GB2362460A - Spectroscope - Google Patents

Abstract

A spectroscope having an optical fibre light input (7) comprises a slit (1) for admission of light, a single focussing lens or mirror (2), a dispersion means such as a prism or grating (3), and a sensing array (5). The optical fibre may be a large diameter (1 mm) plastics fibre and a ball lens may be mounted between its end and the slit (1). A sample may be illuminated by a source and the light from the sample directed through the spectrometer. A separate detector may monitor the source to correct for variation in brightness. Alternatively a portion of the source light may be directed to an unused part of the sensor array (5) through an optical fibre.

Description

2362460 IMPROVEMENTS TO SPECTRONffiTERS AND SPECTROPHOTONTTERS

Spectrometers are used to examine the spectrum of a source of light which may be used as a measure of some property of the material of the source. For instance, the source may be a material at high temperature, or some sort of plasma, the constituent atoms or molecules of which emit light at characteristic wavelengths.

A spectrometer may also be used to measure the spectrum of light from a sparate source which has been transmitted through the material of interest which may absorb certain characteristic wavelengths and sometimes emit at others. In this case, the complete instrument of source, sample and spectrometer is usually called a spectrophotometer.

It is the object of this invention to reduce the number and required accuracy of components in spectrometers and spectrophotometers and to solve some common problems in their construction.

Conventional spectrometers have five main optical components. In addition they contain optical components that generate the light source, which is peculiar to each application, and which need not usually be of the highest accuracy because they do not affect the principal parameter of a spectrometer which is the resolution.

The layout of a typical conventional spectrometer is shown in Figure 1.

The light source which may be a narrow slit, 1. or the end of an optical fibre, or bundle of fibres.

A collimating lens or mirror 2. which produces a collimated beam ftom the divergent beam from the first element.

A Grating, 3. Or prism, which may be reflective, or transmissive, flat or concave, fixed or moving. If it is concave it can remove the need for either one or both of the second and fourth elements.

A focussing lens or mirror 4. which focusses the diffracted beam from the lens onto the final element.

An exit slit, single detector, or array of detectors 5. If this element is a slit, the spectrometer is usually known as a Monochromator.

a It is important in the interests of economy to minimise the number and required accuracy of the components.

The slit of the first element can be combined with the window which is often required to separate the dust-tight enclosure in which the optical elements are usually mounted. In this case the slit is printed in opaque ink or paint, or photographically reproduced on the material of the window, which may be plastic, glass, sapphire, or other suitable transparent material.

The focussing lens or mirror 4. may be dispensed with by reducing the focal length of mirror 2. The beam falling on the grating is then convergent, but still comes to a useful focus. This geometry is not as sensitive as that of figure 1, but is perfectly adequate for many applications. One advantage is that it is physically smaller for an equivalent performance.

Figure 2 shows the layout of a spectrometer showing this principle. Light enters through the slit 1. It is then focussed by a mirror, or lens 2. Not into a collimated beam as in Figure 1, but into a focussed beam. The light is then dispersed by the grating 3 and the dispersed light falls onto the array 5 This layout has to be used very carefully because the plane in which the dispersed light is focussed is not parallel to the grating, and if the geometry of the layout is not carefully chosen, may be too steeply inclined to be practical. It is only practical for narrow wavebands, with carefully chosen geometries.

This invention will be described as it applies to the layout of figure 2, but the same principles may be applied to any arrangement of mirrors and dispersing elements which may act to form a spectrum on a light sensitive array.

In some spectrometers the light enters through an optical fibre. In this case the slit may be dispensed with as in figure 3. Here there is no separate slit, the small size of the fibre, 6, acting as a sufficiently small source. The fibre may be typically 120 micrometers in diameter acting as a slit of the same width. However the vertical size of the source is inherently the same dimension, so the vertical dimension of the image of the source on the sensing array is the same. With CCD arrays with very small sensing elements this imposes severe restraints on the vertical stability of the optical system to prevent the image of the end of the fibre which is cast on the sensing array wandering off the array. The horizontal stability may be compensated by means of a reference line in the incoming light, whose wavelength is known. Movement of the image due to temperature, for instance, may be compensated by sensing the position of the reference line, and offsetting the rest of the spectrum correspondingly. This cannot be done in the vertical direction.

