This invention relates generally to techniques and apparatus for image sensing, and more particularly to a method and apparatus for obtaining a digital measurement of a photo-current.
A major problem in imaging is the limited dynamic range of image sensor arrays. As a result, some portions of the image may be saturated while other portions are under exposed. A typical photodiode optical detector, such a charge-coupled device (CCD), consists of an array of pn junction photodiodes. Each photodiode has a capacitance associated with it, and when light falls on the detector the resulting photo-current charges the capacitance. The charge is thus the time integral of the light intensity falling on the detector. The CCD periodically and sequentially switches the charge to a video line, resulting in a series of pulses, which can be converted into a voltage signal representing the light pattern incident on the array. If the integration time is too long, the device will saturate. If the integration time is too short, the voltage signal will be lost in the noise of the device.
One approach to adjusting sensitivity is therefore to adjust the integration time according to the intensity of light falling on the array. This can be done globally for the whole array by adjusting the switching rate, for example. This allows the array to adjust for various light levels (as in adjusting the exposure time of an analog camera), but does not increase the dynamic range within a single image.
Another approach is to measure the time taken for the capacitor voltage to reach a reference level. An example of such a system is given in xe2x80x9cIntensity Mappings Within The Context Of Near-Sensor Image Processingxe2x80x9d, by Anders xc3x85strxc3x6m, Robert Forchheimer and Per-Erik Danielsson, IEEE Transactions on Image Processing, Vol. 1, No. 12, December 1998. A large time indicates a low intensity of light falling on the array, while a short time indicates a high intensity. The voltage reference level is common to the whole array, but may be adjusted according to the overall intensity falling on the array. This approach is computationally intensive, since the integration time is measured by interrogating the output from a comparator at predetermined intervals. These intervals need to be very close together for accurate measurement. This approach also requires each pixel to have a readout circuit. In addition, each sensor element requires a precharge transistor.
A further device called a xe2x80x9cLocally Autoadaptive TFA Sensorxe2x80x9d is described in xe2x80x9cHigh Dynamic Range Image Sensors In Thin Film On Asic Technology For Automotive Applicationsxe2x80x9d, by M. Bxc3x6hm et al., in Advanced Microsystems for Automotive Applications, D. E. Ricken, W. Gessner (Eds), Springer Verlag, Berlin, pp 157-172, 1998. In this device, the integration time is adapted for each individual pixel according to the local illumination intensity. The capacitor voltage is compared after an initial integration time to a reference voltage that is slightly less than half of the saturation voltage. If the voltage is less than the reference voltage, the integration time is doubled. This process is repeated until the reference voltage is exceeded. Thus, saturation of the capacitor is avoided and the integration time is extended to avoid under exposure. The final voltage and the integration time are together used to determine the illumination intensity. This approach requires circuitry to compare and adjust the integration time and circuitry to measure the voltage at the end of the integration time.
In light of the foregoing discussion, it can be seen that there is still a need in the art to have image sensor arrays resulting in an image representative of the intensity distribution of an object such that portions of the image are not saturated or underexposed.