Apparatus and method for gain correction

1. An electronic method of correcting a column gain mismatch in an image sensor, implemented in circuitry to perform the method, the method comprising:

reading a test signal from at least one test pixel in the image sensor;

comparing the test signal to a target value;

calculating a correction value based on comparing the test signal to the target value;

storing the correction value in a memory;

reading a photo-pixel signal;

determining a black reference value from a black reference pixel, wherein the black reference pixel is a pixel which has not been exposed to light; and

modifying the photo-pixel signal based on the correction value stored in memory and the black reference value, in order to form a modified photo pixel signal; and

outputting the modified photo-pixel signal;

wherein for each column in the sensor, each photo-signal is modified according to:

data out=(data in−blackref)×(1+Δ i )+blackref

where Δ i equals n i /2 s ;

n i is the signed correction values stored in the memory for each column; and

s is a scaling parameter.

2. The method of claim 1 , wherein the steps are performed for each column in an image sensor.

3. The method of claim 2 , wherein the at least one test pixel comprises a plurality of test pixels.

4. The method of claim 3 , wherein the target value is set to an average value of a value of each test signal minus the black reference value.

5. The method of claim 4 , wherein the correction value is a signed correction value.

6. The method of claim 5 , wherein s is set to maximize an available range of values in memory.

7. The method of claim 6 , wherein an initial correction value can be calculated by:

n i =2 s×(target/(data 13 in−blackref)−1).

8. An apparatus for correcting a column gain mismatch in an image sensor, the apparatus comprising:

at least one test pixel;

at least two programmable Digital-to-Analog Converters (DACs) to provide a signal voltage and a reset voltage for the at least one test pixel;

a memory to store a gain correction value; and

a logic circuit to process output signals from the at least one test pixel and image pixel signals from a column in the sensor;

wherein the logic circuit:

reads the gain correction value from memory;

subtracts a black reference value from an input signal;

applies the gain correction value to the input signal;

determines whether the input signal is a test pixel signal or an image pixel signal;

for test pixels signals, compares a corrected input signal with a target value, and based on the comparison, increases or decreases the gain correction value in memory; and

for image pixels, adds the black reference value back to a corrected input signal, and outputs the result;

wherein the logic circuit processes the output signals according to the formula:

data_out=(data_in−blackref)×(1 +Δ i )+blackref

where blackref is the black reference value;

Δ i equals n i /2 s ;

n i is the signed correction values stored in the memory for each column; and

s is a scaling parameter.