GB2427704A - Prism spectrometer with reduced optical aberration - Google Patents

Abstract

A simple prism spectrometer may be realised by omitting a conventionally used collimator. The resulting beam divergence gives rise to prism aberrations, which are substantially cancelled by opposite amounts of aberration present in an offset imaging mirror. Entrance slit 1, dispersing prism 3, focusing mirror 4M, sensor slit 5 and light sensitive pixel array 6 are shown. The mirror 4M may be tilted with respect to an incident beam axis. The angle of tilt may be close to 12{.

Description

Prism Spectrometer Spectrometers acquire and analyse spectral data for

many reasons, e.g. chemical analysis, process control, gas detection, colour measurement.

A large number of spectrometer designs exist on the commercial market.

They can be categorised in several ways, for example: (a) spectral range, (b) spectral resolution, and (c) scheme of acquiring spectral data. The spectral range often characterises the application e.g. infrared spectra are often used to identify organic molecules. Spectral resolution is influenced by application and for reasons of cost is no higher than it need be in any given application.

There are three main methods of acquiring spectroscopic data: (i) grating, (ii) Fourier transform (FT), (iii) prism. Grating is by far the most commonly used method. Fourier transform methods have come to the fore in the last twenty years and are characterised by having high "etendue" or light gathering power, making them suitable in low light level application. The subject of the present invention is a spectrometer using a prism for spectrum splitting. The prism spectrometer is largely superseded by grating and FT methods, but does still have advantages.

The prism spectrometer is free of some of the deficiencies of grating spectrometers such as overlapping spectra. The prism method described here forms the basis of a low cost instrument aimed at education and niche industrial applications.

In the present invention, the optical layout differs from previous prism spectrometers in a way as to be advantageous in terms of performance and cost. The present invention therefore affords a distinct commercial advantage in a competitive market place. The optical design requires just three optical components: slit, prism, and focussing means. In the preferred embodiment of the invention, the focussing means is a single surface concave spherical mirror. In prism spectrometers (prior art) it is common practice to pass collimated light through the prism, thereby requiring the presence of a collimator. After passing through the prism, the collimated light is focussed onto a second slit or sensor array, using a further component. Collimated light ensures the absence of certain optical aberrations such as spherical and coma, and prevents loss of spectral resolution.

Designs are known in which the collimated light, after passing through the prism, is reflected back through the prism for a second time and brought to focus using the collimating means. The arrangement has associated difficulties and does not lead to a low cost system.

In the present optical design, a collimator is dispensed with, and slightly diverging light passes through the prism before being focussed with an off- axis concave mirror. This design contains the essence of the invention. Not only is the design simple and hence low cost, but also the aberrations of the off-axis spherical concave mirror, act to substantially cancel or counteract aberrations of the beam acquired on passing through the prism.

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According to the present invention there is provided a prism spectrometer comprising entrance slit, dispersing prism, focussing means and sensor; light transmitted by the slit passes through and is dispersed by the prism before being focussed onto a sensor by said focussing means, characterised by the focussing means acting to substantially reduce optical aberrations of a light beam acquired on passing through the prism.

It will be appreciated that the absence of a collimator is a cost saving expediency, but one, which gives rise to some optical aberrations being imposed on the beam as it traverses the prism. In the preferred embodiment according to the present invention, a concave reflective mirror of spherical form focuses the beam, after traversing the prism, in an off-axis fashion. The focussing parameters of the concave mirror, together with the orientation and angle of the dispersion prism, are optimised so as to substantially reduce aberrations picked up by the beam on passing through the prism. This cancelling of aberrations is possible by virtue of the aberrations associated with the prism being negative, while those associated with the concave mirror are positive.

It is with some surprise that deficiencies of a low-cost imaging mirror, correct for (or more precisely, cancel) those aberrations resulting from the absence of a collimator.

The invention will now be described more fully with reference to the following figures:

Figures 1. shows an example of prior art.

Figure 2. shows an equivalent arrangement according to the present invention.

Figure 3. shows the preferred embodiment of the present invention.

Figure 1 shows the conventional optical arrangement of a prism spectrometer.

Traditionally, the focussing lens, 4, is provided with cross-wires and telescope and arranged to swivel about a vertical axis passing through the prism, for the purpose visual measurement. The table, upon which the prism is mounted, also rotates about the same vertical axis in order to maintain the angle of minimum deviation. This aspect is not relevant to the present invention and is not shown. Incident light (electromagnetic radiation) passes through slit, 1, and is collimated by lens, 2, before being dispersed by prism, 3. The resulting spectrum is rendered analysable by re-focussing the beam with a lens, 4, onto a suitable sensor (represented by slit, 5). Traditionally, the sensor consists of a scanning slit with a photo-sensor for measuring spectral intensity. Light passing through the prism is collimated in order to eliminate (or significantly reduce) certain geometrical optical aberrations the, so- called, seidel aberrations. In the case of prisms, these are: spherical aberration, coma, and astigmatism.

