GB2370960A

GB2370960A - Partially shielded photodiode array

Info

Publication number: GB2370960A
Application number: GB0100227A
Authority: GB
Inventors: Mark Graves
Original assignee: SPECTRAL FUSION TECHNOLOGIES L
Current assignee: SPECTRAL FUSION TECHNOLOGIES L
Priority date: 2001-01-05
Filing date: 2001-01-05
Publication date: 2002-07-10
Also published as: GB0100227D0

Abstract

An array of photodiodes is provided, at least one of which is shielded from incident signals such as by a lead cover, for x-ray sensing. In a linear array, this photodiode is preferably at the end of the array as it can then be ignored for the purpose of creating an image. In this way, the shielded photodiode always gives a dark output and can be used to provide a running calibration for the remaining photodiodes. Thus, the output of the one photodiode is subtracted from the output of other photodiodes in the array. If the temperature rises (for example) then the signal of all photodiodes will increase but the signal after subtraction will remain the same.

Description

PHOTODIODE ARRAY The present invention relates to a photodiode array. It addresses the difficulty commonly encountered with such arrays that the individual elements are typically sensitive to variations in temperature. In addition, when used for x-ray scanning (usually in combination with a scintillating material), x-rays which reach the photodiode tend to cause damage to the semiconducting structure which accumulates over time.

The use of a linear strip of photodiodes covered with a scintillating layer is widespread in real-time linear x-ray acquisition systems such as those used in bone densitrometry, CT scanning and baggage scanning. A more reliable system would therefore be of wide benefit.

The problem of thermal drift of the photodiodes is usually dealt with by seeking to maintain accurate thermal control over the environment in which the photodioides are placed. This usually involves using an air conditioning device operating under closed-loop control. As the environmental conditions change, the flow of fluid to a cooling device is regulated thereby maintaining a constant temperature at the array.

The cooling device includes a fan which blows air through a coiled conduit containing the fluid and over the array. This is however problematic in that the cost of an air conditioning unit is significant, and photodiodes closest to the fan are significantly cooler than those which are more distant. A temperature gradient therefore exists over the array.

Another approach to this problem is to make an"offset"correction to the output of the photodiodes. A reading is made of the array in the absence of any incident xray signal. The output will therefore represent the dark current, which is temperature dependent. This can be stored for later reference. As the array is used, the stored dark output can be subtracted. This works provided the temperature does not change between calibration and use. The unit must therefore be at its operating temperature before calibration. In practice, a long warm-up period must be observed and frequent re-calibrations must be made.

According to the present invention, an array of photodiodes is provided, at least one of which is shielded from incident signals.

Where the incident signal is x-radiation, the shielding can be by way of a lead cover. An appropriate opaque material can be used for other incident signals.

In a linear array, the at least one photodiode is preferably at the end of the array as it can then be ignored for the purpose of creating an image.

In this way, the shielded photodiode always gives a dark output and can be used to provide a running calibration for the remaining photodiodes. Thus, the output of the one photodiode is subtracted from the output of other photodiodes in the array. If the temperature rises (for example) then the signal of all photodiodes will increase but the signal after subtraction will remain the same.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying figures, in which; Figure 1 is a vertical section through an array of photodiodes according to a first embodiment of the invention; Figure 2 is a schematic view of the drive electronics for the array of figure 1; Figure 3 is a schematic view of the signal handling electronics for the array of figure 1; Figure 4 shows an alternative signal handling electronics; Figure 5 shows a plan view of an array of photodiodes according to a second embodiment of the invention; and Figure 6 shows a vertical section of figure 5 on VI-VI.

Referring to figure 1, a detector for an x-ray image consists of a linear array of photodiodes 10. Each is covered with a layer of scintillating material 12 and a filter 14. Incident x-rays are thus filtered to remove those of the wrong wavelength and strike the scintillator 12. This generates a burst of photons which travel to the photodiode 10. Under suitable electrical bias, the output current of the photodiode can be related to the intensity of light detected and hence to the intensity of incident x rays. Each photodiode therefore acts as a single pixel in a image which is a single pixel wide.

The object being scanned is typically translated past the array at a set speed.