3 One solution to this is for the source to be formed by a number of fibres arranged in a vertical line. This is difficult to build.

Additionally, the 120micrometre diameter of the fibre is not always ideal. In some cases it would be desirable to have narrower source.

This invention uses a large diameter fibre, as shown in Figure 4, typically I min, which may be of plastic. This is much easier to handle than a small glass fibre, being inherently more robust- These large diameter fibres are much easier to trim and connect. The light from the fibre 7, illuminates the slit, 1 to give the same performance as the layout of figure 2. There is no difficulty in aligning the fibre and slit as the fibre is much wider than the slit. The fibre illuminates a Imin high section of the slit, so the image on the array is also of the order of I mm high, eliminating the need for high precision in the vertical alignment.

The layout of figure 4 could be used with a narrowfibre, but the fibre has in this case to be very carefully aligned with the slit.

The end of the fibre 7 shown in figure 4 must be as close as possible to the slit to avoid the loss of light. If this is a problem, a ball lens or other lens of very short focal length may be mounted between the end of the fibre and the slit as shown in figure 6.

The spectrometer may be used as the sensing part of a spectrophotorneter one layout of which is shown in figure 6. A light source 11, Illuminates a sample 9, preferably by means of a lens or mirror 10. The spectrum of the light from the sample is dispersed by the grating 3, and detected by the array 5. However, if it is required to measure the optical density of the sample in addition to the relative densities of the parts of its spectrum, then it is necessary to measure the brightness of the source 12, as this may typically be an incandescent lamp whose brightness may vary during its life, and with variations in the ambient temperature. This is usually accomplished by means of a photocell 12. However, variations in temperature may have different effects on the photocell 12, and the array 5, making the compensation inaccurate unless special precautions are taken.

This invention provides a separate path for a portion of the light from the lamp 11, to an unused part of the array 5, which may conveniently be an optical fibre 13 as shown in figure 7 The spectrum from the array might then have an appearance such as figure 8. Two normal peaks 14, are shown, which might appear in a typical spectrum. In addition there is a peak 15, which is caused by the light from the fibre 13 in figure 7, not by light which has passed through the sample 9 in figure 7. The height of peak 15 is therefore a measure of the brightness of the light source 11 in figure 7. If the heights of the peaks 14 are compared to the height of peak 15 instead of being measured absolutely, then they will be compensated for variations in the brightness of the source 11. In addition variations in the sensitivity of the array which might be caused for instance by temperature variations will affect peaks 14 and 15 similarly and therefore be cancelled out.

In order that the light from the fibre 13 in figure 7 should not occupy too much of the array, and to make it form a reasonably narrow peak, it may be terminated inside a black or darkened hole 20 shown in figure 9 mounted as near as possible to the array so that its light spreads as little as possible. Alternatively as in figure 10, there may be mounted between the end of the fibre and the array a ball lens or short focus lens 18, to focus the end of the fibre onto the array. These precautions are necessary because the sensitive area of the array 17 is behind a window 19 some distance from the fibre end. Alternatively a windowless array may be used.

Claims (6)

1. A spectroscope having a slit for the admission of light a single focussing mirror or lens a dispersion means and a sensing array.

2. A spectroscope as in claim 1 where the light applied to the slit from an optical fibre close to the slit

3. A spectroscope as in claim 2 where a focussing means is interposed between the fibre and the slit.

4. A spectroscope as in claim 1 or claim 2 having a means for bringing a sample of light from a light source to the sensing array.

5. A spectroscope as in claim 4 where the means for bringing a sample of light from the light source to the sensing array causes the light to pass through a narrow aperture close to the sensing array.

6. A spectroscope as in claim 4 where a focussing means is interposed between the means for bringing a sample of light from the light source to the sensing array and the array.