In the past years we conceived and developed the concept of spectrally tunable direct color sensors, based on the principle of the Transverse Field Detector. In this work we analyze the performance of a 64x64 (x3 colors) matrix of such a sensor built in a 150-nm CMOS standard technology for demonstrative purposes.
The matrix is mounted on an electronic board that provides the biasing; the board is inserted into a suitably arranged film back slot (magazine) of a Hasselblad 500C camera. The camera is aligned in front of a transparency Macbeth Color Checker uniformly illuminated by an integrating sphere.
Electrical, noise and colorimetric performance are the object of the ongoing analysis. In particular, the color reconstruction results will be compared to those obtainable through large-area devices based on the same concept.
They are here reported the developments and experimental results of fully operating matrices of spectrally tunable pixels
based on the Transverse Field Detector (TFD). Unlike several digital imaging sensors based on color filter arrays or
layered junctions, the TFD has the peculiar feature of having electrically tunable spectral sensitivities. In this way the
sensor color space is not fixed a priori but can be real-time adjusted, e.g. for a better adaptation to the scene content or
for multispectral capture. These advantages come at the cost of an increased complexity both for the photosensitive
elements and for the readout electronics. The challenges in the realization of a matrix of TFD pixels are analyzed in this
work. First experimental results on an 8x8 (x 3 colors) and on a 64x64 (x 3 colors) matrix will be presented and analyzed
in terms of colorimetric and noise performance, and compared to simulation predictions.
The Transverse Field Detector (TFD), a filter-less and tunable color sensitive pixel, is based on the generation of specific
electric field configurations within a depleted Silicon volume. Each field configuration determines a set of three or more
spectral responses that can be used for direct color acquisition at each pixel position. In order to avoid unpredictable
changes of the electric field configuration during the single image capture, a specific active pixel (AP) has been
designed. In this AP the dark- and photo-generated charge is not integrated directly on the junction capacitance, but, for
each color, it is integrated on the feedback capacitance of a single-transistor charge pre-amplifier. The AP further
includes a bias transistor, a reset transistor and a follower. In this work the design of such a pixel is discussed and the
experimental results obtained on a 2x2 matrix of these active pixels are analyzed in terms of spectral response, linearity,
noise, dynamic range and repeatability.
The Transverse Field Detector (TFD) is a filter-less and demosaicking-less color sensitive device that easily allows the
design of more than three color acquisition channels at each pixel site. The separation of light into different wavelength
bands is based on the generation of transverse electric fields inside the device depleted region, and exploits the properties
of the Silicon absorption coefficient. In this work we propose such a device for the joint capture of visible and near
infrared (NIR) radiation, for possible applications in videoconferencing and 3D imaging. In these applications the
detector is used in combination with suitably generated NIR structured light. The information of the fourth acquisition
channel, mainly capturing NIR signals, can be used both for sampling NIR light intensity and for subtracting unwanted
NIR crosstalk from visible channels thus avoiding the need for the IR-blocking filter. Together with the presentation of a
4-channel sensor, a suitable algorithm for the processing of signals generated in the visible and infrared bands is
described. The goal of the algorithm is to minimize the crosstalk of NIR radiation inside the visible channels and,
simultaneously, to maintain good color reproduction and noise performance for the sensor, while holding a good
sensitivity of the NIR channel up to 900 nm. The analysis indicates that the algorithm reduces the crosstalk of infrared
signals inside R, G and B channels from 31%, 12% and 5% respectively to less than 2%. Concerning noise propagation,
the worst coefficient of the color conversion matrix (CCM) is -2.1, comparable to those obtained for CCM of Bayer
Color Filter Arrays.
In this work the use of the Transverse Field Detector (TFD) as a device for multispectral image acquisition is proposed.
The TFD is a color imaging pixel capable of color reconstruction without color filters. Its basic working principle is
based on the generation of a suitable electric field configuration inside a Silicon depleted region by means of biasing
voltages applied to surface contacts. With respect to previously proposed methods for performing multispectral capture,
the TFD has a unique characteristic of electrically tunable spectral responses. This feature allows capturing an image
with different sets of spectral responses (RGB, R'G'B', and so on) simply by tuning the device biasing voltages in
multiple captures. In this way no hardware complexity (no external filter wheels or varying sources) is added with
respect to a colorimetric device. The estimation of the spectral reflectance of the area imaged by a TFD pixel is based in
this work on a linear combination of six eigenfunctions. It is shown that a spectral reconstruction can be obtained either
(1) using two subsequent image captures that generate six TFD spectral responses or (2) using a new asymmetric biasing
scheme, which allows the implementation of five spectral responses for each TFD pixel site in a single configuration,
definitely allowing one-shot multispectral imaging.
In a recent work, an interleaved sensor that parallels the human retina was proposed to enhance the capabilities to acquire images under very different lighting conditions. We present an implementation of that interleaved sensor based on the new transverse field detector, a CMOS photosensitive device for imaging applications that perform color detection without color filters. This implementation adds some advantages of the filterless detector in terms of color acquisition and spatial resolution in very high or very low lighting conditions. It also adds the flexibility to electronically reconfigure the interleaved sensor pattern, adapting it to the present light conditions.
A new method for White Balance, which compensates for changes in the illuminant spectrum by changing
accordingly the native chromatic reference system, is presented. A set of base color filters is selected in the
sensor, accordingly to the scene illuminant, in order to keep the chromatic components of a white object
independent from the illuminant. On the contrary, conventional white balance methods do not change the
native color space, but change the chromatic coordinates in order to adjust the white vector direction in the
same space. The development in the last ten years of CMOS color sensors for digital imaging whose color
reconstruction principle is based on the absorption properties of Silicon, rather than on the presence of color
filters, makes the new method applicable in a straightforward manner. An implementation of this method with
the Transverse Field Detector, a color pixel with electrically tunable spectral responses is discussed. The
experimental results show that this method is effective for scene illuminants ranging from the standard D75 to
the standard A (i.e. for scene correlated color temperature from 7500 K to 2850 K). The color reconstruction
error specific for each set of electrically selected filters, measured in a perceptive color space after the
subsequent color correction, doesn't change significantly in the tested tuning interval.
The Transverse Field Detector (TFD) is a recently proposed Silicon pixel device designed to perform color
imaging without the use of color filters.
The color detection principle is based on the dependence of the Silicon absorption coefficient from the
wavelength and relies on the generation of a suitable transverse electric field configuration, within the
semiconductor active layer, to drive photocarriers generated at different depths towards different collecting
electrodes. Each electrode has in this way a different spectral response with respect to the incoming
wavelength. Pixels with three or four different spectral responses can be implemented within ~ 6 μm of pixel
Thanks to the compatibility with standard triple well CMOS processes, the TFD can be used in an Active
Pixel Sensor exploiting a dedicated readout topology, based on a single transistor charge amplifier. The
overall APS electronics includes five transistors (5T) and a feedback capacitance, with a resulting overall fill
factor around 50%.
In this work the three colors and four colors TFD pixel simulations and implementations in a 90 nm standard
CMOS triple well technology are described. Details on the design of a TFD APS mini matrix are provided
and preliminary experimental results on four colors pixels are presented.