Gain measurement structure

Abstract

A CCD Device of the type for providing charge gain by impact ionisation has a multiplication register. Gain provided by a subset of the elements of the multiplication register are independently controllable from other elements in the register. This enables the register to be used in one setting with the same gain applied to all elements and a different setting with a subset of elements arranged to provide a different gain. By comparing the two signals, the gain provided by each element and the register as a whole may be derived.

Description

DESCRIPTION OF THE FIGURES

An embodiment of the invention will now be described by way of example and with reference to the accompanying figures in which:

FIG. 1 is a schematic diagram of a CCD imager of known type and which may embody the invention;

FIG. 2 is a schematic diagram of a physical cross section of one element in a multiplication register;

FIG. 3 is a schematic cross section of a single multiplication element showing voltages applied to electrodes at a point in time;

FIG. 4 is a chart showing the relationship between voltage difference between electrodes of an elements and the signal added;

FIG. 5 is a schematic representation of a frame transfer device embodying the invention;

FIG. 6 shows the gain as a function of the ratio of two signals;

FIG. 7 is a flow diagram of the process to determine gain using the invention; and

FIG. 8 is a flow diagram of the process to determine gain using alternate lines using the invention.

Claims

1. A CCD device of the type for providing charge gain by impact ionisation, comprising: a multiplication register for producing charge gain having a plurality of elements, the gain provided by a subset of the plurality of elements being independently controllable from the remainder of the elements of the multiplication register; and an output arranged to produce a first signal derived from the multiplication register with the same gain applied to all elements and a second signal derived from the multiplication register with subset of elements controlled to provide a different gain, whereby the gain of the multiplication register is derivable from the first and second signals.

2. A CCD device according to claim 1, wherein the elements of the subset of elements are arranged sequentially with the remainder of the elements.

3. A CCD device according to claim 1, wherein the elements of the subset of elements are arranged interleaved with the remainder of the elements.

4. A CCD device according to claim 1, wherein each element comprises a sequence of electrodes including a high voltage electrode, the high voltage electrodes of the elements in the subset being connected together but not being connected to the remainder of the elements.

5. A CCD device according to claim 1, wherein each element comprises a sequence of electrodes including a DC electrode, the DC electrodes of the elements in the subset being connected together but not being connected to the remainder of the elements.

6. A CCD device according to claim 1, wherein the second signal is derived from the multiplication register with the subset of elements controlled to provide a substantially unity gain.

7. A CCD device according to claim 1, further comprising an array of light sensitive CCD elements arranged to provide charge to the multiplication register.

8. A CCD device according to claim 7, wherein the output is arranged to produce the first and second signals from charge derived from first and second respective captured images.

9. A CCD device according to claim 7, wherein the output is arranged to produce the first and second signals from charge derived from alternate respective lines within a captured image.

10. A method for deriving the normal gain provided by a multiplication register having a plurality of elements, comprising: deriving a first signal from the multiplication register with the normal gain applied to all elements,deriving a second signal from the multiplication register with subset of elements controlled to provide a different gain from the remainder of the elements, anddetermining the normal gain of the multiplication register from the first and second signals.

11. A method according to claim 10, wherein the second signal is derived from the multiplication register with the subset of elements controlled to provide a substantially unity gain.

12. A method according to claim 10, wherein the first and second signals are derived from charge derived from first and second respective captured images.

13. A method according to claim 10, wherein the first and second signals are derived from charge derived from alternate respective lines within a captured image.

14. A method according to claim 10, wherein the subset of elements are controlled to provide a different gain from the remainder of the elements by altering the potential of the clock high level applied to the high voltage electrode within each element.

15. A method according to claim 10, wherein the subset of elements are controlled to provide a different gain from the remainder of the elements by altering the voltage applied to a DC electrode within each element.

16. A method according to claim 10, wherein the subset of elements are controlled to provide a different gain from the remainder of the elements by altering the phase relationship between the clock waveform applied to the high voltage electrode within each element and the other clock waveforms

17. A CCD imaging device comprising an area array of light sensitive elements and a CCD device according to claim 10.

18. A camera including a CCD device according to claim 10.