The invention relates to an optical sensor consisting of an arrangement of pixel units, each comprising an optoelectronic converter for converting the incident radiation into a photoelectric current that depends on intensity and wavelength, an integrator means for deriving a measured value corresponding to the photoelectric current detected, and a controllable storage means for storing the measured value, and a readout control device for readout of the stored measured values relative to one pixel unit, wherein the image striking the sensor can be composed of the measured values based on pixel units.
The function of optical sensors is to pick up an image scene and convert it to electric signals so that multiple image segments are generated, differing in weighting of the various spectral components emitted by the scene. For the visible spectral range, the break-down into the blue, green and red spectral ranges has proven especially advantageous, because this spectral weighting permits a colored reproduction of image scenes adapted to the human eye.
Known image sensors with color resolution (A. J. P. Theuwissen, Solid-State Imaging with Charge-Coupled Devices, Kluwer Academic Publishers, Dordrecht, chapter 6.2, p. 2, pp. 165 ff., 1995) are characterized in that more than one signal must be detected for each pixel. As a rule, a three-way division is performed according to the three color components red, green and blue. Only these color signals, taken together, yield the complete color information of a pixel which can be reproduced from the three linearly independent individual signals within the context of a signal transmission link.
Technically, there are multiple possibilities of implementing the aforementioned color resolution. The methods of generating the spectrally weighted image segments can be subdivided roughly into two types. The first,type of such color imaging methods (A. J. P. Theuwissen, Solid-State Imaging with Charge-Coupled Devices, Kluwer Academic Publishers, Dordrecht, chapter 6.2.4, pp. 171 ff., 1995) uses multiple image sensors whose spectral sensitivities differ by the introduction of color filters into the respective beam paths of the individual sensors. This method has the disadvantage that the individual beam paths must be adjusted so that they lead to completely identical images of the scene on the image sensors, because otherwise the reconstructed component color images cannot be combined so that they coincide, which leads to color shifts.
The second type of color imaging method (A. J. P. Theuwissen, Solid-State Imaging with Charge-Coupled Devices, Kluwer Academic Publishers, Dordrecht, chapter 6.2.2, pp. 168 ff., 1995) uses special color image sensors where adjacent pixels for different spectral ranges are made sensitive by applying multiple color filters in a mosaic. Although the component color images of these sensors are thus automatically congruent to one another in this way, the effective resolution with this method drops by a factor of three to four, because three or four adjacent sensor pixels are needed to extract the complete color information of a pixel.
Furthermore, it is also possible to combine the types described previously (A. J. P. Theuwissen, Solid-State Imaging with Charge-Coupled Devices, Kluwer Academic Publishers, Dordrecht, chapter 6.2.4, pp. 171 ff., 1995), where one spectral range, for example, the green component, is recorded by one sensor, while the other spectral ranges, namely the red and blue components in this example, are recorded by a mosaic two-color sensor. This compromise has the disadvantages of both of the basic.types, although to a diminished extent.
An alternative to this is formed by sequential recording of three image segments, representing the red, green and blue components, respectively, of the light assigned to the pixel. The complete image must be read out immediately after exposure of an image segment. Meanwhile, the three component color images are separated from one another in time by the unavoidable readout operations. The duration of the readout operations depends on the number of pixels and consequently on the lateral resolution of the image sensor, so that following an increase in resolution and number of pixels, an increase in readout time can also be expected in this regard. The readout cycles inserted between exposures of component color images consequently make it impossible to record scenes in which there is movement, because then the three image segments no longer coincide. Moreover, this mode of operation of a color sensor does not allow a color image to be recorded within the flash time available for an exposure with a flash apparatus.
The object of the invention is to create an optical sensor which permits operation within a short exposure time.
The problem is solved according to this invention by the fact that the optoelectronic converter can be controlled selectively with regard to its spectral sensitivity and that each pixel unit comprises an integrator means and at least two storage means arranged in parallel, such that at least two measured values assigned to different spectral ranges of the incident radiation can be detected and stored during the measurement period and then read out together to form the relevant color information for the pixel element.
According to this invention, each of the pixels of a color sensor is equipped with multiple storage cells which permit temporary storage of the color components. In this way, multiple image segments corresponding to the individual color components can be recorded in succession without having to read out the information of the complete image sensor after exposure of an image segment. The time-consuming readout operations can now be shifted to the end of the entire exposure cycle, consisting of multiple successive individual exposures.
The solution achieved according to this invention thus creates an integrating imaging optical color-sensitive image sensor, where each pixel can record the complete color information according to the light striking it and can make it available in electronic form in each subsequent image cycle. The sensor is characterized by two essential properties: each pixel of the sensor structure, which is organized in a matrix, has a plurality of information storage devices to permit temporary storage of the different color information. In addition, the sensor contains controllable detector elements whose spectral sensitivity can be varied. An image sensor according to this invention thus makes it possible to record all the color information of an image scene by immediate successive exposure of the individual color components without requiring a readout operation between the individual exposures. In this way, short exposure times can be implemented with undiminished resolution at the same time. In particular, the present sensor structure is suitable for recording color images within a single exposure cycle of a short-term lighting system (flash apparatus), for example.