The equivalent arrangement of the prior art according to the present invention is shown in figure 2. It will be noticed that a collimating means is absent, and the beam divergence after the first slit is relatively narrow. The aperture of a focussing mirror, 4M, determines the numerical aperture of this beam.

Dispersing prism, 3, is located between the first slit, 1, and focussing mirror, 4M as shown in figure 2. The beam is folded by the focussing mirror and brought to a focus at sensor slit, 5.

Figure 3 shows the preferred embodiment of the invention in which the second, sensor slit is replaced by an array of light sensitive pixels, 6, such as is found in CCD cameras. Such a device is able to capture the entire spectrum without any moving parts. The full spectrum, created as a result of dispersion of the prism, is focussed simultaneously onto the CCD camera chip. As the spectrum is essentially one-dimensional, the camera chip need only consist of a one-dimensional array of photo- elements, and many such devices are commercially available. The number of applications using two- dimensional imaging devices now far exceeds those requiring one- dimensional devices, and so two-dimensional devices are now less expensive. The preferred embodiment uses a two-dimensional CCD camera chip, which is available on a small circuit board with control and drive electronics enabling it to operate via the USB port of modern PCs.

As mentioned above, optical aberrations in the focussing mirror, 4M, tend to cancel those aberrations due to a divergent beam passing through the prism.

The cancellation of astigmatism is less important than coma. This is because the image resolution, of the spectrum, need only be high in one dimension (that in the direction of the spectrum). Corrective coma is introduced into the imaging mirror by virtue of forming an offset image as indicated in figure 2.

That is, the focussing mirror, 4M, is tilted with respect to the beam axis of the beam incident upon it. In the preferred embodiment shown in figure 3, the mirror tilt-angle is close to 12 degrees, which is just sufficient to allow beam clearance of the prism at the blue end of the spectrum.

The optical design shown in figure 3 results in good cancellation of the coma produced respectively by the prism, 3, and imaging mirror, 4M, across the range of the spectrum. By way of example, the incident face of a 16 mm side equilateral dispersing prism, made of FS1 I glass, was placed 65 mm from a 0.1 mm wide slit. A spherical concave mirror, having a 40 mm radius of curvature, images the spectrum in the fashion shown in figure 3. The sensor array is angled for best focus across the spectrum. The spectral range is between 400 nm and 900 nm. This suits the sensitive range of most silicon based camera image sensing devices.

The preferred embodiment represents a solution within the physical constraints, which not only provides the conditions for good imaging performance but alào makes for a compact optical design.

The preferred optical design shown in figure 3 is essentially empirical in origin, with small adjustments made to angles, object distance and stop position in order to optimise performance. Persons skilled in the art of optical design will appreciate that many "optimum" designs can exist when the parameters, describing the optical components, and the design, are adjusted. The scope of this invention extends to include all such designs.

Another design similar to the one shown in figure 3, but adapted for use in ultraviolet radiation, has a prism fabricated in sapphire. Sapphire has similar refractive index to SL1 I glass used in the preferred embodiment, and therefore allows the same design to be used as a starting point with only minor adjustments needed to obtain an optimum design. Other prism materials having very broad spectral transmittances are series I metal halides.

These have low refractive indices and so considerable revision to the design of figure 3 is needed in order to find an optimum.

The use of a reflecting mirror to focus the spectrum makes it suitable for use with virtually any wavelength band.

Despite coma being substantially cancelled, the spectrometer in the example given above is not diffraction limited. That is, the resolution of the instrument is limited by geometrical aberrations present in the focussed image. This fact can be used to advantage by allowing the optics to be miniaturised. When the optics is scaled down in size, so too do the geometrical aberrations scale down by the same amount. The instrumental resolution does not increase, but the size of the instrument is considerable reduced.

By using a two-dimensional sensor array, with the line scans in the same direction as the spectrum, electronic noise may be suppressed in relation to the signal by adding any predetermined number of scan lines. Electronic noise is further suppressed by performing a rolling time-average on a preselected number of line-scans.

Claims (7)

1. Claims 1. According to the present invention there is provided a prism

   spectrometer comprising an entrance slit, a dispersing prism, a focussing means and a sensor; light transmitted by the slit, passes through and is dispersed by the prism before being focussed onto the sensor by said focussing means; charactensed by the focussing means acting to substantially reduce certain optical aberrations, of a light beam, which are acquired by the light beam on passing through the prism.
2. 2. Prism spectrometer according to claim 1 wherein prism is an equilateral dispersing prism, having all internal angles nominally 60 degrees.
3. 3. Prism spectrometer according to claims 1 or 2 wherein prism is fabricated in sapphire.
4. 4. Prism spectrometer according to any one of claims 1, 2, or 3 wherein focussing means is an offset spherical concave mirror.
5. 5. Prism spectrometer according to any one of the afore mentioned claims wherein sensor is a subdivided addressable one dimensional (linear) camera array.
6. 6. Prism spectrometer according to any one of the claims 1, 2, 3, or 4 wherein sensor is a subdivided addressable two dimensional camera array.
7. 7. Prism spectrometer according to accompanying figures 1 or 2.

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