CMOS ACTIVE PIXEL SENSOR FOR A POLARIZATION-
NSF Summer Undergraduate Fellowship in Sensor Technologies
Gregory J. Barlow (Electrical Engineering) – North Carolina State University
Advisors: Dr. Nader Engheta and Dr. Jan Van der Spiegel
Polarization-sensitive vision is well documented as serving in navigation for many animals, but some types of biological polarization-sensitive vision may enhance object visibility in scattering media. Because neither the human eye nor conventional cameras are polarization-sensitive, artificial polarization vision systems must be designed to exploit the polarization of light; artificial polarization-difference imaging has been shown to be capable of enhancing target detection in scattering media. Previous polarization-sensitive cameras required external processing, were not real-time, and used relatively large amounts of power. A CMOS active pixel sensor is presented for use in a low power, portable, real-time polarization-difference camera. Pixels were designed for integration with a diffractive optical element polarization analyzer. Column readout circuits include fixed pattern noise suppression. In addition, a scaling methodology to enhance system performance and to correct for non-ideal polarization analyzers is presented.
Humans rely heavily on their visual systems to understand the world around them. Human vision is based on brightness and color, which are extremely efficient under normal conditions.  In optically scattering media, however, such as underwater, in fog, or in rain, variations of brightness and color are low, diminishing object contrast and lowering the effectiveness of the visual system.  While polarization-sensitive vision is well documented as serving in many invertebrate navigation systems, Rowe et al. hypothesize that the green sunfish uses polarization vision to enhance target detection underwater.  Artificial polarization-sensitive vision has been shown to enhance the visibility of target objects in scattering media, using a method called polarization-difference imaging (PDI), inspired by the visual system of the green sunfish. 
Polarization-sensitive cameras have been previously demonstrated, but have not been designed to operate at full video rates. [4,5,6,7] In addition, these schemata require the use of significant external computing resources, limiting the portability of the system. Nor have these cameras been designed for low power use. The design of a low power, portable, real-time polarization-sensitive electronic camera-on-a-chip is desirable for use in a variety of applications.
This paper covers a variety of areas related to the design and implementation of a polarization-difference camera. Section 2 gives a brief review of polarization and PDI, background on electronic cameras and diffractive optical elements (DOE), and
information on charged coupled devices and CMOS pixels. Section 3 covers the overall design of the camera. Section 4 addresses the design and layout of the CMOS active pixel sensor (APS) used in this camera, with specific consideration of the design constraints of a PDI camera. Section 5 contains the designs of the readout circuitry for the camera and methods of fixed pattern noise (FPN) reduction. Section 6 details the operation of the camera, with some specifications for control, timing, and drive systems. Section 7 addresses intensity scaling to correct for the non-ideal DOE polarization analyzer. Section 8 gives the simulation results for the designed circuits, while Section 9 contains discussion of results and project conclusions. Future work and recommendations are addressed in Section 10.
Light has three properties detectable by vision systems: intensity, wavelength, and polarization. While the human eye can perceive both intensity and wavelength, it is polarization-blind. For this reason, conventional electronic cameras are not designed to extract polarization information from a scene. 
Light is a transverse electromagnetic wave; its electric and magnetic fields are orthogonal to the direction of propagation. The path traced by the tip of the electric field as the wave propagates defines the polarization of light. Light sources, such as the sun, are usually randomly polarized, while the media that the light encounters tend to alter its polarization.
2.2 Biological Basis
Polarization vision has been extensively studied in many classes of invertebrates. Bees, ants, and other invertebrates use polarization for navigation.  While some vertebrates are capable of extracting polarization information from visible light, the physical mechanism is not as well understood as in invertebrate systems. The sensitivity of the green sunfish to variations in polarization led to the hypothesis that polarization was used to enhance underwater vision.  The potential for vision enhancement makes polarization sensitivity desirable to incorporate into electronic cameras.
2.3 Polarization-difference Imaging
PDI is one method of extracting polarization information from a scene. In this method, images of a scene are captured at two orthogonal linear polarizations. The pixel-by-pixel sum of the two images forms the polarization-sum (PS), and the pixel-by-pixel difference of the two images forms the polarization-difference (PD).  Color is used to map the PD into the visual realm. 