A complete image cycle using a color image sensor according to this invention consists of the following steps, for example:
1. Exposure of the first individual image (e.g., red).
2. Storage of the first individual image.
3. Exposure of the second individual image (e.g., green).
4. Storage of the second individual image.
5. Exposure of the third individual image (e.g., blue).
6. Storage of the third individual image.
7. Readout of the first individual image.
8. Readout of the second individual image.
9. Readout of the third individual image.
Essentially any sequence is possible for recording and storage of the measured values belonging to the individual images and their readout. The individual images may also be exposed and stored simultaneously.
The important advantage of a color sensor that operates according to this mode of operation in comparison with a mode of operation wherein the individual exposures are each interrupted by readout of the individual images is the short total exposure time, which includes only processes numbered 1 through 5 in the above list, while the time-consuming readout processes 7 through 9 are not counted as the actual exposure.
The method is suitable in particular for recording color images using a short-term lighting system, such as a flash apparatus, wherein the total exposure can take place within a single lighting cycle. As a rule, there is no disadvantage to shifting all the readout processes to the end of a complete image cycle.
Readout of the different color information need not necessarily take place in chronological order, but instead may also take place simultaneously with a suitable configuration of the sensor in the form of multiple parallel readout lines, thereby shortening the duration of the readout operation.
A preferred embodiment of a color image sensor according to the invention proposes implementation of the sensor function described above within an optical sensor system in thin film on ASIC (TFA) technology. A TFA image sensor consists of an optically active detector layer in the form of a thin film structure which is integrated vertically into an integrated circuit, such as an ASIC (application-specific integrated circuit). The ASIC contains the pixel structure organized into a matrix, including the required pixel circuits for integration of the photoelectric current, for storage of the color signals and for readout thereof. The optical detector consists of a multi-layer system based on amorphous silicon, which has a spectral sensitivity that can be controlled with the help of the applied electric voltage.
The optical sensor preferably has the following layer structure:
(a) an n-conducting a-Si:H (amorphous hydrogeneated silicon) layer,
(b) a first intrinsically conducting a-Si:H layer,
(c) a p-conducting a-Si:H layer,
(d) a second intrinsically conducting a-Si:H layer, consisting of a first component layer with a higher xcexcTau product the product of the movement xcexc and the lifetime Tau of the charge carrier and a second component layer with a lower xcexc-Tau product in comparison with the first component layer,
(e) an n-conducting a-Si:H layer.
According to the nipin structure described above, a pinip structure can also be produced by exchanging n- and p-conducting layers.
Especially good suitability for the color image sensor described here for short-term exposure is obtained when the following layer structure is used:
(a) a p-conducting a-Si:H layer,
(b) an intrinsically conducting a-Si:H layer, consisting of three component layers (I, II, III) with different dielectric constants, where component layers having a low dielectric constant precede component layers having a higher dielectric constant in the direction of incidence of light,
(c) an n-conducting a-Si:H layer.
It is also especially suitable for the color image sensor described here for short-term exposure when the following layer structure is used:
(a) a p-conducting a-Si:H layer,
(b) an intrinsically conducting a-Si:H layer, consisting of three component layers having different dielectric constants, where component layers having a low dielectric constant precede component layers having a higher dielectric constant in the direction of incidence of light,
(c) a p-conducting a-Si:H layer.
This yields a so-called pi3n or ni3p structure.
Operation of the sensor involves applying the various detector voltages, which lead to the different color sensitivities, in succession, and integrating into the ASIC the resulting photoelectric currents, which are a measure of the different color components of the light striking the respective pixel, and depositing them in corresponding storage cells. For example, the photoelectric currents that are established with three linearly independent color sensitivities (e.g., red, green, blue) may be integrated and stored in three analog storage cells. Each of the storage cells may consist, for example, of a capacitor in which integration of the photoelectric current over time generates a proportional voltage which is then stored in said capacitor until readout of the pixel. The three capacitors may be arranged laterally (side by side) or vertically (one above the other) in the pixel. In this connection, gate capacitors of the MOS transistors or capacitors between different metallization levels of an ASIC, for example, may be used as storage cells. The ASIC technology thus makes it possible to implement multiple storage cells vertically one above the other, so that especially space-saving configurations can be achieved. The important advantage of such an embodiment is that, in contrast with multiple laterally arranged photodetectors, the storage capacitors can be miniaturized without any loss of sensor sensitivity, so that in this way it is in principle possible to achieve a greater resolution without any sacrifice with regard to the dynamic extent. Meanwhile, the entire sensor surface can be utilized optically, because the storage cells are integrated vertically and do not entail any loss with regard to the area filling factor.
