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  • ColorLock™ Filter Stacks Enable Better Wide-Angle Imaging

Photonics Spectra
Feb 2015
Jason Keck, Reynard Corporation

New glass filters overcome considerable design challenges to transmit wide-angle incident light without shifting wavelengths, offering an innovative solution for a variety of applications in which the high transmission afforded by dielectric filters is less important than the consistency of the wavelengths of light coming in from wider incident angles.

Parents lament that their children are not getting enough exercise because they spend too much time playing video games. The edge of this criticism has been somewhat blunted by the use of novel motion-based game controllers for consoles such as the Wii and Xbox. These devices might not require players to expend as much energy as they would in a soccer game, but instead of simply pressing buttons in a sitting position, players use large, natural movements and gestures to control the game. The Kinect module, which sits atop the device like a webcam and functions as the Xbox controller, projects an infrared laser grid into the room, and a camera sensor detects what comes back from the environment. This enables the system to capture the user’s movements and gestures, subtract background stationary objects that the system has been calibrated to ignore, and convert the gestures into commands that control gameplay. This is one example of wide-angle imaging, an increasingly important field.

ColorLock filters do not exhibit wavelength shifting (blue shift) at large incident angles of illumination.

ColorLock filters do not exhibit wavelength shifting (blue shift) at large incident angles of illumination.


Iridescence

One problem that wide-angle imaging systems have to overcome in such exacting applications is iridescence – the change in transmitted or reflected color when seen from different angles. This is a familiar effect, commonly seen in soap bubbles or thin films of oil floating on a wet surface, where light is reflected in a variety of colors because of the way in which the wavelength varies with the viewing angle. This structural coloration, caused by microstructures that distort light, is often found in nature. It is a useful phenomenon for peacocks eager to display their brightly colored plumage to any aspiring mates, but a nuisance for many man-made devices that have precise color requirements.

Many color-sensing applications need wide-angle performance capabilities so that a wavelength can be detected irrespective of the incident angle, a situation in which iridescence could cause a lot of problems if an optical device were to distort the wavelength of light coming from peripheral objects. Coated devices such as dielectric filters deliver high transmission over very narrow cone angles, but beyond angles of 5°, they are prone to iridescence – a change of color (or blue shift) as the angle of the filter changes and the thickness of the optical filter stack increases. This makes them unsuited to wide-angle imaging applications with bright illumination. A new system based on stacking two or more layers of filter glass tailored to meet specific system requirements can overcome this problem.

Manufactured with two or more types of absorbing filter glass, the ColorLock filters can achieve specified performance.
Manufactured with two or more types of absorbing filter glass, the ColorLock filters can achieve specified performance.


The exact composition of the layers in these ColorLock™ filters is determined using proprietary software that determines a merit function and an optical layer thickness. This means that filter stacks can be designed for bandpass, long-wave pass, short-wave pass or any user-specified function. The incident angle can change by as much as 50° without any shift in the transmitted wavelength, while more traditional coated filters operating in the same conditions would see a significant shift toward shorter wavelengths, observable as a change from yellow to green.

Design challenges

Designing such a filter is a complex business, for numerous reasons. The first is the limited choice in filter glass – limited not only by manufacturer availability but also by physics. It is difficult to find filter glass with a cut-on or cut-off wavelength that would be ideal for an application. Where it is available, the designer is limited by what the manufacturer can deliver in reasonable time, because certain melts often must be scheduled in a frequency measured in years, depending on demand. The second problem is nearly the opposite. Although the designer may never find the perfect filter glass for a particular application, numerous vendors offer hundreds of other glass types, from which he can select a viable alternative combination that comes reasonably close to meeting the requirements.

In addition, physical manufacturability requires that no layer be too thin, and the dimensional requirements of the part may limit layer thickness. When these conditions are combined with the possible need to use many filter glass types at the same time, with the optimal thickness for each glass layer, the design of these filters becomes a massively multidimensional, nonsmooth optimization challenge.

However, in-house software at Reynard solves this problem. All of the system requirements are fed in, and the software produces a manufacturable design for a filter containing the necessary combination of materials with the correct thickness for each layer. This enables the quick, efficient creation of a design that meets customer requirements.

Advantages

The fact that the glass itself performs the blocking offers a number of advantages, such as eliminating the need for thin-film surface coatings. While no Fresnel loss occurs as light passes from one internal layer to another, it can still be an issue on the air-to-substrate surface, where a loss of around 8 percent takes place. This can be recovered with the addition of a broadband antireflection coating, the spectral range of which is much wider than the active spectral region of the filter, so changes in angle of the filter will not affect the stability of the transmission band. To create steeper edges for in-band performance, blocking coatings can also be added.

At high incident angles where dielectric filters exhibit color shifting, ColorLock filters maintain color transmission.

At high incident angles where dielectric filters exhibit color shifting, ColorLock filters maintain color transmission.


The spectral range is from the UV to NIR, and transmission at the specified design wavelength can exceed 60 percent, which may not be as high as dielectric filters but is sufficient for applications in which it is more important to have consistent wavelengths from wider incident angles, and illumination can be controlled.

Using glass to do the blocking not only reduces the need for coatings, but also offers durability. The device can be as tough as the substrate itself, which means that it can pass temperature and humidity testing, severe abrasion and salt/fog testing, and durability standards including MIL-PRF-13830B, MIL-C-48487A, MIL-C-675C and others.

ColorLock filters can be designed for wide-angle color-correcting applications with customized filtering performance.
ColorLock filters can be designed for wide-angle color-correcting applications with customized filtering performance.


Applications

The number of applications for this glass is considerable. Agriculture has long been a field in which the color of crops or products reveals vital information. The vegetative index of crops, a measurement of green hue, has traditionally been measured by Earth-observing satellites, but the affordability of aerial drones has brought new possibilities. Now, a drone can be programmed to fly in a fixed pattern over a crop area demarcated by GPS data, taking images at regular intervals and building up a picture of the vegetative index of a specific crop. In such applications, wide-angle imaging offering spectral data with minimal iridescent distortion can give farmers precise control over fertilizer application rates, greatly improving productivity. The advantages over costly conventional aerial photography and low-resolution satellite imagery are obvious.

In digital imaging, colorimeters profile and calibrate display devices. These instruments, which take wideband spectral energy readings, can benefit greatly from consistent wavelengths coming in from all angles.

Hyperspectral imaging has a range of applications in astronomy, agriculture, biomedical imaging and mineralogy. Given the expense of launching such precision instrumentation into orbit, the incident light must go through as little iridescence as possible, and the filters must be robust enough to withstand extreme operating conditions.

Meet the author

Jason Keck is director of technology at Reynard Corp. in San Clemente, Calif.; email: sales@reynardcorp.com.



GLOSSARY
hyperspectral imaging
Methods for identifying and mapping materials through spectroscopic remote sensing. Also called imaging spectroscopy; ultraspectral imaging.
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