Integrated High Resolution Focal-Plane Polarization Imager

The polarimetric vector is a more general descriptor of light than intensity information alone, and it contains physical information about the imaged objects in a scene that traditional intensity based sensors ignore. Polarimeters – devices that measure polarization – are used to extract physical features from an image such as specularities, occluding contours, and material properties. Scientists use polarization information to perform difficult tasks such as image segmentation and surface reconstruction, object orientation, material classification, atmospheric and solar analysis. We will present an integrated CMOS sensor/imager that uses a unique polymer-based polarizing filter to sense two orthogonal directions of linear polarization. The CMOS imager uses analog translinear circuitry to compute, in real-time on the focal-plane, polarization contrast: a measure of the orientation and degree of linear polarization in an imaged scene. We will present the microfabrication technique that enables us to apply CMOS fabrication technology to bulk manufactured poly(vinyl alcohol) linearly polarizing films. This technique allows us to define lithographically micron-scale linearly polarizing regions in polymer-based polarizing filters in order to make a high resolution polarization contrast imager. POLARIZATION CONTRAST The electric field (e-vector) of electromagnetic waves can be expressed as the superposition of two orthogonal components, Ex and Ey, and can be written as E → = ˆ xExe − j (ωt − kz) + ˆ yEye − j ( ωt − kz + φ) , (1) where ω is angular frequency, k is the wave-vector, and φ is the phase between the orthogonal electric field components. If the phase is deterministic and φ=0, light is linearly polarized; in the case of φ≠0, light is elliptically polarized. While polarized light sources are rare in nature, reflected or scattered light is almost always partially linearly polarized, which means that it has an unpolarized component and a linearly polarized component. Intensity based imagers, such as CCD cameras and Active Pixel Sensors[1] cannot distinguish between polarized and non-polarized light, and therefore lump the orthogonal e-vector components into one intensity term by summing the energies of Ex and Ey. Our polarization contrast imager analyzes the orthogonal electric field components of incident light at the pixel level and can image temporal and spatial polarization changes in a scene. Since the degree and orientation of polarization of reflected or scattered light is intimately related to the physical properties of the reflecting or scattering object (especially its surface properties, index of refraction, and spatial orientation), our polarization imager can offer physical cues which intensity or color imagers neglect. Below, we will show how polarization information can be used to obtain shape information in an imaged scene. This type of operation is important in the field of image understanding and object recognition.[2][3][4] We define polarization contrast using the following equation: Figure 1 . Lithography on the polarizing polymer film poly(vinyl alcohol) (PVA). a) Cross-section of a typical dichroic PVA polarizer, b) Acetone is used to dissolve the acetate layer on one side to expose dichroic PVA, c) Photoresist is spun on to PVA, d) Photoresist is exposed to UV light through a chrome mask, e) As the photoresist is developed, iodine begins to dissolve from the PVA. After 1 minute in the developer solution, only a residual layer of unexposed photoresist remains, f) After 2 minutes in the developer, iodine is completely dissolved from the PVA, leaving a micron-scale pattern of polarizing and non-polarizing PVA. polarization contrast = TR⊥ − TR|| TR⊥ + TR|| , (2) where TR || and TR⊥ represent the transmitted radiances through orthogonal linear polarizers whose transmission axes are orthogonal relative to each other. Polarization contrast is a relative measure of the degree and orientation of linear polarization in a scene.[5] In the following sections, we describe how we extract orthogonal polarization components at the pixel level using specially processed linearly polarizing dichroic polymer film. We also describe a custom fabricated CMOS imager that uses translinear circuits to compute polarization contrast. PVA FILM LITHOGRAPHY Linearly polarizing films – iodine doped poly(vinyl alcohol) (PVA) dichroic films in particular – are manufactured through a bulk process that is not amenable to delineating micron-scale features. The manufacturing process for a widely used polarizing film, the H-sheet, dissolves potassium-iodine solution into a sheet of poly(vinyl alcohol) (PVA).[6] When the iodine-doped PVA is stretched, the polymer chains of PVA are axially coordinated along the stretching direction. Stretching of the PVA film causes an abundance of polyiodine complexes of I3 and I5 to form in linear chains that lie parallel to the polymer molecules. These conducting complexes form a light-absorbing axis in the entire film along the stretching direction, imparting dichroism to the otherwise transparent polymer sheet. We have developed a lithographic process to fabricate a linearly polarizing focal-plane filter suitable for a high resolution CMOS imager using commercially available, bulk-manufactured PVA films. The technique uses masking and ‘etching’ steps similar to those in VLSI silicon fabrication processes to undope selectively the iodine complexes which impart dichroism to the otherwise transparent PVA. Figure 2. Photomicrograph of focal-plane polarizing filter. Two linearly polarizing polymer (PVA) films with alternating transparent and polarizing lines are aligned to form one focal-plane polarizer filter. This photomicrograph shows two views of the same filter with the microscope polarizer set at 0o (left) and 90o (right). The white dot is provided as a stationary point of reference.