Image sensor with built-in thermometer for global black level calibration and temperature-dependent color correction
A semiconductor image sensor is provided that includes an on-chip temperature-sensitive element. The signal output of the temperature-sensitive element is used to determine a black level value for the image sensor and to calculate a color correction value to be applied to the signal output of the semiconductor image sensor. The signal output of the temperature-sensitive element may be determined by time-averaging a series of signal outputs from the temperature-sensitive element. The temperature-sensitive element signal output may also be determined by combining, e.g., averaging, the signal outputs of a plurality of on-chip temperature-sensitive elements.
1 . A semiconductor image sensor, comprising:
at least one pixel;
a temperature-sensitive element configured to output a signal related to a sensed temperature of the image sensor; and
a black level setting unit configured to use the output signal of the temperature-sensitive element to calculate a black level to be applied to an output signal of the at least one pixel.
2 . The semiconductor image sensor of claim 1 , wherein the black level setting unit comprises:
a temperature-sensitive element output-to-temperature unit configured to convert the output signal of the temperature-sensitive element to a corresponding temperature;
a temperature-to-dark current unit configured to convert the temperature to a temperature-induced dark current value; and
a dark current-to-black level unit configured to convert the temperature-induced dark current value to the black level.
3 . The semiconductor image sensor of claim 2 , wherein the temperature-sensitive element output-to-temperature unit is configured to use a linear relationship between the output signal of the temperature-sensitive element and the corresponding temperature.
4 . The semiconductor image sensor of claim 2 , wherein the temperature-to-dark current unit is configured to calculate the dark current value by using a relationship between the dark current value and a probability for exciting an electron from a top of a valence band to a bottom of a conductance band.
5 . The semiconductor image sensor of claim 2 , wherein the dark current-to-black level unit is configured to convert the dark current value to a charge value, and to convert the charge value to the black level.
6 . The semiconductor image sensor of claim 1 , wherein the temperature-sensitive element comprises a plurality of temperature-sensitive elements whose signal outputs are combined for use by the black level setting unit.
7 . The semiconductor image sensor of claim 6 , wherein the signal outputs of the plurality of temperature sensitive elements are combined by an averaging process.
8 . The semiconductor image sensor of claim 1 , wherein the black level setting unit is configured to input a time-averaged signal output of the temperature-sensitive element.
9 . The semiconductor image sensor of claim 1 , further comprising a color correction unit configured to use the signal output of the temperature-sensitive element to calculate a color correction value to be applied to the signal output of the at least one pixel.
10 . The semiconductor image sensor of claim 1 , wherein the black level setting unit is further configured to use a lookup table to determine the black level that corresponds to the signal output of the temperature-sensitive element.
11 . A method of operating a semiconductor image sensor, comprising:
measuring a temperature of the semiconductor image sensor; and
setting a black level to compensate pixel signal outputs of the semiconductor image sensor, the black level being determined from the measured temperature.
12 . The method of claim 11 , wherein the measuring act further comprises:
outputting a analog temperature-dependent signal from a temperature-sensitive element;
converting the temperature-dependent signal into a digital signal; and
transforming the digital signal into a temperature.
13 . The method of claim 12 , wherein the digital signal is transformed into a temperature using a linear relationship between the temperature and the digital signal.
14 . The method of claim 11 , wherein the setting a black level act further comprises:
calculating an amount of dark current that corresponds to the measured temperature; and
converting the amount of dark current to the black level.
15 . The method of claim 14 , wherein converting the amount of dark current comprises converting the amount of dark current to a charge value, and then converting the charge value to the black level.
16 . The method of claim 11 , wherein the measuring act comprises:
measuring a plurality of temperatures using a plurality of temperature-sensitive elements; and
averaging the plurality of temperatures to determine the temperature of the semiconductor image sensor.
17 . The method of claim 11 , wherein the measuring act comprises:
measuring a plurality of temperatures using a single temperature-sensitive element; and
averaging the plurality of temperatures to determine the temperature of the semiconductor image sensor.
18 . The method of claim 11 , further comprising using the measured temperature to calculate a color correction value to be applied to the pixel signal outputs of the semiconductor image sensor.
19 . An imaging system, comprising:
an array of pixels for capturing an image;
a processing circuit for processing an image captured by the pixel array; and
a temperature compensation circuit, comprising:
a temperature-sensitive element configured to output a signal related to a sensed temperature of the array of pixels; and
a black level setting unit configured to use the signal output of the temperature-sensitive element to calculate a black level for the array of pixels.
