Sensor system

Details Sensor system Sensor system

2000-03-10

2002-07-02

Kim, Robert H. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S205000, C250S2140LS

Reexamination Certificate

active

06414294

ABSTRACT:

This invention relates to a sensor system for sensing radiation from a scene. It also relates to a method of sensing such radiation.
Radiation sensor systems are well known in the prior art. They find widespread application, for example, in portable consumer video and digital cameras and also in thermal imagers as employed by emergency services.
A typical system incorporates a sensor comprising a two-dimensional array of elements, each with an associated signal processing circuit. Radiation from a scene is projected onto the array where each element responds via its associated processing circuit with an output S
k
as given in equation [1]; the index k here is used to identify elements uniquely, i.e. S
k
is the output from the k
th
element circuit. The output S
k
includes unwanted artefacts which arise either from the scene itself or are generated within its associated element or processing circuit:
S
k
=A
k
(
Rf
k
,Rped
k
)
+B
k
+N
k
  Eq. 1
where
S
k
=output generated from k
th
element via its associated processing circuit;
A
k
=k
th
element responsivity function;
Rf
k
feature information or scene contrast radiation from the scene received at the k
th
element;
Rped
k
=background radiation from the scene received at the k
th
element;
B
k
=offset signal generated within the k
th
element and its associated processing circuit; and
N
k
=noise signal generated within the k
th
element and its associated processing circuit.
The outputs S
k
from each element are combined to provide a sensor signal. The sensor including its associated circuits may be based on charge coupled devices (CCDS) or metal oxide semiconductor (MOS) devices. When MOS devices are employed in particular, it is found that there is an undesirable variation in element responsivity function A
k
, namely the elements have differing responsivities and give different outputs S
k
in response to the same received radiation intensity. This variation is often larger than that of sensors incorporating charge-coupled devices (CCD). It has prevented widespread use of sensors incorporating MOS devices in consumer video cameras in preference to sensors incorporating CCDs despite a long-felt want to do so in order to benefit from the compatibility of MOS detection and processing circuitry. In this connection, providing power supply and control signals for operating MOS devices tends to be less complex and less expensive compared to providing them for operating corresponding CCDs; this is because circuit parameters such as supply voltages are compatible in the former case. The variation gives rise to fixed pattern noise (FPN) in the outputs S
k
which results in the corresponding sensor signal depicting a speckled scene. Moreover, the elements also have differing values of the offset signal B
k
amongst the elements. For sensors employed to respond to low level radiation intensity, noise N
k
generated within their detector elements and associated processing circuits often becomes a problem, in particular flicker noise contributing to N
k
which has a noise spectral density which increases inversely relative to frequency.
When the sensor receives radiation from the scene, the output S
k
contains an unwanted pedestal component, corresponding to a general background radiation from the scene, together with a feature or contrast component which corresponds to features within the scene. This is particularly pertinent when:
(i) the sensor is detecting infra-red radiation;
(ii) the scene is at an ambient temperature of approximately 300 K; and
(iii) the temperature variations within the scene Rf
k
giving rise to the feature component are less than 1 K.
The pedestal component may be a factor of one thousand or more larger than the feature component. This results in poor signal contrast which may render the temperature variations difficult to identify in the outputs S
k
unless further signal processing is applied thereto.
The presence of the pedestal component imposes constraints and limitations on design and performance of a sensor system for sensing emissions from a scene, especially infra-red emissions therefrom. It often results in a sensor signal representing an image of the scene which is dominated by offset errors and artefacts with respect to scene contrast.
A solution which addresses the problem of pedestal component described above is disclosed in U.S. Pat. No. 5,155,348. This describes a read-out circuit for a sensor comprising a two-dimensional array of 128×128 photodetector elements responsive to infra-red radiation where each element is connected to a respective read-out circuit. The circuit has calibration and measurement phases.
During the calibration phase, a calibration image is projected onto the elements. The image may correspond to a featureless calibration object of similar temperature to a scene to be viewed or a totally blurred featureless uniform image of the scene. Each element generates a signal in response to the calibration image and its respective circuit is arranged to store a calibration signal on a storage capacitor C
c
incorporated within it corresponding to a signal generated by its respective element in response to the calibration image. This provides a correction for pedestal component across the array.
During the measurement phase, a focused image of the scene is projected onto the array and generates a measurement signal at each element. The calibration signal is subtracted from the measurement signal for each element to provide a difference signal. The difference signal is integrated to provide an output signal. The circuits produce respective output signals scanned by a multiplexer to give a compound sensor signal.
This solution reduces the dynamic range of the compound sensor signal by removing pedestal component at each element. It eases dynamic range performance requirements of remote circuits receiving the sensor signal from the multiplexer, for example allowing use of analogue-to-digital converters of 8-bit resolution instead of 12-bit resolution.
A technique for reducing FPN is described in a U.S. Pat. No. 5,373,151. The specification is concerned with an optical system which projects focused and periodically defocused images of a scene onto a multielement focal plane array for generating at each element corresponding focused and defocused signals respectively. A difference signal for each element is derived by subtracting its associated defocused signal from its focused signal. Difference signals from the elements are combined together to provide a system output in which FPN artefacts have been reduced.
Despite use of FPN correction in the prior art, it is found in practice that image quality in the prior art does not remain as good as the initial correction would suggest and it is necessary to recalibrate repeatedly. Reasons for this inconsistency are not presently disclosed in the prior art.
It is an object of the invention to provide a sensor system with more effective FPN correction.
According to the present invention, a sensor system is provided for generating a sensor signal corresponding to a filtered image of a scene, the system incorporating:
(i) detecting means incorporating a plurality of detector elements for generating first and second element signals during first and second detection phases respectively; and
(ii) processing means for deriving a difference signal from the element signals for use in generating the sensor signal,
characterised in that the processing means incorporates sensing means for sensing at least one environmental factor influencing responsivity of the elements and the system is arranged to repeat both of the first and second phases in response to environmental change.
The invention provides the advantage that the sensor system undergoes recalibration to reduce FPN when only when necessary when changes in any one of environmental factors influencing responsivity of the system occurs.
A first problem with the detection circuit described in the U.S. Pat. No. 5,155,348

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