GB2313206A - Optical component for distributing radiation to foci - Google Patents

Abstract

An optical element which transmits, reflects or refracts incident radiation can be used as a radiation homogeniser, and may be a concave reflecting surface which is modified by concentric undulations that have the effect of deviating incident radiation so as to distribute foci in a predetermined zone. Bumps and facets can also be used instead of undulations to achieve a similar effect. The element may be used in an infrared gas detector (Fig 1).

Description

OPTICAL COMPONENT FOR DISTRIBUTING RADIATION This invention relates to an optical component for distributing radiation in accordance with a predetermined format or pattern. In a particular embodiment of the invention, which is useful in infrared gas detectors, the optical component may be used in producing a homogenous distribution of illumination. In such an embodiment, which may be termed an "illumination homogeniser" the optical component is capable, either by its own action, or with the aid of another optical component, of uniformly distributing incident radiation (i.e. illumination) in a predetermined zone. However, the invention may be used more broadly, i.e.

in accordance with the optical effect required for a particular application.

Generally speaking, many different optical techniques involve focusing radiation on a given point. For example, radiation from a source may be focused at a point where a photoelectric detector is located, whereby the detector generates a signal representing some measurable quantity of the incident radiation. While this enables the radiation to be concentrated at the focal point, thereby increasing the signal generated by the photoelectric detector, it can also lead to problems due to errors in focusing. These errors may be due to positional errors of optical components, or to misalignment of optical components, for example, in an optical path between the radiation source and the detector.

Such errors or misalignment may occur in the course of manufacture, particularly if one attempts to employ standard engineering tolerances without subsequent adjustment, or in the course of attempting accurate assembly of components.

There may then be a need for final adjustments or "tuning" in order to ensure accurate alignment. However, even though an instrument may be accurately aligned at the final stage of its manufacture, its environment may cause similar positional errors and misalignment to occur during use.. For example, alignment may be affected by vibration, shock and temperature. When instruments are subjected to harsh environments, they would need to be removed for maintenance or adjustment and this is not always convenient, nor possible. Consequently, it may be necessary to calibrate instruments frequently to ensure that their response is correct, accurate and reliable.

In imaging apparatus, it is important to focus an image accurately on the receiver. However, with nonimaging apparatus, it is not necessary to focus an image sharply on a receiver, because the receiver (such as a photodetector) need only respond to a level of incident radiation, since the level of the detector signal will vary in accordance with the level of radiation. As the detector will be highly sensitive to small changes in position when it is adjacent to a focal point of incident radiation, its signal will vary in accordance with the focusing errors and the problem mentioned above.

Various attempts have been made, in the prior art, to deal with this problem but without complete success. These prior art attempts include the use of scattering or diffusing materials, the use of diffractive optical elements (DOE) or holographic optical elements (HOE), and the use of defocusing. These techniques will be described briefly below.

The technique of using light scattering or diffusing materials relies on the interaction between light and microroughness or micro-irregularities, either on the surface, or within the bulk material of an optical component. For example, a radiation diffuser can be located in the optical path so as to scatter or to diffuse the radiation before it reaches a receiver (photodetector). The radiation is then distributed over a wide area so that focusing errors should make less difference. Typically, the roughness and irregularities of the diffusing surface are considerably smaller than the wavelength of the incident light. The resulting scatter is a complex function of the size and distribution of scattering centres, as well as the wavelength and angle of incidence of illumination. The scatter produced tends to exhibit significant angular nonuniformity and another major drawback is that the light is scattered over far wider area than is necessary and hence the amount of light received by a detector is drastically reduced. Hence, the signal level of the detector may be reduced to a point at which noise becomes a serious problem.

Thus, the inefficiency and non-uniformity of this technique makes it undesirable for most applications.

As an alternative, a DOE or HOE may be introduced into the optical path. Such an element has a microscopically fine pattern of phase or amplitude variations, which are smaller than the wavelength of light, recorded on its surface on within its bulk material. These phase or amplitude variations interact with incident radiation to produce a diffraction pattern. With closely defined illumination conditions, it is possible to produce holographic elements where the diffraction pattern can perform a huge variety of optical functions. Holographic elements which produce uniform illumination from a filament source have already been produced for work at visible wavelengths. However, materials and fabrication techniques for infra-red holograms are not fully developed and the cost of such components is uncertain. Additionally, DOEs and HOEs are wavelength and orientation sensitive and quite commonly introduce significant insertion losses which are thus serious drawbacks.