The array is sampled at intervals, and a two-dimensional image can be built up from the plurality of one-pixel wide images.

According to the invention, an outermost pixel 1 Oa is shielded from incident xrays by a lead foil 16. The lead foil should be sufficiently thick to reduce the x-rays incident on the scintillating layer to substantially zero. Thus, this photodiode will give a dark reading. As it is on the edge of the linear array, its output can be ignored in preparing the two-dimensional image, and used only to calibrate the other pixels.

As shown, the photodiode has a filter 14 and a scintillating layer 12, although these will not be strictly necessary as the lead foil 16 removes substantially all the incident x-rays. However, it may be simpler to manufacture the array with these in place.

Figure 2 shows the bias circuitry for the array of figure 1. Each of the photodiodes 10 is reverse biassed by a supply voltage Vb, as which is smoothed by the RC network formed by As and Cd-Incident light from the scintillators or thermal effects cause the generation of charge carriers in the photodiodes and allows a current to flow under reverse bias.

The output current of each photodiode is converted to a voltage as shown in respect of the edge photodiode 1 osa. An operational amplifier 18 is supplied with the output current of the photodiode 10a and provided with negative feedback via a resistor Re. In the style of a simple inverting amplifier, therefore, the output voltage Vout of the op-amp 18 is proportional to the output current of the photodiode 1 osa.

This output voltage is then fed to a voltage shift and amplification circuit shown in figure 3. This is of known design and results in a modified output voltage mod suitable for further processing. The modified voltage is then converted to a digital signal by an analogue-digital converter and placed in a computer memory.

Once a complete set of data has been read into the memory, the value derived from the reference pixel can be subtracted from the value of each other pixel to give an output corrected for temperature variations.

At each scan, a fresh calibration signal is taken from the reference pixel and therefore the temperature correction is always up to date. The warm-up period can be minimised, and downtime due to re-calibration is eliminated.

Figure 4 shows an alternative calibration circuit using analog hardware. The current output of each photodiode is converted to an analog voltage signal by converters 20 which correspond to those of figure 2. A shift/amplification circuit 22

is then provided for each photodiode 10 other than the reference photodiode 1 osa. The circuit corresponds to that of figure 3 but uses the output voltage from the reference photodiode 10a as its reference voltage. Thus, instead of subtracting a steady voltage, the potentially varying voltage of the reference photodiode is subtracted to automatically eliminate the dark signal of each photodiode 10. The corrected output Vcorr is then used for further processing, including A-D conversion and assembly into a two dimensional image, as before.

Figures 5 and 6 show an alternative array, from above. Each pixel 24 has a photodiode 26, scintillator 28 and filter 30 as per the pixels 10 of figure 1. Adjacent each such pixel 24 is a reference pixel 32 which is as per the reference pixel 1 osa of figure 1, and thus has a photodiode 34, scintillator 36 and filter 38, over which is provided a lead foil 40 which is sufficiently thick to reduce the incident x-ray intensity to substantially zero. The reference pixel 32 thus acts as a reference for pixel 24 only. Each pixel has a reference of its own. Any thermal gradient over the array will be accounted for automatically.

The output of the array shown in figures 5 and 6 can be handled as described above in relation to figure 1. It is likely that the hardware correction shown in figure 4 will be preferred as the use of a reference value per pixel will impose a higher processing load than a single value. However, this is not essential.

It will be appreciated by those skilled in the art that many variations may be made to the above-described embodiments without departing from the scope of the present invention.

Claims

CLAIMS 1. An array of photodiodes for sensing incident radiation, at least one of which is shielded from incident signals.
2. An array of photodiodes for sensing incident x-radiation, at least one of which is shielded from incident signals, at least the remainder being covered by a layer of scintillating material.
3. An array according to claim 2 in which the at least one photodiode is shielded by a lead cover.
4. An array according to any preceding claim in which the at least one photodiode is at the end of the array.
5. An array according to any preceding claim in combination with processing electronics adapted to subtract the output of the at least one photodiode from the output of other photodiodes in the array.
6. An array of photodiodes substantially as described herein with reference to and/or as illustrated in the accompanying figures.