Image sensor including real-time automatic exposure control and swallowable pill including the same
An imager and a method for real-time, non-destructive monitoring of light incident on imager pixels during their exposure to light. Real-time or present pixel signals, which are indicative of present illumination on the pixels, are compared to a reference signal during the exposure. Adjustments, if necessary, are made to programmable parameters such as gain and/or exposure time to automatically control the imager's exposure to the light. In a preferred exemplary embodiment, only a selected number of pixels are monitored for exposure control as opposed to monitoring the entire pixel array.
1. An image sensor comprising:
an array of imaging pixels arranged in a plurality of rows and columns;
a circuit for determining the amount of light received by the array of imaging pixels during an exposure period, the determining circuit non-destructively reading pixel signals from a set of imager pixels of the array which is less than the total number of imaging pixels in the array; and
a circuit for performing at least one of controlling the gain of pixel signals read from the array, and controlling the illumination period of a light source for illuminating a scene imaged by the array, in response to the determining circuit,
wherein the determining circuit compares each of the non-destructively read pixel signals to a reference signal and provides an output indicating whether a read pixel signal has a value which exceeds the reference signal, and the performing circuit performing the at least one of controlling the gain of the read pixel signal and controlling the illumination period of light source when the number of the read pixel signals which exceed the reference signal is greater than a predetermined value and wherein the reference signal is adjusted by a reference voltage generator to compensate for supply voltage variations occurring during an exposure period and pixel readout.
2. An image sensor as in claim 1 , wherein the determining circuit comprises a plurality of comparators, each coupled to at least one respective column of the array for reading imager pixels from the set, each of which compares a non-destructively read pixel signal to the reference signal to provide an output indicating whether a read pixel signal has a value which exceeds the reference signal.
3. An image sensor as in claim 2 , wherein each comparator can be selectively coupled to one of a plurality of columns of the array.
4. An image sensor as in claim 3 , wherein the comparators are operated in consecutive phases, wherein in a first phase the comparators are respectively connected to a first subset of columns to which the comparators can be connected and in a second phase the comparators are respectively connected to a second subset of columns to which the comparators can be connected, the second subset being different from the first.
5. An image sensor as in claim 4 , wherein in one of the first and second phases, positive and negative inputs to the comparators respectively receive input from a column and the reference signal and provide an output at a first logical output of the comparator and in a second one of the first and second phases positive and negative inputs to the comparators respectively receive the reference signal and input from a column and provide an output at a second logical output of the comparators which is complementary to the first logical output.
6. An image sensor as in claim 2 , wherein the determining circuit further comprises a circuit for detecting the number of pixels in the set which have a value which exceeds the reference value.
7. An image sensor as in claim 6 , wherein the determining circuit further comprises a counter for counting the number of pixels in the set which exceed the reference value.
8. An image sensor as in claim 1 , wherein the performing circuit controls the gain of pixel signals read from the array.
9. An image sensor as in claim 1 , wherein the performing circuit controls the illumination period of the light source.
10. An image sensor as in claim 1 , wherein the performing circuit controls the gain of pixel signals read from the array and the illumination period of the light source.
11. An image sensor as in claim 1 , wherein the reference voltage generator sets an initial value of the reference signal at the beginning of the exposure period and adjusts the value of reference signal during the exposure period.
12. An image sensor as in claim 11 , wherein the initial value of the reference signal is a full scale signal for a pixel value divided by a maximum gain which can be applied to a pixel signal.
13. An image sensor as in claim 12 , wherein the adjusted value of the reference signal can be to a value of the full scale signal or a value of the full scale signal divided by a gain value which is less than the maximum gain.
14. An image sensor as in claim 1 , wherein the gain is controlled to be one of a plurality of possible values.
15. An image sensor as in claim 14 , wherein the gain is controlled to be at least one of a minimum gain, a maximum gain and an intermediate gain between the minimum and maximum gain.
16. An image sensor as in claim 1 , wherein the determining circuit operates in a time period which is no greater than the exposure period.
17. An image sensor as in claim 1 , wherein the reference voltage generator determines a mean reset value (V rst, mean ) from a set of dummy pixels prior to the beginning of the exposure period and determines a full scale level (V fs ) from a voltage supply source and generates the reference signal (V ref ) using (V rst , mean) and (V fs ).
18. An image sensor as in claim 17 , wherein the reference voltage generator generates the reference signal (V ref ) as (V ref =V rst , mean−V fs /x), where x is a gain value.
19. An image sensor as in claim 1 , wherein the imaging device is part of a swallowable pill.