In the case of defocusing, a detector is located at a point some distance from the nominal focus, i.e. where the illumination (incident radiation) is spread over a larger area than at the focus. As the distance between the focus and the detector is increased, the illuminated area increases and uniformity improves. This simple defocusing technique is inexpensive, because it requires no additional components. It is also efficient, because the illumination is confined to a well-defined area. However, the amount of defocus realised is dependent upon the precise distance between the focus and the plane of the detector. For modest amounts of defocusing, the focal displacement required can be less than the production tolerances on optical and mechanical components. As a result, it is almost impossible to achieve repeatable results without careful adjustment.

Furthermore, small movements of components along the optical axis will result in significant changes in the intensity of radiation falling on the detector, hence affecting the detector signal. Thus, whilst defocusing has certain advantages, it also has drawbacks which means that it is not a fully satisfactory solution to the problems noted above.

The present invention seeks to solve these problems by providing an optical element for distributing radiation and which transmits, reflects or refracts incident radiation and which also modifies the transmission, reflection, or refraction of the radiation so that it is, or it can be, brought to a multiplicity of foci in a predetermined zone.

Preferably, the optical element has a surface which is modified by a plurality of features that are dimensioned and located for geometrically directing (e.g.reflecting or refracting) incident illumination onto respective foci in the predetermined zone. These features and their relative spacing are significantly larger than the wavelength of the incident illumination.

These features may be surface features, like undulations, facets or bumps, or they may be variations in the bulk material of the optical element, such as changes in refractive index.

In the case of using a modified reflecting surface, for example, the angle of incidence is equal to the angle of reflection at the modified reflecting surface. Hence, there are no effects due to dimensions or spacings less than the wavelength of incident illumination, which would otherwise lead to interaction between phase or amplitude variations (as with the DOEs and HOEs mentioned above). The features can be designed in this way to produce a predictable geometric reflection format or pattern of foci in the predetermined zone. This differs radically from the known techniques mentioned above. The features can be made geometrically similar and can be geometrically spaced in such a way that the distribution of radiation in the predetermined zone is made homogenous. Such geometric similarity and spacing may not be uniform, because the features may differ in size and spacing in some geometric way, e.g. becoming smaller or larger in a given direction (as in the arrangements described in more detail below).

The possibility of homogenising the radiation in this way is a further considerable advantage over the known techniques described above.

The optical element can be a concave mirror which would usually reflect incident radiation on to a point focus, but which has its reflecting surface modified so that the radiation is brought to a multiplicity of foci in a given focal plane. This "multiplicity of foci" may be finite, or it may be infinite, in the sense that there is a continuum of foci in the focal plane. The multiplicity of foci are preferably uniformly distributed so that the radiation is made homogenous in the predetermined zone.

This provides the advantage of spreading the radiation smoothly across the zone. In addition, the size of the zone can be controlled by the design of the surface modification of the concave mirror, so that the homogenised radiation occupies only a predetermined limited area in the focal plane. This has the advantage of confining the radiation into the zone of interest only, so that little or none is lost around a target area. (In the case of using a diffuser, much radiation would be lost in this way and the radiation would not be uniform.) A reflecting surface may be modified in various ways.

For example, a reflecting surface of a circular concave mirror can have features such as concentric undulations radiating outwardly from its geometric centre (which lies on the focal axis). These undulations may be sinusoidal with a change in frequency (spacing) and amplitude in the radial direction which is designed to produce a required degree of homogeneity and a required confinement of radiation into the predetermined zone. However, instead of using undulations, facets or bumps could be provided on the radiating surface of an otherwise generally concave mirror in order to achieve substantially similar results.

Instead of using a concave mirror to reflect and to focus the radiation, a lens may be used. In this case, the lens surface and/or material may include variations such that the radiation is brought to a multiplicity of foci in a predetermined zone. For such an implementation, the radiation is both transmitted and refracted by the lens.

