The present invention relates to retroreflectors, and to methods of manufacture and use of such retroreflectors. Embodiments of the present invention are particularly suitable for, but not limited to, providing a retroreflector having both a large acceptance angle and a relatively large scattering cross-section.
A retroreflector is a device that reflects incident light or other electromagnetic radiation from a source back towards the source. Retroreflectors function over a range of angles of incidence, in contrast to plane mirrors, which only function as retroreflectors if the mirror is exactly perpendicular to the incident light beam. The acceptance angle is the range of incident angles over which the retroreflector will reflect light or other radiation back from where it came.
Retroreflectors are utilised in a range of applications, particularly for providing positional information. For example, cats-eye retroreflectors located along the centre of a road-surface provide a convenient reference for vehicle drivers at night. Retroreflectors are also utilised in electronic distance measurement and object tracking over a variety of ranges. Typically, lasers are utilised to provide a collimated incident beam for reflection by the retroreflector, which ideally provides a collimated return beam. To maximise the range of operation of the apparatus, it is desirable that the retroreflector provides efficient reflection of the incident beam.
Two broad categories of retroreflector are commonly used: retroreflectors formed of corner-cube mirrors or prisms, and cats-eye retroreflectors.
A corner-cube mirror or prism provides an almost perfect collimated return beam, but typically provides a limited angle of operation (acceptance angle) e.g. +/−10° at a loss of −3 dB for a corner cube mirror retroreflectors, and +/−15° at a loss of −3 dB for a typical glass prism. As described within the article by Handerek V. and Laycock L. “Feasibility of retroreflective free-space optical communication using retroreflectors with very wide angle of view”, in “Advanced Free-Space Optical Communication Techniques and Technologies (Ross M and Scott A. M. Eds.)”, Proceedings of SPIE Vol. 5614, 2004, such corner-cube devices suffer from greatly reduced performance at larger angles.
In many applications it is desirable to utilise a device having a much larger acceptance angle, so that the retroreflector may be targeted from a range of different positions. Example applications include free-space optical communications, electronic distance measurement (particularly building and construction surveys) and laser tracking.
A cats-eye retroreflector has a wide viewing or acceptance angle. The article by Takatsuji et. al. “Whole viewing angle cat's eye retroreflector as a target for laser trackers”, Measurement Science and Technology, Vol 10, pp N87-N90, 1999, describes a spherical retroreflector.
The article by Handerek V, McArdle H. Willats T., Psaila N. and Laycock L. “Experimental Retroreflectors with very wide Field of View for Free-Space Optical Communications”, 2nd EMRS DTC Technical Conference, Edinburg 2005, describes how a retroreflector can be formed using a ball lens of lower refractive index (i.e. potentially cheaper) material.
The article by Burmistrov V. B. et al., “Spherical Retroreflector with an Extremely Small Target Error: International Experiment in Space”, 11th International Workshop on Laser Ranging, Deggendorf, Germany, 21-25 Sep. 1998, describes a multi-layer spherical retroreflector.
It is an aim of embodiments of the present invention to provide a retroreflector that addresses one or more of the problems of the prior art, whether described herein or otherwise. It is an aim of a particular embodiment of the present invention to provide a retroreflector that provides a relatively high efficiency return signal over a wide range of acceptance angles.
In a first aspect, the present invention provides a retroreflector comprising at least two concentric spherical layers, a first of said layers being of uniform refractive index n1 surrounding a second of said layers of uniform refractive index n2, wherein the refractive indices satisfy the criteria n1>n2.
The present inventor has appreciated that spherical aberration severely limits the performance of many prior art spherical retroreflectors. A retroreflector, in which the first (outer) layer has a higher refractive index than the second (inner) layer, provides a curved surface (formed between the layers) having a negative power. The inventor has appreciated that such a curved surface can be utilised to compensate for spherical aberration, thus allowing a significant increase in the performance of the retroreflector. The layers can be arranged to provide either partial or full correction of spherical aberration, depending upon the desired application. Such retroreflectors can be formed from materials having a refractive index significantly less than 2 i.e. allowing the use of relatively cheap materials to provide relatively efficient retroreflectors having large angles of acceptance.
The second layer may be a sphere.
Said concentric spherical layers may provide a negative optical power to incident radiation of a predetermined wavelength to compensate for spherical aberration.
The retroreflector may further comprise a reflective coating of predetermined thickness located on the external surface of the outermost of said layers.
The retroreflector may further comprise a partially-reflective coating of predetermined thickness extending uniformly around the complete outer surface of the outermost of said layers.
Said concentric spherical layers may be dimensioned such that collimated radiation incident on an outer surface of the retroreflector at a first position is brought to a focus on an outer surface of the retroreflector at a second position opposite to the first position.
Said first and second positions may be located on the external surface of the outermost of said layers.
The retroreflector may further comprise a concave reflective surface positioned a predetermined distance from the outermost concentric layer.