The capacitors also serve to integrate the photoelectric current at the same time, i.e., to convert it into a voltage signal, and to store this voltage signal. In addition to being dependent on the photoelectric current, as described above, the voltage level is proportional to integration time and inversely proportional to integration capacity. Different gains can be achieved in the individual color channels through the use of capacitors with different capacities for integration of the three photoelectric current signals and/or through a choice of different integration times, to compensate for any differences in color sensitivity that might be inherent in the detector.
In principle, through a suitable design of the pixel circuits, it is possible for the photoelectric currents to be converted in the sensor pixel into digital signals which are then stored digitally in the respective storage cells.
In the embodiment of a color image sensor in TFA technology, the photoelectric current signals corresponding to the color components are integrated and stored in multiple storage cells that are integrated laterally or vertically into each pixel of the ASIC, whereas the photoelectric currents themselves are generated with the help of a color-selective and electrically controllable optical detector in thin film technology applied vertically to the ASIC.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that all pixels are exposed simultaneously with respect to each of the color components. In comparison with an exposure slit traveling over the image sensor, this method has the advantage that there is no distortion of the image due to non-synchronous exposure of the individual color images.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that it is possible to select a mode of operation wherein the total exposure time for all individual images is so short that a complete color image is exposed within a single lighting phase of a short-term lighting system. In addition, the possibility of using multiple, individual, chronologically successive, colored flashes in synchronization with the exposure of the individual images is not ruled out, and the option of controlling the color sensitivity of the optical detector may be omitted in this case. Furthermore, it is also possible to use controllable optical filter layers which vary the spectral component striking the sensor, again in synchronization with the exposure of the individual images. Again in this case, it is not absolutely necessary to be able to control the color sensitivity of the optical detector.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that lighting is provided by a known so-called linear flash which has flash times of up to 20 ms with an intensity that is mostly constant over time. This method is suitable in particular when longer exposure times are required due to transient effects with regard to color detectors.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that in addition to the light- and color-sensitive detectors, the sensor also has incorporated into it other radiation-sensitive elements which have a sensitivity with regard to the infrared or ultraviolet spectral range, for example. In this case, additional storage elements in which are stored the detected signals corresponding to the spectral components outside the visible range may also be integrated. With regard to the mode of operation of the sensor as well as the chronological sequence of integration, storage and readout, the same schematics also apply accordingly as for a sensor which is sensitive only in the visible range.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that the individual color channels have a selective gain. In this way, different color sensitivities of the detector can be compensated. Selective gain can be achieved, for example, by appropriate dimensioning of the integration and storage capacities or by selecting different integration times in integration of the photoelectric currents.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that a color correction can be performed, for example, by the known measure of a linear linkage in the form of weighted addition of color signals. In this way, the spectral selectivity of the individual color channels can be improved so that color distortion can be reduced. Appropriate electronic circuits for color correction can be accommodated within the pixel electronics or in the peripheral electronics of the color image sensor.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that the number of readout lines matches the number of color memories provided in each pixel, so that a synchronous readout of all color information can be performed with the goal of shortening the total required readout time.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that the integrator device is provided with a period-controlling device for determining the end point of the integration period such that higher radiation intensities lead to shorter integration periods and lower radiation intensities lead to longer integration periods. To increase the dynamic extent, a so-called global or local-adaptive or local-autoadaptive control of sensitivity is utilized, according to which the sensitivity of each pixel is adapted automatically or otherwise to the image brightness prevailing locally. With respect to the use of this method in color sensors, it should be pointed out that sensitivity must be controlled separately for each of the different color components, with additional storage elements optionally being provided in the sensor pixels.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that the color image sensor is designed as a line sensor or as a two-dimensional pixel matrix. In the case of a linear sensor structure in the form of a line sensor, very small pixel sizes (for example, 10xc3x9710 xcexcm2) and pixel spacings (for example, 2 xcexcm) may be achieved by using the technologies currently available, because the pixel electronics can also be positioned next to the optically active areas.
Another advantageous embodiment of the color image sensor according to this invention consists of the fact that additional optical elements are integrated into the light-sensitive layer. For example, an arrangement of micro-lenses structured in a pixel grid system may be used for optical input of the beam into the sensor without requiring any additional expensive imaging optical systems. In this way, it is possible to implement compact imaging color sensors with integrated imaging optical systems (facet eye) which are suitable for use as retinal implants (artificial retina) in the human eye.
Another preferred embodiment provides for each storage means a comparator device in which a reference signal for a preselected color can be deposited. This variant makes it possible to achieve a so-called xe2x80x9ccolor trackerxe2x80x9d where a certain color, e.g., a certain red color value, is preselected as the reference value. If this color value then appears in the image recorded, this is ascertained by the comparator device which can then issue the proper signal.
Another advantageous embodiment of the device described above is illustrated in the following figures and explained in greater detail. The embodiment is a pixel for short-term exposure according to this invention in a TFA technology implementation.