The pixel-scale patterning of optical filters directly onto sensors has the potential to drastically simplify and scale down the cost of real-time multispectral imaging. Without moving parts, a more robust and compact system can be deployed into harsher environments than have been previously accessible for multispectral imagers.
Current multispectral imaging systems provide data about specific spectral regions. Similar to the common spectrometer, information about an object or scene can be obtained across UV-VIS-IR wavelengths. Imaging techniques, however, offer decided advantages over point spectral readings because of the ability to utilize the full capacity of an advanced imaging sensor. There is a broad range of applications where multispectral imaging and sensing techniques are utilized, including agriculture, machine vision/sorting, chemical and biological detection, art conservation, art and document authentication, and biomedical sciences (see Figure 1). The primary drawback with current systems is that they are not as portable or compact as would be desired for many applications. With patterned pixel sensors, those concerns can be addressed in a more cost-effective package.
Dielectric filters allow for unparalleled flexibility in spectral selection. A dielectric filter that passes only a narrow band of red laser light while blocking the rest of the visible spectrum, for example, is just as feasible to fabricate as one that passes blue starlight from hot stars while blocking light pollution from broad-spectrum home lighting. The most commonly encountered dielectric filter is the anti-reflective (AR) coating found in vision-correction glasses, home windows, and cell-phone cameras. For cameras and glasses, clarity is improved by eliminating glare. Cameras also acquire images more quickly, as more light reaches the sensor. Home windows can be coated to reflect the invisible infrared radiation that brings unwanted heat, while allowing more visible light through. Additionally, all of these coatings can be scratch- and chip-resistant when made with hardened materials. More complex coatings produce the assortment of application-specific filters found in biological imaging, chemical identification, and laser physics, ranging from filters passing only light produced from green fluorescent proteins to the specialized laser-line selectors (see Figure 2).
With dielectric filters placed prior to cameras, inexpensive monochrome imaging sensors can now be “sensitized” to only specific spectral bands allowed through the filter. Traditionally, dielectric filters are deposited onto a rigid, transparent substrate like glass or hard thermoplastics. Dielectric filters operate on the principle of interference: Light travels through media as waves, and by adjusting the phase of a reflected wave relative to its forward traveling counterpart, the amplitude of a specific frequency (or wavelength) at the output of the filter is modulated. The filter material is deposited in sub-wavelength stacks, the composition and arrangement of which dictate the spectral transmission properties. The aforementioned AR coating stack results in constructive interference across the visible spectrum. An improved, averaged index gradient between air and glass lessens the discontinuity in the refractive index, reducing the amount of light reflected at the air-glass interface. Spectral bands can be likewise eliminated by tuning the stack for destructive interference across the desired spectral subset.
Ultimately, dielectric filters improve imaging contrast by rejecting light outside of the spectral band(s) of interest, as well as improve the transmission of light within the band of interest. The transmission improvement is only possible with dielectric filters, and not with the dye filters typically employed in the ubiquitous RGB sensor.
Deposition on Sensors
In principle, depositing interference filters onto substrates other than glass only requires accounting for the different refractive index of the substrate. Of particular interest is the direct application of a dielectric filter to the surface of an active device. Coated glass can be utilized as well, and placed in front of the sensor. Direct application, however, has a significant advantage in that there is one less optical surface in the light path. Additionally, internal Pixelteq researchers have calculated a 3-10% theoretical increase in light throughput to the sensor when the filter is deposited directly, compared to being on intermediary glass. The features of the device that make it active — such as bond pads on photodiodes — are the very source of the challenge in successfully applying a patterned filter onto a sensor. These sensitive areas must be protected from the bombardment of high-energy atoms in a coating chamber.