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errr,

username?

password?

:)
 

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I'm guessing this is a puzzle. So, ummmh


username: Brandon B

password: swordfish


BTW, am I the only one who thought this was an Ohlson post at first, just based upon the title? :)



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Palladin


No matter where you go, there you are
 

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hehe! good one!

Look for buzzwords like:

LASER

GLV

SXRD

LCoS

LEDs

4kx2k

etc. etc. :D
 

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WanMan,


Try "Loser" in the password field.


-Mr. Wigggles
 

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Quote:
Originally posted by MrWigggles
WanMan,


Try "Loser" in the password field.


-Mr. Wigggles
Okay, I can see where this thread is headed. :rolleyes:

Waitress, check please.


BTW Brandon, when you can get us a link that doesn't require your 'cookie', try to post it in a new thread, as it looks like the Jets & the Sharks are about to rumble in this one.

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Palladin


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I feel like I just witnessed a fine lady slap someone across the face while at a nice lounge. Only problem is, I don't know if I had too much to drink because I don't know who got slapped, me or Brandon.
 

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Discussion Starter · #10 ·
Oops, sorry. Haven't been back online since lunch yesterday. Here's the text:


New Scientist vol 181 issue 2431 - 24 January 2004, page 24



They look like simple slabs of rubber or plastic. Yet they bend light like a lens - and change their focus at the flick of a switch. Jeff Hecht investigates a new kind of optics, and why the Pentagon is so interested



THE lens in Ed Rietman's lab looks like something a basement inventor put together at home. It consists of a circular slab of transparent rubber a little smaller than a jam-jar lid, held in an aluminium frame. Light seems to pass through it in just the same way as through any other transparent material. And the view through the lens is nothing special, just a power cable on the other side of the office. It is hard to believe that something so modest-looking could change optics for ever.


But when Rietman switches on the power to his lens, something strange happens. The flat slab of rubber doesn't move, change shape or rotate as a more conventional lens might, but the view through it changes dramatically. Suddenly, the cable on the other side of the room shrinks. When Rietman turns off the power, the view returns to normal. Not only is his rubber slab acting as a lens, it is also changing its power.


Rietman's work is one of a number of projects commissioned by the US Defense Advanced Projects Research Agency aimed at developing a revolutionary new family of lenses. The programme is the brainchild of Len Buckley, a chemist from the Naval Research Laboratory in Washington DC. The lenses Buckley is aiming for could change everything from high-powered telescopes and military guidance systems to cheap disposable cameras.


"If you look at what's been done with lenses over the past several hundred years, it hasn't changed very much," Buckley says. As he prepared to join DARPA in early 2002, Buckley began to think about ways to move on from this medieval technology. DARPA asks incoming project managers to plan fresh approaches to important technical problems, and Buckley decided to tackle the complexity of modern optical systems.


Periscopes, viewfinders, telescopes and camera lenses are made up of many individual optical components. They are masterpieces of ingenuity, but they are also cumbersome and delicate. Zoom lenses in particular can be extremely unwieldy compared to the size of the camera. Designers have wrestled for decades over the problem of how to reduce this complexity and bulk while maintaining high performance. But Buckley has a new approach that he hopes will change everything.


The key property of any material used to make lenses - apart, of course, from its transparency - is the ability to bend light. The strength of this bending effect is summed up in a parameter known as its refractive index, which is essentially a measure of how much more slowly light moves through the material compared with its speed in a vacuum. For example, the glass used in conventional camera lenses is said to have a refractive index of 1.5, because light travels 1.5 times as fast in a vacuum as it does in the glass.


It is the change in speed as a light ray crosses the boundary between one material and another that causes its path to bend. The amount by which the ray bends depends on the angle at which it crosses the boundary, so the optical behaviour of a conventional lens is fixed by its shape. If a lens is convex, meaning thicker at the middle than at the sides, parallel light rays are bent so that they come together, like sunlight focused by a magnifying glass. If the lens is concave, or thicker at the sides, the light rays are spread apart.


