GB2344166A - Filter based spectrophotometer - Google Patents

Abstract

A radiation sensor comprising two graded filters E and F positioned upstream of an array of electromagnetic detectors G. The filters have a transmission characteristic which varies along a physical dimension B, C, and are positioned such that their said dimensions are inclined with respect to one another by an angle A. The result of this arrangement is that spectral information is distributed across the array, so that the detectors view a range of centre wavelengths at varying bandwidths.

Description

Versatile Filter Based Spectrophotometer The present invention relates to spectrophotometry instruments, ie : instruments which are able to analyse the spectral characteristics of amples.

An analytical instrument may be constructed in which the principal components are: a wide band radiation source (eg. a hot filament), a sample cell, an optical filter with a defined centre wave length and band width and a radiation detector.

If the optical filter is chosen to match the absorption characteristics of the intended sample then the detector output will be very sensitive to the sample concentrations.

An example would be an instrument that measures carbon dioxide concentrations through its strong absorption near 4. 3/m.

Known drawbacks to this technique are as follows: (a) Sensitivity will vary with inevitable changes in the components or obscuration of any of the optical elements, I (b) Absorption bands of different gaseous species often overlap so careful wavelength selection is required and limitations will exist inherent in filter technology, (c) Temperature and pressure changes will necessitate recalculations or recalibrations.

Numerous methods are used to overcome these drawbacks, including the use of multiple detectors and wavelengths and the creation of a reference channel.

However, these may not be sufficient for all applications and the cost and complexity may become unacceptable. A generally improved instrument would almost certainly use a multiplicity of detectors and filters to gather information over a wide spectral range; this range would include the full absorption band of the designated gas itself as well as those of interfering species and reference wavelengths that are not subject to attenuation.

One instrument variant that achieves some of the aims described above uses a so called"graded"or"variable"filter in which the transmission characteristics vary along a defined physical dimension. This filter will be closely overlaid on a detector array so as to give wavelength discrimination along the length of the array. The remaining components of a complete analytical instrument will be as described above.

The present invention aims to provide an even more versatile radiation sensor.

In one aspect the present invention provides a radiation sensor comprising an array of detectors of electromagnetic radiation, and first and second filters positioned in front of the array whereby to limit the wavelength response of the detectors, each filter having a transmission characteristic which varies along a physical dimension, said filters being positioned such that their said dimensions are inclined with respect to each other.

As a result of the inclination or offset of the filters, the individual detectors of the array will be screened by different filter combinations such that a range of centre wavelengths at a range of band widths can be viewed by the array.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a detector array in front of which two filters are positioned ; Figure 2 illustrates graphically the effect of combining two filters whose characteristics are similar at the point of overlap. This would be the case at array position (9,9) in Figures 4,5,6 and 7 below; Figure 3 is a graphical illustration of how the combination of two filters of similar bandwidth and different centre wavelength combine to produce a narrower bandwidth centred on a slightly shifted wavelength. This would be the case at array position (9,15) in Figures 4,5,6 and 7 below; Figures 4 and 5 are tables of centre wavelengths and bandwidths respectively for a 20 X 20 array positioned behind two graded filters of known characteristics inclined at 10 ; Figures 6 and 7 are three dimensional graphical representations of the figures given in the tables of Figures 3 and 4 respectively; and Figure 8 is a graph of percentage absorption versus wavelength for carbon monoxide and carbon dioxide.

In Figure 1, G indicates a two dimensional array of detector elements.

E is a variable filter whose centre wavelength varies in the direction of arrow B.

The bandwidth of the filter will be determined by the manufacturing design and will ideally have a rectangular profile. The bandwidth will also additionally increase with the length of filter exposed in direction B.

F is a second variable filter inclined at an angle angle A with filter E and whose centre wavelength varies in the direction of arrow C. This filter could be identical to filter E.

The radiation seen by an arbitrary detector D through the combination of filters E and F will have an exactly calculable centre wavelength and bandwidth determined by angle A, the known characteristics of filters E and F, the offset between them and the position of D within the matrix. The overall result of this arrangement is to distribute spectral information across the two dimensional array -one direction will be a substantially centre wavelength gradation whilst an approximately perpendicular direction will present the same centre wavelength but at different bandwidths. This effect becomes clearer from an examination of Figures 2 to 7.