It is not essential to use a focusing element, such as a concave mirror or a convex lens. A plane optical element may achieve the same purpose either by surface modification, or by changes in the material from which it is made, or both. For example, a flat optical plate could be modified in this way, so that the light it transmits is refracted or reflected in different predetermined directions and a focusing element of conventional design could then be used to focus the transmitted light onto a target area. However, the plane element may also have concentric surface deformations which provide, or contribute, to the focusing effect.

Embodiments of the invention will now be described with reference to the accompanying schematic drawings, in which: Fig. 1 represents an infra-red gas detector; Fig. 2 illustrates the conventional focusing arrangement, whereby infra-red radiation is focused onto a photodetector; Fig. 3 is a graph illustrating the sharp response of a photodetector located at the focal point in a conventional detector; Fig. 4 is another graph showing the flattening of the response due to using a diffusing element (with consequent loss of intensity); Figs. 5 and 6 are respectively a plan view and a section through a concave reflecting mirror; Fig. 7 is an enlarged view of a diametrical section of the mirror shown in Figs. 5 and 6; Fig. 8 is a detail of the enlargement shown in Fig. 7 and also showing how radiation is brought to a multiplicity of foci with homogenous distribution; Fig. 9 is a ray diagram for the purpose of showing how a concave mirror, according to the invention, provides a multiplicity of foci in such a way that the position of a detector, along the optical axis, is far less sensitive to change; Fig. 10 is a diagram showing the difference between a point focus S and an extended foci region F in a mirror modified in accordance with the preferred embodiment of the invention; and Fig. 11 is a diagram illustrating a typical facet on a concave reflecting surface.

Referring to Fig. 1, this schematically illustrates a gas detector which is typical of a kind used in the petrochemical industry and which is required to signify the presence of dangerous gases, or vapours, in hazardous environments. Such detectors are expected to provide continuous reliable service, despite environmental conditions, so that dangerous conditions are detected as soon as possible and steps can be taken to alleviate the situation. Whilst the gas detector will be described, by way of example, it is to be understood that the invention has a wider application and its use is not confined to this type of apparatus. However, the invention finds particular utility and application with gas detectors.

The detector shown in Fig. 1 includes walls 1, which define a gas chamber, a concave mirror 2, closing the end of the chamber, at least one source of radiation 3 and a corresponding detector 4 and various electronic circuitry for processing the detector signal and for producing a signal which is used to signify the presence and amount of gas which is detected. This electronic circuitry, which is of conventional construction and operation, forms no part of the invention and hence it will not be described in detail. Attention only need be given to the gas chamber defined by walls 1 and to the way in which the radiation from source 3 is reflected, by mirror 2, onto the detector 4. The gas chamber communicates with the environment, for example, through a port (not shown), either naturally, or with the use of a pump, so that the gas in the chamber affects the radiation and hence the signal generated by the detector 4. The source is usually a broad band source of radiation, filtered to provide signals which correspond to absorbing and non-absorbing regions of the absorption spectrum of the gas to be detected. The ratio of the signals received at these distinct wavelengths provides the basis for high accuracy gas detection and measurement. As this technique is well-known in the art, it requires no further explanation.

Referring to Fig. 2, radiation from the source 3 would normally be brought to a sharp focus 5 by a conventional concave mirror 2A. This focus lies on the optic axis 6 for the sake of illustration. A detector 4 needs to be located accurately at the focal point because any displacement, one way or the other, will mean that the sensitive element of the detector will move away from the sharply defined focused radiation. Fig. 3 seeks to illustrate this by showing a sharp peak at the focal point (displacement on either side of the sharp peak causing a rapid fall-off in signal level).

Fig. 2 also illustrates the position of a diffuser 7, shown by the broken lines. When diffuser 7 is used, it intercepts radiation on the optical path and scatters it over a wide area. The effect of this is illustrated very schematically by the graph of Fig. 4. In this case, the illumination level is like a plateau extending over a wide area, but the level of the plateau is much less than the peak response shown in Fig. 3. Therefore, although some degree of smoothing is obtained, far less light is received by the photodetector 4, whereby the signal/noise level is very low.