In modern cameras, images are projected onto photographic film or electronic imaging chips by assembling two or more lenses and moving them back and forth to keep the image in focus. The limit to how much a lens or system of lenses can be moved in a camera or microscope restricts the range of distances an optical system can cope with. It can be extended with extra lenses and clever design, and part of the lens designer's art is to balance this complexity against cost and practicality.


But Buckley knew another way. The human eye has only one lens, yet it does a superb job of focusing on objects over a range of distances. Instead of moving, the eye's lens focuses by changing its shape. Turn from this page to look far away, and the lens of your eye becomes thinner and flatter to keep the image in focus. Turn your gaze back to the page, and your lens becomes rounder and fatter again.


This got Buckley thinking about another way of changing the power of a lens: not by changing its shape, but by changing the refractive index at different distances throughout the lens. Buckley knew that in some materials, such as certain silicones, the refractive index can be changed, just slightly, by passing ultrasonic pressure waves through them. As the waves interact with the material, they push atoms together then pull them apart, making the material alternately denser and less dense. This produces a moving pattern of alternating zones of high and low refractive index as the waves pass through the material.


The ability of ultrasonic waves in solids to modulate a beam of light and make it flicker has been known for decades, but it had never been used in focusing. Rietman and his colleagues Keith Higginson and Mike Costolo at the company Triton Systems in Chelmsford, Massachusetts, have been commissioned by Buckley to how work out to exploit this effect to bring light to a focus and so create a useful lens.


The frame around the silicone rubber disc is the key. It houses a set of piezoelectric actuators that generate high-frequency ultrasonic waves. Waves generated by each actuator reflect back and forth across the frame, setting up an interference pattern. The patterns from each actuator cross in the middle of the lens, and by tweaking the frequency of the actuators it is possible to produce a static interference pattern that makes the centre of the silicone disc less dense than the edges, thereby lowering its refractive index. The result is that the refractive index of the silicone increases further from the centre, so light rays that strike it spread out. As the eye traces the rays back, the object appears smaller and further away.


There is, however, a long way to go before the rubber lens can be put to practical use. So far, only an area a few millimetres across at the very centre of the lens shows any change in refractive index. Increasing the wavelength of the ultrasound would extend the area, and Rietman's team is currently trying to build actuators that are powerful enough at these wavelengths. They are also working on liquid lenses. He and Higginson show off a 7-centimetre glycerine lens between two glass plates. The refractive index change is smaller than in the rubber lens, so they make it visible by turning down the lights and aiming a green laser beam through the centre. The beam spreads to a blur on a screen 1.5 metres away until they switch on the power and the blur becomes a bright spot.


To make these into useful lenses, the team will eventually have to overcome one big hurdle - chromatic aberration, which also blights conventional lenses. Simple materials like glass bend light of different colours by different amounts: blue light more than red light, for example. This is what makes a prism separate white light into its component colours. But in a simple lens it means that different colours are brought to a focus at slightly different points, causing coloured fringes around objects in cheap telescopes and fuzzy images in cheap cameras.


Lens designers can get around this problem because different types of glass disperse colours to a different extent. For example, flint glass has about twice the dispersive power of crown glass. So by combining a convex flint lens with a concave crown lens, the dispersive effects can be made to more or less cancel out. But this comes at the price of vastly increasing the complexity of devices such as telescopes and camera lenses.


The human eye has a very different solution to the problem of chromatic aberration. Instead of using numerous lenses made of different materials, the eye's lens is made up of between 2000 and 3000 layers of cells stacked on top of each other. Some layers are thicker than others and have different refractive and dispersive properties, and they are arranged in just the right way to cut out chromatic aberration. Buckley believes that this might point the way to other methods of minimising chromatic aberration, and is funding research into materials inspired by the best features of natural lenses. "We can't use the materials nature has because they're not very robust," he says. But there are artificial nanostructures that may be able to compensate for chromatic aberration in a different way. One idea is to fill the pores in a porous material with certain optical properties with a second material that has different optical properties.