20 . The system of claim 19 , wherein the black level setting unit comprises
a temperature-sensitive element output-to-temperature unit configured to convert the output signal of the temperature-sensitive element to a corresponding temperature; and
a temperature-to-black level unit configured to convert the temperature to the black level.
21 . The system of claim 20 , wherein the temperature-sensitive element output-to-temperature unit is configured to use a linear relationship between the signal output of the temperature-sensitive element and the corresponding temperature.
22 . The system of claim 20 , wherein the temperature-to-black level unit is configured to calculate a dark current value by using a relationship between the dark current value and a probability for exciting an electron from a top of a valence band to a bottom of a conductance band.
23 . The system of claim 22 , wherein the temperature-to-black level unit is configured to convert the dark current value to a charge value, and to convert the charge value to the black level.
24 . The system of claim 19 , wherein the temperature compensation circuit comprises a plurality of temperature-sensitive elements whose signal outputs are averaged for use by the black level setting unit.
25 . The system of claim 19 , wherein the black level setting unit is configured to input a time-averaged signal output of the temperature-sensitive element.
26 . The system of claim 19 , further comprising a color correction unit configured to use the signal output of the temperature-sensitive element to calculate a color correction value for a signal output of the array of pixels.
27 . A processing system, comprising:
an array of pixels for capturing an image;
a temperature compensation circuit, comprising:
a temperature-sensitive element; and
a black level setting unit; and
a processing circuit configured to use the black level setting unit and a signal output of the temperature-sensitive element to calculate a black level for the array of pixels.
28 . The system of claim 27 , wherein the processing circuit is further configured to sample the signal output of the temperature-sensitive element a plurality of times over a set time period and determine an average signal output of the temperature-sensitive element.
29 . The system of claim 27 , wherein the temperature compensation circuit comprises a plurality of temperature-sensitive elements and the processing circuit is configured to calculate a black level by using an average signal output for all of the plurality of temperature-sensitive elements.
30 . The system of claim 27 , wherein the processing circuit is further configured to:
convert the signal output of the temperature-sensitive element to a corresponding temperature;
calculate a dark current value that corresponds to the temperature; and
transform the dark current value into the black level for the array of pixels.
31 . The system of claim 30 , wherein the processing circuit is configured to use a linear relationship to convert the signal output of the temperature-sensitive element to the corresponding temperature.
32 . The system of claim 30 , wherein the processing circuit is configured to calculate the dark current value by calculating a probability for exciting an electron from a top of a valence band to a bottom of a conductance band.
33 . The system of claim 30 , wherein the processing circuit is configured to transform the dark current value into a charge value, and then transform the charge value into the black level.
34 . The system of claim 27 , wherein the processing circuit is further configured to use the signal output of the temperature-sensitive element to calculate a color correction value for a signal output of the array of pixels.
35 . The system of claim 27 , wherein the processing circuit is further configured to use a lookup table to determine the black level that corresponds to the signal output of the temperature-sensitive element.
36 . A digital camera, comprising:
an image sensor, comprising:
at least one pixel array;
a temperature-sensitive element positioned adjacent to the at least one pixel array; and
a black level setting unit; and
a processing circuit configured to use the black level setting unit and the signal output of the temperature-sensitive element to calculate a black level for the at least one pixel array.
37 . The digital camera of claim 36 , wherein the processing circuit is further configured to sample the signal output of the temperature-sensitive element a plurality of times over a set time period and determine an average signal output of the temperature-sensitive element.
38 . The digital camera of claim 36 , wherein the at least one imager comprises a plurality of temperature-sensitive elements and the processing circuit is configured to calculate a black level by using an average signal output for all of the plurality of temperature-sensitive elements.
39 . The digital camera of claim 36 , wherein the processing circuit is further configured to:
convert the signal output of the temperature-sensitive element to a corresponding temperature;
calculate a dark current value that corresponds to the temperature; and,
transform the dark current value into the black level for the at least one pixel array.
40 . The digital camera of claim 36 , wherein the processing circuit is further configured to use the signal output of the temperature-sensitive element to calculate a color correction value for a signal output of the at least one pixel array.
41 . The digital camera of claim 36 , wherein the processing circuit is further configured to use a lookup table to determine the black level that corresponds to the signal output of the temperature-sensitive element.
42 . The digital camera of claim 36 , wherein the camera is a still digital camera.
43 . The digital camera of claim 36 , wherein the camera is a video digital camera.
44 . The digital camera of claim 36 , wherein the camera is a cell-phone camera.
45 . The digital camera of claim 36 , wherein the camera is a handheld portable digital assistant (PDA) camera.