac.il/~itzik/ImagingSystems2/Projects/P19_Wavefront%20coding.pdf

Bill

[coppercone2]

I mean like the lens that nikon sells for high end cameras, that offers quality at a shorter physical length of optics.

also used for optical photos of atoms some how.

[Vipitis]

From what I understand, Niko uses "phase fresnel" compact chromatic aboration. In thermal imaging it's not that huge of an  issues became we image in single band most of the time (or do the dual purpose mw/lw cameras allow for dual band sensitivity with dual band palettes?).

The pixels are much larger then what you find in visual cameras, and so are the wavelengths.

The idea of new types optical designs and material is still very interesting. Fresnel are usually uses to gather light for detection or to focus and distribute light, not an image. In photography fresnel elements have been for ground glass viewfinders to illuminate edges. But they weren't part of the optical imaging path.

You can find some papers on applications for fresnel elements in color photography, but it's very basic experiments.

I believe that the near future will bring new technologies in coatings and very fine manufacturing of synthetically cristals for optical applications. One example would be micro lenses for lightfield photography and computational imaging that uses several sensors and lenses for one final image.

They don't exist today. Not for the public. Current plastic fresnel sheets aren't useful for anything imaging and barely work.

[Bill W]

In thermal imaging it's not that huge of an  issues became we image in single band most of the time

You would be surprised how much chromatic does matter in designing lenses for 8 - 14µm, especially shorter focal lengths.  Get it wrong and you can lose half the resolution at pixel frequencies.  This is why you will often get ZnSe and Ge in the same lens unit.


Bill

[Ultrapurple]

You would be surprised how much chromatic does matter in designing lenses for 8 - 14µm, especially shorter focal lengths.  Get it wrong and you can lose half the resolution at pixel frequencies.  This is why you will often get ZnSe and Ge in the same lens unit.

Ah - interesting! I had always assumed there was a completely different reason for mixing ZnSe and Ge, namely cost.

I can imagine that designing a lens to operate over a 2:1 wavelength range must be challenging at best, particularly when you remember that the material properties change with ambient temperature (hence athermalized lenses). Very few visible light lenses can match that sort of performance.

Classic visible light lenses are optimised for perfect focus at two or sometimes three wavelengths (colours); even with the wide range of materials available to the modern lens designer, not to mention the computer power that can be harnessed to do the sums. According to Wikipedia, a simple two-element air-spaced lens has nine variables (four radii of curvature, two thicknesses, one airspace thickness, and two glass types). The number of variables rises rapidly with the number of elements and, for a complex design, could number in the hundreds.

One of the few visible light photographic lenses I know of that is genuinely optimised for a comparable wavelength range, near-infrared through to near-ultraviolet (below 900nm to at least 220nm) is the Nikon UV-105, 105mm f/4.5 Multispectral Imaging Lens, "an exceptionally sharp and distortion-free 105mm f4.5 variable diaphragm (iris) conventional manually operated lens" - there's a heck of a lot of information on that page and it's well worth a read, though it's of very limited relevance to thermal imaging. And the last time I checked it cost something in the region of US$7000. A fabulous lens if you need it, but you really have to need it.

[coppercone2]

btw as far as war stories go, I made a giant *(but good) pcb Video microscope using old optics from a high end 80's film camera (thats like a foot long) and some magnifier meant for metalurgical inspection that glued to the other optic or maybe the camera (ended up dismantling it because my focus system required a gear that I did not have in that point in time and it was a complete dog to adjust two screws on a heavy assembly by hand, required special construction.

It was good because you had a very nice working length and it felt very real time because it was analog but I noticed that I only got working results using IR or Red light (ended up using red light). When I tried to use it with violet/UV it became unfocused looking despite the fact there was no mechanical change other then ring light being changed. However if you have lab room and just use it for inspection and focus it by moving the PCB and not the microscope it was very very good, despite being black and white.

Is this Chromatic Aberration? It used a old school 480 line CCD camera (scientific) without its original optics, just something I glued on.

[Ultrapurple]

No, that's probably not chromatic aberration as such. Many (most) lenses have a subtly different focus point at different wavelengths; if you focus on something using blue light you will probably have to adjust the focus to see it in deep red light. Many photographic lenses have a red dot on the distance scale to show where they focus for (near) infrared; they acknowledge that focus is different for different wavelengths.

As mentioned earlier, different types of lenses with different numbers of elements can be corrected for one, two, three or more specific wavelengths. Typical performance curves for various lens designs look like this:




You can change the shapes of the curves, and make the lines straighter and straighter for a given wavelength range, by adding more optical elements of carefully-calculated characteristics. But broadly speaking, the flatter you make the response over a given (wanted) range, the faster it all falls apart outside that range.

Have a look at the Wikipedia article on chromatic aberration. It's very informative. Compare these two images - the top has no chromatic aberration, the bottom has bad chromatic aberration.

[Vipitis]

LWIR is a very wide band and the transmission and detection isn't constant throughout the spectrum. It depends on your optics, coatings and the sensor.

Germanium also has a vastly different refractive index when compared to glass for visible light optical systems.

What really stun me lately was that in analog large format view cameras, you need to adjust your focus scale depending on your media. Typical silver film or paper is most sensitivie near the red part of of the visible spectrum. IR film is sensitive in the NIR part of up to around 1080nm. And wetplates are sensor to blue and vis-UV.

Some cameras got a focus indicator scale with note that IR needs 2mm further out for example.

[eKretz]

I don't think that has anything to do with the film. It's the light. Different frequencies of light focus through the same lens to different points due to varying wavelength. This is the principle behind achromatic lenses to help focus all the relevant visible light spectra at as close to the same point as possible and minimize chromatic aberration. Light in the infrared spectrum and light in the visible spectra will very rarely focus to the same point through the same lens.