Sterling McBride of the Sarnoff Corporation in Princeton, New Jersey, is working on moving transparent fluids inside regularly structured materials known as photonic crystals. As the fluid moves into the pores, it changes the overall refractive index. McBride has changed the focal length of an experimental lens by a factor of 2 in this way, meaning that the distance to the lens' image doubled, without the lens or object having to move. The system acts only on infrared light, but it is promising for the infrared wavelengths in military night-vision systems, McBride says.


Another related approach is the work Buckley has commissioned from Shin-Tson Wu at the University of Florida in Orlando. Wu is working with liquid crystals, which are made up of cigar-shaped molecules that tend to align themselves with an electric field. Light travelling in the direction of the long axis of the molecules experiences a different refractive index than light shining across the long axis. Wu can vary the strength of the field to change the refractive index of the material as more molecules become aligned.


It has proved difficult to make liquid crystal lenses big enough for imaging devices, but by suspending nano-sized droplets of liquid crystal in a polymer, Wu has been able to create larger variable lenses. He varies the distribution of the liquid crystal droplets in the polymer so that the refractive index changes with distance from the centre of the lens. Applying an electric field changes the refractive index of the liquid crystals, with the largest changes in areas where the crystals fill more space.


Another approach commissioned by Buckley avoids the problem of chromatic aberration by concentrating on a narrow range of wavelengths. Instead of liquid crystals, SBA Materials of Goleta, California, is using photochromic materials, like those used to turn spectacles dark when the light is bright. These materials consist of molecules that can exist in two states - a light-absorbing state that makes the lenses look dark, and a light-transmitting state that makes them clear. The molecules are switched between these states by light itself. Switching the molecules to a light-absorbing state also increases their refractive index at wavelengths close to the one that is absorbed. The materials being investigated by SBA are photochromic to visible and infrared light, but when they switch between states, they change the refractive index for the nearby blue or red visible light. This means that they could form the basis of UV or infrared-activated variable-focus lenses for visible light. Like Wu's team, SBA is also working on materials that change state in an electric field rather than when bombarded with light.


Demonstration of variable refractive power in single lenses is only a first step. Buckley is already thinking up ways in which the technique could be exploited for more complex systems and has put together some designs showing what optical systems of the future might do. One application he has in mind is as a replacement for complex, heavy and delicate zoom lenses. While a single variable refractive index lens could not do the job of zooming by itself, because the image could not be both magnified and remain in focus, Buckley has shown that two of them separated by a fixed distance could. That could make zoom lenses much simpler and more compact than they are now.


Buckley also has some more exotic ideas, one of which he presented at a conference of the SPIE, the International Society for Optical Engineering, in Rochester, New York, last year. In the human eye, the central region of the retina has the highest density of light-sensing cells and so provides us with our detailed, high-resolution vision. While we can vaguely perceive objects in our peripheral vision, we have to move our eyes onto them if we want to study them. Buckley wants to mimic this ability of the human eye but without the lenses having to move. The idea is to control the refractive index across the lens so that, while one part of it focuses closely on one spot, the rest of the lens watches the surrounding area in less detail. If anything interesting happens elsewhere, the focus could shift.


A zoom lens made from variable refractive index lenses would be able to magnify part of its field of view without having to move, allowing a camera to zoom in for a close-up while keeping its peripheral vision alert for other action. In optical systems linked to image processing computers, the technique could dramatically reduce the processing power needed to analyse images by allowing a processor to gather fine detail on the most interesting parts of a scene while ignoring the rest. And because the lens does not have to move, optical systems could change their focus, gather data and move to decisions far more quickly. A car fitted with an infrared night vision system could zoom in and out quickly, spotting different objects on the road while moving at speed.


Just how fast all this could happen is anyone's guess. It'll be some years before these concepts can be turned into the high-quality lenses needed by commercial imaging systems. But once that happens, the design of optical systems will undergo a revolution and lenses will never be quite the same again.

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