(i,j) and I(i,j), where (i,j) If the two image intensity distributions are symbolized as I12
represents the pixel location and I and Ihave orthogonal linear polarizations, then the 12
PS and PD are as follows:
I(i,j) = I(i,j) + I(i,j) (1a) PS12
I(i,j) = I(i,j) - I(i,j) (1b) PD12
The PS image is equivalent to a conventional image if the linear polarizer is ideal. For non-ideal linear polarizers, corrective scaling must be implemented. It should be noted that the PD image depends on the polarization axes. 
PDI is qualitatively better than conventional imaging for target detection in scattering media; detection enhancement has been demonstrated at observable degrees of polarization of less than 1%.  PDI is inherently capable of common-mode rejection for background light, which further enhances target detection. Because PDI only requires relatively simple computations, it is extremely suitable for use in a polarization sensitive electronic camera.
2.4 Polarization Camera
Previous polarization-sensitive cameras were not real-time, though near-video rates have been achieved. [4,5,6,7] Previous polarization-sensitive cameras have also required intensive external processing. PDI is a suitable method for use in a real-time electronic camera-on-a-chip which is polarization-sensitive.
A polarization-difference camera extends the functionality of a normal digital electronic camera. A conventional electronic camera generally consists of eight stages.  These are (1) optical collection of photons via a lens, (2) discrimination of photons, generally based on wavelength (3) detection of photons via a photodiode or photogate, (4) readout of detectors, (5) timing, control, and drive electronics, (6) signal processing electronics, including FPN, (7) analog to digital conversion, and (8) interface electronics. The order of these stages is not necessarily fixed; some signal processing may occur after analog to digital conversion.
2.5 Diffractive Optical Elements
A DOE is a pattern of microstructures that can transform light in a predetermined manner.  For example, a Fresnel zone lens DOE can be used to focus light. While the Fresnel zone lens requires a variation of the height of the surface, a sub-wavelength binary DOE can exhibit the same behavior.  Figure 1 shows images of both types of lenses. In addition, sub-wavelength binary DOEs are polarization selective, which is advantageous for this project. The DOE designed for this project focuses the incoming light along a line parallel to the grooves of the DOE. This DOE is polarization sensitive.
Figure 1 - (a) Fresnel zone lens and (b) sub-wavelength binary lens
2.6 Image Sensors
The imager technology used in an electronic camera is instrumental in determining the capabilities of the final system. Low noise, large array sizes, high frame rates, and power dissipation are preferred for a polarization-difference camera.
2.6.1 Charge-coupled Device Image Sensors
Charge-coupled device (CCD) technology, currently the most popular sensor technology, is capable of producing high-quality images.  Small, low-resolution CCD cameras are also relatively inexpensive. CCD technology's relative freedom from FPN is one of its most attractive characteristics. However, CCD-based systems often consume several watts of power, can be accessed only a single pixel at a time, and are difficult to integrate with processing circuitry.
2.6.2 CMOS Image Sensors
MOS image sensors were demonstrated in the 1960's, but work fell off with the introduction of the CCD, which displayed much less FPN than MOS sensors.  The need for smaller and less expensive imaging technology has led to a resurgence in the popularity of CMOS image sensors. CMOS imager technology also has the advantage of low power, random and row based pixel access, and easy integration with processing circuitry. There are three main approaches to CMOS pixels: passive pixels, photodiode APSs, and photogate APSs. APSs can be designed for operation in either voltage or current-mode.
2.6.3 CMOS Passive Pixel Sensors
The passive pixel sensor is very simple, consisting of a photodiode and a transfer transistor.  Passive pixel sensors have high quantum efficiency and extremely small pixel size; however, noise levels are quite high, and this pixel type does not scale well.
2.6.4 CMOS Active Pixel Sensors
An active pixel includes at least one active transistor within the pixel cell.  An active amplifier within the pixel helps to improve performance over that of the passive pixel, allowing larger arrays and faster readout speeds. Since the amplifier within a pixel draws power only when the pixel is being read out, power dissipation remains low. Many APSs have been designed, some with very high quality and extremely high frame rates. [13-23] Active pixels are either photodiode or photogate based; readout is either voltage-mode or current-mode.
2.6.5 Photodiode APS