Machine vision technology has gained significant importance due to its ability to automatically inspect, analyse and control processes. Its deployment only stands to increase with the development of industry 4.0 and further automation of manufacturing processes. In modern machine vision, high speed imaging is increasingly important. In this article, we will discuss how the unique properties of liquid crystals means they are the ideal basis for shutters and polarisation components and how these can make a critical contribution to the development of effective imaging systems.
With the ongoing improvement of imaging sensors and image processing software, the potential for high-speed imaging is expanding. Effective, and rapid, control of light levels and polarisation is fundamental to the efficient functioning of imaging systems as they capture high-quality images for analysis. The ability of liquid crystal (LC) materials to rapidly alter their molecular structure makes them the ideal foundation for fast optical shutter and polarisation modulation devices.
The properties of liquid crystals lend themselves to efficient light control. The rod-like crystals exhibit a different refractive index on the long (extraordinary), and short (ordinary) axis, a characteristic known as “birefringence”. This means that light travelling through a liquid crystal cell is subjected to a phase shift, or “retardation”, depending on whether it is travelling along the extraordinary or ordinary axis of the rods. Effectively, randomly polarised light travelling through an LC cell is polarised, whereas polarised light is either unaffected, or its polarisation rotated, as it travels through the LC cell, depending on the alignment of the LC rods.
A second characteristic is that the liquid crystal rods will align their extraordinary axis to an applied electric field, meaning the polarising effect is voltage dependant. This is known as “voltage controlled optical anisotropy”.
Fast Optical Shutters
A fast optical shutter (FOS) is a liquid crystal cell positioned between two polarising films, each aligned at 90° to the other. Under normal operation, with no voltage applied to the LC cell, the polariser on the input face (0°) selects a single polarisation from randomly polarised incoming light and this polarisation is rotated by 90°, so that it can pass through the exit polariser. This is referred to as “Normally White” or “Normally Open” operation. When a voltage, typically 4V, is applied the LC rods realign extremely quickly (~30µs), rotating the polarisation of the transmitted light so that it will be blocked by the exit polariser.
When the applied voltage is removed the LC rods realign with a slower timescale in the region of a few 10s of milliseconds – this it the “elastic relaxation” phase. Hence it can be seen that a standard FOS exhibits asymmetric switching of the order of 10s of microseconds to close and 10s of milliseconds to open.
Between 0V and ~4V the transmitted light is elliptically polarised at an angle between 0° and 90°, meaning that only the component matching the output polariser exits. This then allows for grayscale operation of the FOS.
The ability of a liquid crystal-based FOS to provide precise, high-speed, control of optical intensity, enables improved performance in many imaging applications. With fast switching speeds, high contrast and operation over a wide wavelength range, fast optical shutters offer improved signal-to-noise and overall performance compared to traditional mechanical shutters. These devices play a key role in swiftly controlling light levels, crucial for capturing clear images of fast-moving objects.
The lack of moving components ensures reliable and rugged vibration-free operation, lack of wear and tear and a compact footprint. A range of optimised shutter configurations are available including visible (400-700nm); near-infrared (400-2500nm); high speed; contrast enhanced (100k:1); variable neutral density and imaging quality designs.
Fast optical shutters offer rapid response times while maintaining precise control, outperforming mechanical shutters prone to wear and tear. LC FOS provide flexibility and enhanced image quality, making them indispensable in high-speed machine vision and other applications including laser welding intensity control.
Fast Polarisation Modulator (FPM) devices
The incorporation of polarisation control in machine vision systems allows the extraction of supplementary physical and structural characteristics of objects, unobtainable through standard imaging techniques. Such information might include reduced glare, the detection of stress or strain in transparent glass and plastic materials and enhanced contrast and edge detection.
Compared to a FOS structure the FPM features only one polarising film. Applying a drive voltage reorients the birefringent LC molecules, changing the phase retardation of light passing through the LC cell. This results in a change in polarisation of light passing through the full modulator structure.
A standard FPM is designed to output a linear polarisation at 0° and 90° with elliptical polarisations between. The Variable Polarisation Rotator (VPR) is a variation which incorporates a quarter wave plate to allow a linear output polarisation at intermediate angles.
Liquid crystal-based devices provide accurate and fast polarisation control with low power consumption. They allow machine vision system designers to add polarisation imaging with a single, low-profile device with no moving parts instead of complex and bulky configuration requiring rotating, or multiple, polarisers; prisms; multiple image sensors or the loss of sensitivity inherent in image sensors with integrated pixel-level polarisers. This makes them ideal for advanced machine vision applications requiring precise and dynamic polarisation control.
Imaging Quality
Since standard LC-based optical components feature a laminated plastic polarising film they have inherently low wavefront quality which can result in image distortion. Our products incorporate rugged, optically flat, low aberration, cover sheets with 60/40 scratch/dig characteristics and a high-performance antireflection coating.
This “imaging quality” design provides superior imaging accuracy as a result of minimal wavefront distortion. The flat surfaces preserve the integrity of the optical wavefront and resulting in crisp, distortion-free images.
Components
AP Technologies represents Swedish liquid crystal pioneers and component specialists LC-Tec. Their extensive experience and expertise enable them to specialise in designing and manufacturing leading-edge liquid crystal solutions renowned for their superior performance.
The LC-Tec fast optical shutter range comprises various models optimised for different applications:
- FOS for visible (400-700nm) wavelengths with reduced light leakage,
- FOS(G2) with improved switching times
- FOS(G2)-CE with high contrast ratio
- X-FOS(G2) for extra-fast switching
- PolarSpeed®-S for ultra-fast symmetrical 30µs switching at up to 540fps
- FOS-NIR for broadband visible and NIR (400-2500nm) wavelengths
Visit the FOS product page for more information
The polarisation control range includes a variety of fast polarisation modulators (FPMs) and a variable polarisation rotator (VPR) to suit different applications:
- FPM(L) provides high contrast and fast switching
- X-FPM(L) for high frequency switching between two polarisation states
- FPM(L)-NIR(1100) is optimised for near-infrared operation.
- PolarSpeed®-M(L) offers 30µs symmetrical switching for high frame rate (540fps) applications.
- VPR allows continuous analogue rotation of linear polarisation output states.
Visit the FPM product page for more information
Conclusion
Liquid crystals are an ideal material for fast optical switching and polarisation modulation due to their birefringent nature resulting in their ability to exhibit anisotropic permittivity.
The combination of compact size; lack of moving parts leading to reliable, vibration-free operation; speed and accuracy of liquid crystal components means they are a cost-effective solution, well suited for deployment in modern imaging systems demanding high speed switching or polarisation.control.
Embracing liquid crystal technology can enhance machine vision systems across industries.
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Contact Martin Sharratt to talk to us about shutter and polarisation control for your machine vision applications.
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