GB2382134A

GB2382134A - Spectrophotometer with two edge filters

Info

Publication number: GB2382134A
Application number: GB0127670A
Authority: GB
Inventors: Christopher Frederick Carter
Original assignee: Infrared Integrated Systems Ltd
Current assignee: Infrared Integrated Systems Ltd
Priority date: 2001-11-19
Filing date: 2001-11-19
Publication date: 2003-05-21
Anticipated expiration: 2021-11-19
Also published as: GB2382134B; GB0127670D0

Abstract

A spectrophotometer comprising an array of electromagnetic radiation sensors G and first and second 'edge filters', H and J, located in front of the array, which in combination limit the wavelength response of the detectors. The filters comprise one high-pass filter having a cut-on wavelength that varies along a dimension K, and one matched low-pass filter having a cut-off wavelength that varies along a dimension N. The filters can be rotated so that said dimensions are inclined by an angle X with respect to each other. As a result of this inclination or offset, individual detectors will be screened by different filter combinations such that a range of centre wavelengths at a range of bandwidths can be viewed by the array. In a preferred embodiment the detectors are sensitive to infra-red radiation.

Description

Improved Versatile Filter Based Spectrophotometer The present invention relates to spectrophotometry instruments, i. e.: instruments which are able to analyse the spectral characteristics of samples.

Various instruments of this type are discussed in GB 2344166A. In particular this patent discloses an infrared sensor comprising an array of detectors having two filters positioned in front of it. The filters are band pass filters and each filter has a centre wavelength which varies along a straight line traversing the filter as well as a varying bandwidth. The effect of a single such filter is that different detector elements"see"different pass bands. The two filters are positioned with their lines of variation inclined with respect to each other and as a result the elements see a wider variation of pass bands and bandwidths which can be varied according to the angle of inclination of one filter with respect to the other.

The present invention is based on the realisation that a similar effect can be achieved much more simply using graded"edge filters". The expression"edge filter"is a generic expression for cut-on (high pass) and cut-off (low pass) filters.

Thus, the present invention provides a radiation sensor comprising an array of detectors of electromagnetic radiation, and first and second edge filters positioned in front of the array whereby to limit the wavelength response of the detectors, each edge filter having an edge wavelength which varies along a dimension of the filter, said filters being positioned such that their said dimensions are inclined with respect to each other. Preferred features of the invention are described in claims 2 to 8.

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 is a schematic diagram similar to Figure 1 using different types of filters; and Figure 3 illustrates graphically the effect on one detector element of the filters used in Figure 2.

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. Means Z schematically indicated in Figure I may be provided for rotating filter F with respect to filter E. Such means may include a motor drivingly connected to means supporting filter F.

EXAMPLE The instrument described above and in GB 2344166A employed two overlapping but angularly displaced graded bandpass filters; in these filters the centre wavelength of the bandpass changes along one axis of the filter but is constant along the other axis. An embodiment of the present invention uses two superimposed graded"edge"filters : preferably one long wave pass (cuton) and one matched short wave pass (cut-off) filter as shown in figures 2 and 3. In these latter filters the wavelength of the"edge"or cut on/off wavelength changes along one axis of the filter and is also constant along the other axis.

The matching is understood to mean that the wavelength of the edge would occur at a corresponding point on both filters if they were exactly overlaid.

A practical arrangement to give an instrument according to this embodiment of the invention is shown in figure 2.

In figure 2 G represents an array of detector elements as in figure 1. H and J are two filters. H is a cut-on filter whose cut-on wavelength varies in the direction K and is constant in the perpendicular direction. J is a cut-off filter whose cut-off wavelength varies in the direction N and is constant in the perpendicular direction.

The cut-on/cut-off wavelengths of the two filters would initially be exactly overlaid along one edge of the array M-M so giving near zero overall transmission everywhere. One filter would now be rotated with respect to the other by angle x about the mid point of this edge, point 0, so as to give the required maximum band pass where the opposite edges cross at point P. This will happen because the cut-on wavelength of one filter will be separated from the cut-off wavelength of the second filter as indicated in figure 3, this separation increasing from zero at point 0 to a maximum at point P. In the arrangement described the centre wavelength of the band defined by the two edge filters will vary in the graded direction M-M and the band width will increase in a perpendicular direction. The result of overlapping the two filters is shown schematically in figure 3 for an arbitrary point in the middle array.

A typical specification might be Size of each filter 10 mm x 10 mm Top filter: cut-on (long wavelength pass) wavelength 3, um to 5 um Lower filter: cut-off (short wave pass) wavelength cm to 5 urn Tilt angle. for the above geometry the bandwidth of the filter combination at a tilt angle of x degrees for a distance y mm from the aligned edge will be approximately: 0.2. y. tan (x) micrometers Hence a 0. 17, um bandwidth will exist 8 mm from the aligned edge for a tilt angle of 6 degrees.

Claims

Claims: 1. A radiation sensor comprising an array of detectors of electromagnetic radiation, and first and second edge filters positioned in front of the array whereby to limit the wavelength response of the detectors, each edge filter having an edge wavelength which varies along a dimension of the filter, said filters being positioned such that their said dimensions are inclined with respect to each other.
2. A radiation sensor as claimed in claim 1 in which one of the first and second filters is a low pass filter and the other is a high pass filter.
3. A radiation sensor as claimed in claim 2 in which the filters are matched such that when their variation directions are parallel the cut-on wavelength of the high pass filter is greater than or equal to the cut-off wavelength of the low pass filter at any point whereby no radiation would reach the detectors.
4. A radiation sensor as claimed in claim 1, 2 or 3 in which the edge wavelength of the or each edge filter varies along a straight line traversing the filter.
5. A radiation sensor as claimed in claim 4 in which the edge wavelength is constant in a direction perpendicular to said variation direction.
6. A radiation sensor as claimed in any preceding claim in which the array is a two dimensional array.
7. A radiation sensor as claimed in any preceding claim in which the detectors are sensitive to infrared radiation.

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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. A radiation sensor substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.