(In practice, the light scattered by the diffuser 7 is rarely homogenous).

Referring now to Figs. 5 and 6, a radiation homogeniser according to the invention will now be described with respect to the example of a spherical concave mirror.

In this example, the concave mirror is machined from a solid block of material with a specular finish and so that there are a series of annular undulations 8 which are represented by lines in Fig. 5. These lines form concentric circles although they are not visible as such in the finished mirror. The positions of the lines represent only the positions of circles which would be coincident with the standard concave mirror surface (i.e. at the zero point between the peaks and troughs of the undulations). These undulations extend radially from the centre to the circumference of the mirror.

Fig. 5 indicates how these undulations increase in frequency in the radial direction (from the centre to the circumference of the circle). Fig. 7 is an enlarged section (on a diameter) showing how the amplitude of the undulations also varies (diminishes) as they progressively extend outwardly in the radial direction. These undulations may be modulated in any way, to suit the purpose -of the application. Thus, they may be frequency modulated, amplitude modulated, or both. The amplitude modulation may be such that the amplitude is modulated like the envelope of a carrier wave in AM radio transmission. The degree of modulation is predetermined in accordance with mathematical and optical principles as will be described in more detail below.

Turning first to Fig. 8, this shows a small part of the enlarged section of Fig. 7 and the effect of the undulating surface on a point source of radiation S. The mirror surface 6 causes ray bundles to be brought to a multiplicity of foci F. In fact, the multiplicity of foci is infinite, in the sense that it extends continuously in a given plane P. It will be understood that Fig. 8 is for the purpose of explanation only, since a point source of illumination is not used and the drawing serves only to illustrate the principles involved.

The effect of the undulations on the concave mirror surface is best illustrated by Fig. 9.

Fig. 9 illustrates both the point focus 5 of a conventional concave mirror 2, as well as the extended focus 5A of a concave mirror 2A modified in accordance with the teachings of the invention. In the former case, the point focus 5 is sharply defined by the point to point contact of the cones of radiation 9 on each side of the focal point 5.

However, as the incident radiation on the modified mirror 2A is brought to a multiplicity of foci 5A, in the plane of the focal point 5 of the conventional mirror, the light is homogenously distributed over a controlled wider zone, which is centrally located in a waist-shaped region 10 between the cones of radiation 9A. Thus, a detector (not shown) which is on 5A, can be displaced, on either side of the point focus 5, along the optical axis 6, without substantially affecting the amount of light falling on the detector. This is due to the shape of the "waist" which permits displacement along the optical axis 6 without unduly affecting the cross-section of the "waist" and hence the amount of radiation received by the detector. A technique employed to design the number and shape of the undulations on the concave mirror surface, in order to obtain homogeneity of radiation, will now be described with reference to the mathematical concepts underlying this embodiment of the invention.

It will first be understood that the undulations are large compared to the wavelength of incident radiation.

Thus, the deviation of the optical rays can be calculated using conventional, geometrical optical ray tracing techniques. No complex diffractive or scattering effects are introduced, such as would be the case with, for example, an optical grating, where the spacing is comparable with the wavelengths of light and where diffraction effects can occur.

The amplitude and orientation of the undulation at each point on the surface of the mirror determines the deviation produced in the ray path. By spatially varying the deviation introduced by the undulation at a uniform rate, it is possible to produce uniform spreading of the radiation in the focal plane. Cycling the deviated rays back and forth over the same area improves the uniformity and further breaks up any image which the optics might otherwise attempt to form. Control over the amounts of homogenising is achieved by controlling the amplitude and rate of change (frequency) of the undulations. More particularly, if SAG Z represents the displacement, from mean zero, on either side of the usual concave mirror surface, then the parameters of the undulations may be calculated as follows: SAG Z =

where #Z = mn(Rc(n) - r) + dn, (2) (-1)n-1 x 5x10-3 mn = gradient coefficient = (rn - rn-1), (3) Rc(n) = radius of zone centre = rn + rn-1, (4) 2 dn = peak zone displacement = (1-)n x 1.25x10-3x (rn - rn-1), (5) n = zone number, rn = boundary radius zone n, r = current radial position, and c = radius of curvature.

