TECHNOLOGY FOCUS LIQUID CRYSTAL FILTERS
Visible improvement C
ameras, eyes, telescopes, binoculars and night vision goggles can all be dazzled and damaged by high energy light pulses, but a new
liquid crystal filter could block that while still providing enough light for good picture quality. Over recent years the availability of high power portable laser torches has increased dramatically. Today anyone can buy a burning laser for less than £150 which is a blinding hazard at ranges of over 250m. This increase in the availability of lasers has led to a dramatic increase in the number of incidents in which, for example, pilots have been illuminated with dangerous green laser radiation. However, the liquid crystal filter is not for pilots, who would prefer to have self-activating windows. The researchers behind the filter set out to develop a single solution to protect military and civilian sensors from all types of optical radiation. Several governments have continued the development of purpose-built laser weapons capable of causing permanent blindness from hundreds of miles away. Traditional methods of dealing with laser radiation rely on strongly absorbing filters, which block certain wavelengths of light. While these filters work well against these known laser threats, they are useless against novel lasers emitting multiple frequencies or white light such as that from the Sun.
Light valve
The new light activated filter that is optimised for laser protection is called an Optically Activated Light Valve. A valve because it can control the range of brightness/intensity reaching an optical sensor, whether it is in
Rob Coppinger discovers how liquid crystal defence technology could put an end to blinding lasers and improve imaging
cameras, telescopes, binoculars or night vision goggles. Its potential commercial applications include CCTV defence and automatic number plate recognition in all light conditions. This is because CCTV can be defeated with very bright lights and a car’s headlights while driving on a dark road can stop cameras from seeing the licence plate.
Scientists at the UK government’s Defence Science and Technology Laboratory (DSTL) have developed the filter. Developed by DSTL scientists Christopher Burgess and Mark Herrington, the research is referred to as broadband dazzle protection technology. Burgess is the technical lead for the work and he is also a senior electro-optical protection scientist. He says: ‘This grew out of laser protection. It is still in development. It’s a light valve and by changing the voltage you can pick which intensity you want to start blocking. If you have zero volts on it you would see everything and as you increase the voltage you block [the more intense light sources].’
The filter effectively allows a user to take a picture of anything that is very bright.
The intensity of the bright source can be diminished allowing other features in the scene to be seen. While a typical camera iris would close to reduce the light level stopping dimmer objects in the same scene as the bright one being seen, the filter would allow all the objects to be visible, irrespective of their brightness levels.
Behind the lens According to Burgess the filter could go behind the lens of night vision goggles or a telescope or between binoculars’ lenses. The filter is a special layer of liquid material called a twisted nematic. It is sandwiched between a glass slide and a light sensitive crystal, both of which have thin electrodes deposited on their surfaces. A twisted nematic is a polarisation modulator. Polarised light passing through the material twists as it goes. By applying a strong electric field to the twisted nematic layer the ordered
The filter would allow all the
objects to be visible, irrespective of their brightness levels
Lasers can dazzle a target blinding people or incapacitating cameras. Image courtesy of Defence Science and Technology Laboratory
42 ELECTRO OPTICS l FEBRUARY 2012
With the filter on lasers can no longer dazzle. Image courtesy of Defence Science and Technology Laboratory
structure of the liquid crystal molecules can be broken down undoing the light twisting effect. The combination of a light sensitive material with a liquid crystal material creates a system whose transmission of light changes depending on the strength of the light. The interaction of the light and the photosensitive layer is governed by complex nonlinear equations that are solved using a computer. Burgess and Herrington have prototypes that work and can produce the filters in their laboratory. DSTL, in conjunction with its technology transfer company, Ploughshare Innovations Ltd, are working with a potential UK manufacturer that could produce the device commercially. However, it looks like the filter may not be made in the UK. Burgess says: ‘There isn’t much UK capability in the area of liquid crystal manufacture as far as I am aware. There’s a lot in other countries. The scale of it [that we would need], would be one lab and two or three guys. It wouldn’t need much setting up and the technology is there, it’s all known science.’ l
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