GB2312295A - Integrating sphere for spectroscopic analysis with reflector and baffle - Google Patents

Abstract

An integrating sphere, for use in optical spectroscopy, accommodates a reflector 20 which can direct analysing radiation towards a sample located at a sample position 16 or 17 and a radiation detector 18 which can receive radiation reflected from the sample and the internal surface 12 of the sphere. A baffle 22 is associated with the reflector and is designed to constrain the analysing beam in such a way as to reduce the amount of diffusely reflected radiation which reaches the detector.

Description

INTEGRATING SPHERE FOR SPECTROSCOPIC ANALYSIS This invention relates generally to field of optical spectroscopy. In particular it relates to spectroscopic apparatus of the type known as an integrating sphere.

An integrating sphere comprises a block shaped housing having a central spherical hollow volume the walls of which are diffusely reflective to analysing radiation typically that in the near infrared (NIR) region of the electromagnetic spectrum. The housing has two sample positions and a detector for detecting the analysing radiation. In use analysing radiation is directed towards one of the samples and the radiation scattered from the sample is internally reflected from the internal surfaces of the spherical volume and ultimately received at the detector.

The hollow volume effectively acts an optical integrator by virtue of the fact that radiation reflected from the sample is multiply reflected from the internal reflecting surface and the detector is able to collect equal proportions of the radiation reflected from the sample. This results from the spherical nature of the volume and the fact that the internal surface is diffusely reflective.

It is known in such an arrangement to provide a mirror in the path of the incoming radiation, the mirror being switchable between two positions and arranged such that in one position radiation is directed towards one of the sample positions, whilst it is other position the radiation is directed to the other sample position. The radiation employed will typically be in the near infrared region of the electromagnetic spectrum.

Integrating spheres are employed because they are relatively simple devices and because the light scattered over all solid angles is effectively collected they are relatively efficient. One of the sample positions is usually at the top of the housing and the other at the bottom.

This can be useful since some materials are more easily analysed when placed in the upper position, whilst others are more appropriately placed in the lower position. Another way in which the sphere can be used is to provide a reference sample in one of the sample positions and to compare a spectrum of that reference sample with a spectrum obtained from an unknown sample in the other position.

A problem with existing arrangements is that radiation reflected from the mirror is not only specularly reflected, but also diffusely reflected. This can give a rise to stray radiation being detected when no sample is present in the sample position. This can be between 1 to 3% of the reflected radiation depending upon the condition of the surface of the mirror.

The present invention is designed in order to reduce the problem caused by such diffused reflection.

According to the present invention there is provided an integrating sphere for use in optical spectroscopy, said sphere defining at least one sample position and having a reflector which can be positioned to direct radiation towards said sample position, and said sphere having an internal reflecting surface which can receive radiation reflected from a sample and direct it towards a radiation detector, wherein baffle means are associated with said reflector, said baffle means being so positioned and arranged as to constrain the radiation incident on and reflected from the reflector such that diffused reflection of the radiation is minimised.

The sphere may define two sample positions and the reflector may be arranged so that it can direct radiation to one or other of the sample positions.

The reflector and baffle means may be mounted intemally of the integrating sphere.

The baffle means may comprise an angle-shaped element having first and second limbs extending at an angle of substantially 90 , each limb having an aperture, one of said apertures providing an opening through which radiation incident on the mirror can pass, and the other aperture providing an opening through which radiation reflected from the reflector can pass.

The integrating sphere may comprise a block having an internal spherical volume, first, second, third and fourth openings extending through the block walls into the volume, said first opening providing a path for radiation into the volume, a second opening accommodating a detector, and the third and fourth openings providing said positions for said samples.

The first and second openings may be at diametrically opposite positions. The third and fourth positions may be at diametrically opposite positions on a diameter extending orthogonally to the diameter between the first and second openings.

The reflector may be located on the diameter through the first and second openings.

The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Figure 1 is a schematic side view in section of an integrating sphere in accordance with one embodiment of the present invention; Figure 2 is a schematic plan view in section of the integrating sphere, and Figure 3 is a perspective view of a baffle member used in the integrating sphere.

Referring to the drawings, an integrating sphere in accordance with an embodiment of the present invention is formed from a block shaped housing 10. The housing is formed with a central hollow spherical volume 11 and the internal surfaces 12, which define the spherical volume, are formed from material which is highly reflective to radiation used in analysis of samples. Typically the housing will be formed in two parts, each of which has a semi-spherical recess so arranged that when the two parts are brought together the spherical volume 11 is formed. The detailed way in which this is achieved does not need to be described in the present specification and will be known to those skilled in the art.