TABLE "A" ZONE BOUNDARY RADII Zone radius Zone radius Number (n) rn Number (n) 0 -4.472 17 18.439 1 4.472 18 18.974 2 6.324 19 19.494 3 7.746 20 20.000 4 8.944 21 20.494 5 10.000 22 20.976 6 10.954 23 21.448 7 11.832 24 21.909 8 12.649 25 22.361 9 13.416 26 22.804 10 14.142 27 23.238 11 14.832 28 23.664 12 15.492 29 24.083 13 16.125 30 24.495 14 16.733 31 24.900 15 17.320 32 25.298 16 17.889 By way of an example, a concave mirror, modified in accordance with an example of the invention, was machined from solid aluminium, which was later protected, after machining was complete, with a SiO2 coating. First, a disc was produced which was 8 mm thick and which had a diameter of 54 mm. Next, the mirror surface was machined so as to produce the concave form modified by the surface undulations having peaks and troughs above and below the standard or conventional concave surface C (Fig. 7), i.e. positive and negative going regions +R and -R extending on respective sides of the zone boundary, where each zone boundary is represented by the corresponding concentric circle in Fig.

5. Each zone boundary lies on the conventional concave surface and it can be considered as a zone of zero displacement. The central zone, which is bounded by only the first zone boundary, may be either a peak or a trough.

Assuming the concave mirror to have a generally spherical surface with an outer diameter of 50.596 + 0.1 mm, and assuming that the number of zones (n) that are chosen is 32, then equation (1) represents the undulations, with the displacement from the zone boundary of SAG Z, above and below the zero displacement zone boundary. Each zone is defined by an inner and an outer radial boundary, i.e. which are adjacent to one another. The zones are circularly concentric and preferably have equal areas. This results in the outer zones being much thinner than the inner zones, although each zone contributes equally to the homogeneous spread of radiation in the focal plane. The displacement of the surface from the conventional spherical form is described by the equations (2)-(5), whereby the radius rn of each boundary zone can be calculated, along with the peak zone displacement dn. The slope deviation at each zone boundary alternates between + 5.0 mrad and - 5.0 mrad (see Fig. 8). There are no discontinuities, or abrupt changes in slope at the zone boundaries. In the example, the maximum macroscopic slope error was 2.5 mrad.

It may be considered that the effect of the undulations is to deviate an incident ray by + 10 mrad from its undeviated focal point position in the case of a conventional spherical concave surface. This can be seen in Fig. 8 where for example the spread of the foci is + 10 mrad on each side of a central point at which the point source would otherwise be brought to a focus (with a standard concave surface). In view of the doubling effect caused by the angle of incidence being equal to the angle of reflection, an incident ray is deflected through twice the angle of incidence in the focal plane in which the foci are spread by the homogeniser. Those skilled in the art will appreciate, from the geometry and symmetry of Fig. 8, that the undulations can be designed so as to produce a uniform homogeneity with a predetermined spread where the multiplicity of foci extend in the focal plane of the mirror.

It will be appreciated that the schematic diagrams described above serve to illustrate the principle of the particular aspect of the invention relating to undulations on a concave mirror surface. In practice, the concave mirror (2) reflects incident infra-red radiation received from source (3) onto an infra-red detector (4) in the gas detector shown schematically in Fig. 1. The detector is located in the focal plane so that its sensitive surface receives the homogenised radiation. In order to allow for some latitude in the manufacture and assembly of the components of the detector, the spread of the homogenised radiation may be slightly larger than the sensitive area of the radiation detector (3). 'Edge effects' are primarily associated with the use of an extended IR source as opposed to a point IR source. The 'edge effect' roll-off corresponds to the dimensions of the IR source. With appropriate design it is possible to compensate for these 'edge effects'. (This has not been done in the current design). Accordingly, a peripheral band of radiation (in which these edge effects may occur) may surround the radiation receiving surface of the infra-red detector, so that edge effects" can be ignored.