Referring firstly to Figure 2, combining two filters of the same bandwidth and centre wavelength obviously has no effect on the overall bandwidth and centre wavelength but Figure 3 shows that if the bandwidth and centre wavelength are different, the overall effect is a narrowing of the bandwidth and a shift in the centre wavelength.

Figures 4,5,6 and 7 illustrate calculated results for centre wavelength and bandwidth using the characteristics of two identical filters inclined at 10 and specified below: Centre wavelength spread 3.0 to 5.0, um Bandwidth at any point of a single filter 0.20 i., im Figure 6 gives an overview of the variation of centre wavelength across a 20 X 20 array, with the centre wavelength being indicated on the vertical scale. From position (1, 1) to position (1, 19) along the array the centre wavelength is substantially constant but in a perpendicular direction the centre wavelength increases significantly with increasing distance from row 1.

As Figure 7 shows, the bandwidth varies very little along the columns of detectors, eg (l, 1) to (20,1) but it increases sharply and then decreases equally sharply in a perpendicular direction, with a maximum"apex"approximately aligned with a central column of detectors.

A sensor array of the type described would have a number of unique advantages and could be used in different ways : (a) Certain elements will gather plentiful wide band radiation but with low spectral detail; this can be measured with a high signal to noise ratio. In contrast, other elements will see narrow band radiation having high spectral detail, but the signal levels will be low and the measurement signal to noise ratio will be poorer. By using these complementary measurements interactively it will be possible to deduce the shape of the spectral image with higher accuracy than would be possible with a single graded filter and linear array. The spectral characteristics of this spectral image are effectively being mapped at a high resolution whilst retaining the good signal to noise characteristics of a high throughput low resolution system.

(b) If the same spectral image is mapped for two or more different values of the angle A it will be observed that the optical environment of the detector elements D will change in that neighbouring elements will see a changed bandwidth. This provides a means to eliminate optical crosstalk in situations where there is scatter or the incoming radiation is not perfectly collimated.

(c) Certain detector arrays of the appropriate type for the intended applications may require intensity modulations to function effectively. Normally this is achieved by modulating the source intensity directly or by interposing a mechanical chopper. The described invention opens the possibility of modulation by rotation or oscillation of the second variable filter. Bandwidth modulation of a detector element would have selectivity advantages.

(d) The instrument symmetry is such as to duplicate every measurement point ; this gives a means for eliminating mechanical or uniformity defects in the complete instrument.

EXAMPLE A frequent analytical requirement is to measure carbon monoxide in the presence of carbon dioxide. As will be seen in Figure 8 the absorption bands in the 4 to 5 micrometer range overlap; the greatest measurement accuracies will thus be achieved if both bands can be mapped in full. This can be achieved using a filter array of the type described above and to the following specifications: Size of single filter 10mon x 10mm Centre wavelength spread 4.0 to 5. 0, am Bandwidth at any point of a single filter 0.15, um Angle of tilt between two identical filters as above 15 Calculated centre wavelength range across detector array 4.05 to 4.95, um Bandwidth spread across substantially most of the array in an approximately perpendicular direction to the above 0.15 to 0. 04/m

Claims (9)

1. Claims: l. A radiation sensor comprising an array of detectors of electromagnetic radiation, and first and second filters positioned in front of the array whereby to limit the wavelength response of the detectors, each filter having a transmission characteristic which varies along a physical dimension, said filters being positioned such that their said dimensions are inclined with respect to each other.
2. 2. A radiation sensor as claimed in claim I in which the two filters are identical.
3. 3. A radiation sensor as claimed in claim I or 2 in which the first and/or second filter is a band pass filter whose centre wavelength varies along a dimension of the filter.
4. 4. A radiation sensor as claimed in claim 3 in which the centre wavelength of the first and/or second filter varies along a straight line traversing the filter.
5. 5. A radiation sensor as claimed in any preceding claim in which the band width of the first and/or second filter varies along a dimension of the filter.
6. 6. A radiation sensor as claimed in any preceding claim in which the array is a two dimensional array.
7. 7. A radiation sensor as claimed in any preceding claim in which the detectors are sensitive to infrared radiation.
8. 8. A radiation sensor as claimed in any preceding claim including means for rotating at least one of the filters with respect to the other.
9. 9. A radiation sensor substantially as herein before described with reference to the accompanying drawings.