The wall of the block-shaped housing 10 is formed with four openings identified by reference numerals 14, 15, 16 and 17. The housings 14 and 15 are formed at opposite ends of a horizontal diameter through the block, whilst openings 16 and 17 are formed at opposite ends of a generally vertical diameter through the block. The opening 16 provides a path through which incoming radiation, typically radiation in the near infrared range of the electromagnetic spectrum, can pass. The radiation typically will be produced by a source in associated spectroscopic equipment such as that manufactured and sold under the CRmi) name "PARAGON"t, The way in which the radiation is directed to the input 14 is not critical and can, for example, be by way of an optical fibre coupling or by way of a suitable mirror arrangement. The opening 15 accommodates a detector 18 of the radiation used to investigate the samples. This detector, in response to incident radiation, produces an electrical output signal which can be fed to conventional analysing circuitry.

The openings 16 and 17 each define a position for a sample to be investigated. In operation of the device the openings 16 and 17 will accommodate a container in which a sample is located.

Internally of the volume 11, a mirror 20 is located on an axis extending between the openings 14 and 15. The mirror can be moved between a first position in which it reflects incoming radiation propagating through opening 14 towards opening 16 and a second position in which it deflects radiation towards the opening 17. In this way the mirror can be used to allow radiation to be incident upon either a sample in opening 16, or a sample in opening 17.

Associated with the mirror and movable with it is a baffle member 22. The baffle member is generally angle-shaped and comprises first and second limbs 23, 24, which extend generally at right-angles to each other. Each limb has formed therein an aperture 26, 27. The aperture 26 defines an opening through which incoming radiation has to pass in order to be incident upon the mirror, whilst aperture 27 defines an opening through which radiation reflected from the mirror can pass towards a sample. Thus, the apertures constrain the analysing radiation to paths defined by the size of their openings. In this way the baffle reduces the amount of diffusely reflected light, which becomes internally reflected within the volume 11. Preferably the baffles should have a matt non-reflecting surface and the apertures should be so designed that they do not clip the incoming radiation, nor the specular component. In order to achieve this the periphery of the outside of the apertures 26, 27 should be matt and non-reflecting.

The remainder of the outside of the baffle can be a good diffuse reflector so that optical throughput is not significantly affected.

Thus, in use of the apparatus, a sample is located in one or both of the sample positions 16 and 17. The mirror 20 is positioned so that incoming radiation through the opening 14 is directed towards one of those sample positions. The radiation scattered from the sample is internally reflected by the internal reflecting surface 12 of the volume 11 and ultimately received by the detector 18 which generates an output signal for analysis. The baffle member 22 constrains the analysing beam of radiation in such a way that it reduces the amount of diffusely reflected radiation from the internal mirror 20 reaching the detector and thus improves the performance of the apparatus.

Claims (9)

CLAIMS:

1. An integrating sphere for use in optical spectroscopy, said sphere defining at least one sample positions and having a reflector which can be positioned to direct radiation towards said sample position, and said sphere having an internal reflecting surface which can receive radiation reflected from a sample and direct it towards a radiation detector wherein baffle means are associated with said reflector, said baffle means being so positioned and arranged as to constrain the radiation incident on and reflected from the reflector such that diffused reflection of the radiation is minimised.

2. An integrating sphere according to claim 1, wherein the sphere defines two sample positions and the reflector is arranged so that it can direct radiation to one or other of those positions.

3. An integrating sphere according to claim 2, wherein the reflector and baffle means are mounted internally of the integrating sphere.

4. An integrating sphere according to any preceding claim, wherein the baffle means comprise an angle-shaped element having first and second limbs extending at an angle of substantially 90 , each limb having an aperture, one of said apertures providing an opening through which radiation incident on the mirror can pass, and the other aperture providing an opening through which radiation reflected from the reflector can pass.

5. An integrating sphere according to any one or claims 2 to 4 and comprising a block having an internal spherical volume, first, second, third and fourth openings extending through the block walls into the volume, said first opening providing a path for radiation into the volume, a second opening accommodating a detector, and the third and fourth openings providing said positions for said samples.

6. An integrating sphere according to claim 5, wherein the first and second openings are at diametrically opposite positions.

7. An integrating sphere according to claim 6, wherein the third and fourth positions are at diametrically opposite positions on a diameter extending orthogonally to the diameter between the first and second openings.

8. An integrating sphere according to claim 6 or claim 7, wherein the reflector is located on the diameter through the first and second openings.

9. An integrating sphere substantially as hereinbefore described with reference to and as shown in the accompanying drawings.