Instead of forming concentric undulating ripples on the surface of the concave mirror, it would also be possible to use a concave surface with multiple facets or bumps which are designed to produce the required focal deviations. In this case, finite foci will be gathered into a predetermined zone, rather than a continuum (as in the case of the undulations). However, homogeneity is preserved in the sense of equally spaced points in a given area, rather than a continuously uniform effect.

In the case of using plane facets, it can be assumed (with reference to Fig. 11) that for a generally concave mirror 6 having a radius of 120 mm, for example, at a distance of 120 mm, a 1 mm dia. plane facet 10 will produce a 2 mm dia. beam waist. Allowing for extended source illumination and optical astigmatism, this will produce a 3 mm dia. homogenized illumination zone 11. The surface could be made of 1600 1 mm dia. plane facets superimposed on the generally concave surface of radius 120 mm.

Instead of using plane facets, spherical facets could be used. For example, using 7.5 mm dia. facets with a different radius of curvature to the base radius of the surface it is possible to produce a design with less facets than for a plane facetted design. In the case of the gas detector described herein, for example, the facet radius of curvature should be 180 mm for 2.5 mm of homogenising dispersion. (R = 163.64 mm for 2.0 mm of homogenising dispersion).

Whilst this may appear to combine multiple facet techniques with defocus, the very gentle convergence of the beam from a 7.5 mm facet to a 2 - 2.5 mm dia. waist (in 120 mm) will be relatively insensitive to change in the facet radius of curvature.

The surface would comprise approximately 45 spherical facets (diameter 7.5 mm) superimposed on a concave surface of radius 120 mm.

The design of the reflecting surface may be modelled by a computer which is programmed with appropriate optical formula and data. However, it should also be possible to produce the reflecting surface with a profile which creates a desirable effect by a process of trial and error, based on results achieved from mathematical analysis.

It is also envisaged that instead of using a reflecting surface, a lens may be used on which surface variations produce a similar effect. It may also be possible to use materials having different refractive indices for the same purpose.

It is also feasible to use a plane surface having variations which cause the required effect and which may require other optical components in order to produce some form of focusing effect.

When applied to a gas detector, like that shown in Fig. 1, the modified concave mirror greatly reduces sensitivity to misalignment of optical components (that might otherwise lead to the problems resulting from the focusing errors described above, i.e. where conventional mirrors are used to produce a point focus). This reduced sensitivity enables the gas detector to operate correctly, despite small changes in alignment due to vibration, shock and temperature. Furthermore, it can simplify production by allowing assembly of components to standard engineering tolerances, without the need for adjustment or alignment.

Claims (11)

1. An optical element which transmits, reflects or refracts incident radiation and which also modifies the transmission, reflection or refraction of the radiation so that it is, or it can be, brought to a multiplicity of foci in a predetermined zone.

2. An optical element according to claim 1 which has a plurality of features that are dimensioned and located so as to direct the radiation to said respective foci, said features and their relative spacing being significantly larger than the wavelength of incident illumination.

3. An optical element according to claim 2 in which the features are geometrically similar and are geometrically spaced.

4. An optical element according to claim 2 or 3 in which the features are surface features, including any one or more of undulations, facets or bumps.

5. An optical element according to claim 2 or 3 in which the features are variations in the bulk material of the optical element, including changes in refractive index.

6. An optical element according to claim 1 which is in the form of a concave mirror that usually reflects incident radiation into a point focus, but which has its reflecting surface modified so that the radiation is brought to a multiplicity of foci in a given focal plane.

7. An optical element according to claim 6 in which the concave mirror has a generally spherical concave shape, the surface of the mirror being modified by concentric undulations which radiate outwardly from its geometric centre.

8. An optical element according to claim 7 in which the undulations occur between zone boundaries, the zone boundaries representing positions where there is zero displacement of the surface from the conventional spherical form; zones between said boundaries having substantially equal areas.

9. An optical element according to claim 6 in which the reflecting surface is modified with facets.

10. An optical element according to claim 6 in which the reflecting surface is modified by bumps.

11. A gas detector including the optical element according to any one of the preceding claims and a radiation receiver located at said predetermined zone in which incident radiation is brought to said multiplicity of foci.