Method and system for determining the rate of non uniformity of bolometer based systems

1. A focal plan array system of the bolometer type having means for determining the rate of non uniformity which comprises:

a) an array of i×j pixel detectors of the bolometer type for sensing scenery radiation;

b) a case which accommodates said array of detectors, said case having a front window that provides exposure to the sensing element of all the i×j pixel detectors to radiation coming from the scenery;

c) at least a portion of column j+1 of blind detectors that are shielded from the scenery by a homogeneous reference surface;

d) reading circuitry for reading indication for the scenery radiation as sensed by each of the i×j detectors of the array, and for reading indication for the non uniformity as sensed by said detectors in column (j+1);

e) a register for recording the radiation values as read from all the detectors within column (j+1); and

f) a processing unit for finding the standard deviation σ of all the values in said register, and for comparing the same to a predetermined threshold Q, wherein a standard deviation above Q being an indication for a non uniformity above the allowed level.

2. A focal plane array system according to claim 1 , further comprising mechanism for providing a homogeneous image to all the FPA detectors, and a calibrating circuitry for offset calibrating all the detectors of the FPA, said mechanism and circuitry are activated when said indication for non-uniformity is observed to be above said predetermined threshold Q.

3. A focal plan array system according to claim 1 wherein said homogeneous reference surface is baffle extending from the case.

4. A focal plan array system according to claim 1 , wherein all the focal plan array detectors, including those within column (j+1) perform differential measurement of radiation.

5. A focal plan array system according to claim 4 , wherein each of the detectors has a structure of a Wheatstone bridge type.

6. A focal plan array system according to claim 5 , wherein each of the i×j Wheatstone bridge type detectors, and each of the detectors within column (j+1) has two branches, as follows:

a) a first branch comprising a first resistor R m which is thermally shorted to a substrate which supports the array and is common to all the decoders of the array and a second resistor R r common to each of the decoders within one of the rows i and which is shielded by said at least one homogeneous reference surface; and

b) a second branch comprising a third resistor R c which is thermally shorted to the said supporting substrate and which is common to all of the decoders within each of the columns j and a fourth resistor R p which is unique for each decoder and which is exposed to the scenery.

7. A focal plan array system according to claim 1 , wherein the reading circuitry comprises a row selector for selecting at each time a row of the array, and j+1 column amplifiers for correspondingly receiving and amplifying the sensed radiation by each of the decoders within the selected row.

8. A method for determining the rate of the non uniformity in a focal plane array of the bolometer type, comprising the steps of

a) providing a focal plane array of i×j bolometer type detectors, all being exposed to the scenery;

b) providing at least a portion of an additional column (j+1) of bolometer type detectors, all said detectors in column (j+1) being masked from the scenery by means of a homogeneous surface;

c) predefining a non uniformity threshold Q, relating to the non uniformity of behavior of the decoders;

d) sequentially reading in a row by row manner the values as sensed by the i×j detectors of the FPA and as sensed by the detectors in the (j+1) column;

e) recording at a column register the readings that are sensed by the decoder of column j+1;

f) calculating the standard deviation of all the values in said column register and comparing the standard deviation with the threshold Q;

g) if said standard deviation is found to be below Q, continuing to normally operate the FPA; and

h) if, however, the standard deviation is found to be above Q, stopping the operation of the FPA, and performing a non uniformity calibration of the FPA decoders.