If you think quartz and sapphire are birefringent, wait until you try and hold a plastic lens in place with anything other than anti-gravity dilithium crystals.
If you think quartz and sapphire are birefringent, wait until you try and hold a plastic lens in place with anything other than anti-gravity dilithium crystals.
Klaus,
I have looked into UV lenses for a while now for a couple of different reasons, coming from a slightly different viewpoint. I'm using excimer radiation (248nm) to image mask systems for materials processing. I have also briefly looked into making a UV enlarger for AltProcesses a while ago. I have not looked at pictorial uses as such, but I think this isn't too far off. Just don't expect a performance as of a modern plasmat type desing
I have not found any historical lenses (well, I never really looked), and I think at least at the wavelength I am looking at, it would be difficult to make true achromats, purely since I am not aware of any cement that transmits suitably at these wavelengths.
Lens materials I considered (am considering) is fused silica (UV spec), CaF2 and MgF2. I never considered Al2O3 simply due to cost: a standard window is more expensive than a SiO2 lens. With the above materials, it should be possible to make a kind of "airspaced achromat", probably from silica and MgF2. This in turn would allow you to make Dialyte designs or even a Tessar. A Cooke triplet is certainly a distinct possibility. For the enlarger I was thinking a simple Double Gauss design. Keeping the number of elements down is probably key to performance and cost... Do you have access to an optics design system (Zeemax, etc.) Even Linos' free Winlens may be a good starting point.
If you want to go the homegrown route, good sources for optics are CVI techoptics on the Isle of man. Custom optics, and AR coatings for whichever wavelengths you want. One off lenses are a bit more expensive though... If you can stick to standard focal lengths things aren't too silly. I found Kingsley's book on lens design quite useful while I was looking into this.
Our project for the imaging lens is on hold at the moment (more important things to deal with), but I'd like to resurrect this at some point.
I hope I haven't gone too far off topic...
Marc
Klaus, thanks for the graphs. I didn't know there was a special UV-PMMA, as well as another plastic.
Struan, stress birefringence in plastics could be a problem, no question. Anything injection molded will show it. Not sure about the value of Delta n for that vs. the natural birefringence in the crystal materials in question.
Marc, OSLO has a freeware version of its software, called OSLO Edu, which allows up to 10 surfaces (includes the image plane and the aperture plane), that is a maximum of 4 freestanding lenses. That allows a dialyte, a simple double Gauss, a Triplet or a Tessar construction.
A rough figure culled from people who try to make acrylic waveguides on the surfaces of silicon chips is a difference in refractive index of about 10^-4 for a 1-2% strain. Pure acrylics will also have at least that degree of birefringence from residual molecular orientation. The latter can be reduced with co-polymers, but at the expense of UV transmission.Originally Posted by Arne Croell
Quartz and Sapphire have a natural birefringence about a hundred times stronger. The figure for acrylic is of the same order as that for optical glasses, but of the opposite sign. Acrylics are softer, so the same deforming force gives more birefringence.
As I understand it though, the real problem isn't the birefringence per-se, it is that both the stress and orientational birefringence tends to be non-uniform. Good enough for eyeglasses (and, I think, the lenses on Kodak Disk cameras :-), but I would be doubtful about LF lenses working at short wavelengths.
Quartz lenses- would mostly be of an ancient origin. If someone makes an UV lens (for any format), there are special glasses available that should do.
See for example: http://www.freepatentsonline.com/5547904.html
Does anyone know of the practical feasibility of photographing with light <300nm without vacuum? Apart from the transmission of shorter wavelength UV through our atmospheric surroundings, there is also this minor problem of the sensor (or film) sensitivity to UV below 300nm.
Last edited by Vivek Iyer; 13-Dec-2006 at 10:11. Reason: grammar
Vivek, there are lots of solutions for solid-state UV detectors below 300 nm, and regular silicon photodiodes work fine provided their packaging does not absorb the radiation. Research into better detectors is mostly concentrated on making the sensor blind to visible light, or specific to particular absorbtion lines for, say, pollution detection.
Analogue film is trickier because the gelatin absorbs a lot of the light. Low-gelatin emulsions or bare silver halide crystals on a support used to be the way of coping with this, but these days everyone uses solid state detectors.
As a rough rule of thumb physics experiments move into vacuum somewhere between 300 and 200 nm. There are bright UV lasers for cutting and welding that operate at low wavelengths in air, but even with them you try to keep the in-air path length as short as possible. For photography you have to also consider your light source: the black body solar spectrum cuts off pretty sharply before you get to 200 nm, even before you take into account atmospheric absorbtion.
Struan, Yes. Barring some special formulation (may not be accessible nowadays), B&W film (or plates) won't work below 300nm.
Solid state sensors: Truly astonishing developments! Back thinned CCD sensors can do as low as 160nm! Currently, only very small area chips of this kind are easily available (for a price, of course!). These are all monochrome devices.
As for practical photography below 300nm outside a lab, and for longer distances, I do not see any hope at all. Terrestrial imaging at infinity may not be possible.
So, who needs Quartz?
Doesn't x-ray which has shorter wavelength than UV expose silver BW film?
Yes. Gelatin, plastic base, etc do not absorb X-Ray.
Vacuum UV begins at 200nm and UV ends at about 30nm. A pure nitrogen atmosphere will pass UV below 200nm, as it is the oxygen that is opaque to UV.
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