In this manner, a monochrome sensor can become sensitive to multiple spectral bands without the need for external filter switching — as in the case of wheel-based multispectral cameras — or scanning over space, as in the case of hyperspectral imagers. Eliminating moving parts removes the costly temporal delay associated with transitioning between filters or sampling different locations in space, enabling true video-rate multispectral imaging. Additionally, removing the moving parts enables deployment into harsh environments where the rate of failure increases dramatically. Weight, power consumption, and design costs are all reduced with a directly-coated sensor.
Applying a patterned filter onto glass, then bonding the glass to a sensor with optical-grade epoxy, would seem to accomplish the same goal. There are two distinct drawbacks, however, to the approach. First, the index match between sensor and glass substrate is imperfect, and results in some degree of light loss. The loss is further compounded by the already poor efficiency of silicon sensors, even with AR coatings. Secondly, the dielectric coating — which was previously stated to be sensitive to angle of incidence — is further removed from the sensor plane by the thickness of the glass substrate, requiring the delivery of collimated light to the surface of the glass, not the image plane, potentially introducing aberrations that reduce image quality.
Applications of Patterned Pixel Devices
The improvement in frame-rate and the elimination of moving parts comes, however, with a cost. The functional resolution of the system is reduced as the number of filters applied increases. A nine-band camera, for example, would produce nine simultaneous images, but each with a resolution oneninth of the full frame. That information is not fully lost, and adapting image interpolation algorithms can aid in restoring the perception of resolution, as is currently performed in common RGB cameras.
Video-rate multispectral imaging with coated sensors opens up a previously inaccessible application space. Aerial inspection techniques benefit greatly from a system lacking both moving parts and environmentally sensitive optical epoxy. The imager benefits primarily from improved vibration resistance, and could be mounted onto any number of manned or unmanned observation platforms. Operational longevity is also improved, as the coated sensor platform is lighter and smaller than existing multispectral tools. Applications in the visible wavelengths include crop inspection, illicit drug enforcement, and a wide variety of biomedical research and clinical tools. Each application relies on the improved image contrast produced from spectral differentiation, as well as rapid image acquisition.
For all multispectral imaging, spectral channels are highly application-specific and may be functionally defined by use of a flexible platform (interchangeable filter channels) such as the Pixelteq SpectroCam (see Figure 3). A siliconbased imaging sensor, for example, may be customized to acquire three specific visible wavelength regions, as well as a near infrared channel for a particular IR-emitting fluorophore.
Indocyanine green (ICG) is one such biologically relevant dye that has significant clinical and research applications due to it being FDA approved. ICG binds tightly to proteins in blood vessel walls and, as such, can be a very powerful tool for visualizing blood flow in a variety of biomedical use cases.
Applications within the security and defense space extend the spectral range from the visible and near infrared (NIR) into the short-wave infrared (SWIR) band of 900nm-1.7 microns (see Figure 4). The applications range from color night-vision to aerial surveillance and remote detection. Simultaneous acquisition of multiple spectral channels across the SWIR wavelength range is a unique ability facilitated by depositing selective bandpass filters at the pixel level on a SWIR (InGaAs) sensor. Additionally, multispectral imaging can prove a powerful tool in authentication applications, allowing innovative new methods of analysis for documents, artwork, antiquities, currency, pharmaceuticals, uniforms, and many others.
In the past few years, multispectral imaging has developed a great deal as a technique. For most applications, wavebands of interest can be narrowed to less than ten, which makes data acquisition and analysis much more manageable than with hyperspectral systems. Current innovations in coating techniques allow deposition of filters down to the single pixel level, and thusly, high-speed imaging simultaneously using multiple specific spectral channels is born. With proper selection of application-specific filters, pixel-scale patterned sensors will deliver multispectral imaging to many new users.
This article was written by Steve Smith, PhD, Product Manager, PIXELTEQ (Golden, CO). For more information visit http://info.hotims.com/45604-141.