Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-03-10
2002-05-28
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140LS
Reexamination Certificate
active
06396045
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a sensor apparatus for sensing radiation from a scene. It also relates to a method of sensing such radiation.
2. Discussion of Prior Art
Sensor apparatus for sensing radiation from a scene 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 apparatus 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 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 artefacts generated within the sensor may arise, for example, from offset potentials generated within its processing circuits; these offsets can arise from circuit device semiconductor bandgaps or from transient charge injection effects when processing signals within the circuits.
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, it is found that there is a noticeable variation in responsivity function A
k
amongst the elements, namely the elements have differing responsibilities 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; 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. 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, there is also a variation in offset signal B
k
amongst the elements, and the responsivity function A
k
and the offset B
k
are often dependent upon sensor temperature. For sensors employed to respond to diminished radiation intensities, noise N
k
generated within their 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 infra-red radiation from the scene, each output S
k
is found to comprise an unwanted pedestal component, corresponding to a general background temperature of the scene, together with a desired feature component, namely corresponding to temperature variations within the scene. This is particularly pertinent where:
(i) the scene is at an ambient temperature of approximately 300K; and
(ii) the temperature variations within the scene giving rise to the feature component are less than 1K.
The pedestal component may often 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 apparatus for sensing emissions from a scene, especially infra-red emissions therefrom. The apparatus may, for example, need to incorporate analogue-to-digital conversion circuits providing a large dynamic range corresponding to 12-bits or more so that both the pedestal component and the feature component may each be resolved in data provided by the circuits. Moreover, the unwanted pedestal component may result in problems of saturation in sensor apparatus which analogue integrate photodetector signals in order to provide improved apparatus signal-to-noise performance.
A solution which addresses the problem of pedestal component described above is provided in a U.S. Pat. No. 5,155,348 which 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 its respective read-out circuit. In U.S. Pat. No. 515,534, the circuit is described as operating in 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 measurement image of the scene is projected onto the array. A measurement signal generated at each element in response to the image has subtracted from it the calibration signal for that element to provide a difference signal. The difference signal is integrated within the circuit onto an integration capacitor C
s
incorporated therein to provide an output signal. The circuits each produce a respective output signal which is multiplexed for generating a compound sensor signal.
This solution provides an advantage that the dynamic range of the compound sensor signal is reduced as a result of removing a pedestal component generated at each element. This 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 problem arises with the sensor described in U.S. Pat. No. 5,155,348 when scene contrast radiation Rf
k
is greatly diminished relative to the background radiation Rped
k
, for example where the sensor is used to view a substantially uniform scene incorporating a distant faint object. Inaccuracy when subtracting the calibration signal from the measurement signal can result in the contrast radiation Rf
k
being masked by subtraction inaccuracies. One source of inaccuracy is transient charge injection within the element circuits which arises when the circuits are being switched between calibration to measurement phases. Transient charge injection can be conventionally reduced in a circuit as described in the U.S. Pat. No. 5,155,348 by reducing junction capacitances of MOS devices incorporated therein and increasing capacitance of its storage capacitor incorporated therein to provide a modified circuit. This results in a problem that the modified circuit occupies more space when fabricated in monolithic form and its speed of operation is degraded. Reduced speed of circuit operation can
Ballingall Ronald A
Collins Stephen
Lees David J
Marshall Gillian F
Kim Robert H.
Nixon & Vanderhye P.C.
QinetiQ Limited
Thomas Courtney
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