Said concave reflective surface may be a hemispherical reflector having a radial centre concentric with said spherical layers.
Said second position may be located on concave reflective surface.
Said at least two concentric spherical layers may consist only of first and second layers, the first layer having an external radius of r1 and the second layer having an external radius of r2, which satisfy the criteria
The retroreflector may further comprise a third of said layers having a uniform refractive index n3 surrounding the first of said layers, wherein the refractive indices satisfy the criteria n1>n2>n3.
Said at least two concentric spherical layers may consist only of the first, second and third layers, the first layer having an external radius of r1, the second layer having an external radius of r2, and the third layer having an external radius of r3, the refractive indices and radii of said layers being arranged to provide a predetermined degree of spherical aberration compensation.
The retroreflector may further comprise at least a third and a fourth of said concentric layers.
The retroreflector may have a scattering cross section of at least 5,000 m2.
The retroreflector may comprise one or more surfaces with a total negative optical power of greater than 10% of the total positive power.
Said criteria may be satisfied for at least two different wavelengths of incident radiation.
The materials forming said layers may substantially satisfy the athermal condition over a predetermined temperature range.
The retroreflector may further comprise an optical modulator arranged to modulate incident radiation.
According to the second aspect of the present invention there is provided a method of manufacturing a retroreflector comprising at least two concentric spherical layers, the method comprising:
providing a first spherical layer of uniform refractive index n1 around a second spherical layer of uniform refractive index n2, wherein the refractive indices satisfy the criteria n1>n2.
The method may further comprise the steps of:
calculating the radii and refractive indices of said concentric layers required to provide a predetermined degree of at least one of spherical aberration compensation and scattering cross-section; and
forming the layers having the calculated radii and refractive indices.
According to a third aspect of the present invention there is provided a method of operation of a retroreflector comprising at least two concentric spherical layers, a first of said layers being of uniform refractive index n1 surrounding a second of said layers of uniform refractive index n2, wherein the refractive indices satisfy the criteria n1>n2, the method comprising:
directing a radiation beam to reflect from the retroreflector; and
measuring a predetermined property of the radiation beam reflected from the retroreflector.
According to a fourth aspect of the present invention there is provided a retroreflector comprising a sphere having a predetermined refractive index and radius such that spherical aberration is at least partially compensated for by primary defocus.
The present inventor has realised that the performance of single layer spherical retroreflector devices can be improved by selecting the radius of the device such that spherical aberration of the device is corrected by primary defocus i.e. such that the optical performance of the retroreflector is actually enhanced by spherical aberration. This allows the provision of simple, but relatively cheap retroreflectors (i.e. formed of materials have a refractive index less than two), that have reasonable performance and wide acceptance angles.
The sphere may be formed of S-LAH79 optical glass.
The sphere may have a radius of less than 6 mm.
The sphere may have a scattering cross-section of at least 5 m2.
According to a fifth aspect of the present invention there is provided a method of manufacturing a retroreflector comprising a sphere, the method comprising:
designing a sphere to have a predetermined refractive index and radius such that spherical aberration is at least partially compensated for by primary defocus; and
manufacturing the designed sphere.
According to a sixth aspect of the present invention there is provided a method of operation of a retroreflector comprising a sphere having a predetermined refractive index and radius such that spherical aberration is at least partially compensated for by primary defocus, the method comprising:
directing a radiation beam to reflect from the retroreflector; and
measuring a predetermined property of the radiation beam reflected from the retroreflector to determine the position of the retroreflector.
According to a seventh aspect of the present invention there is provided a spherical retroreflector, comprising at least one concentric spherical layer, the values of the refractive index and radius of each of said at least one layer correcting for spherical aberration within the retroreflector for incident radiation of predetermined wavelength. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
The present inventor has realised that the performance of spherical retroreflectors can be improved by correcting for spherical aberration. By controlling the geometry of the layer(s) of spherical retroreflectors, and by selecting materials of appropriate refractive index for the layer(s), the present inventor has realised that spherical aberration can be corrected for within such retroreflectors, increasing the performance of (i.e. magnitude of the reflected beam from) spherical retroreflectors. The correction for spherical aberration is ideally full correction, but it will be appreciated that any partial correction of spherical aberration can result in improved performance of the retroreflector. Correcting for spherical aberration within the retroreflector ensures that the radiation beam, at the point of reflection, has less longitudinal spherical aberration (preferably negligible), and hence reflects more efficiently.
For example, multi-layer retroreflectors can be formed that include a curved surface having a negative power, this curved surface then being used to correct the spherical aberration. The curved surface having a negative power can be formed at the interface between two of the layers of a multi-layer spherical reflector, by ensuring that the outermost layer has a higher refractive index. Such multi-layer retroreflector devices can be of a structure akin to the layers of an onion i.e. with each layer contacting one or more adjacent layers, such that the interface between adjacent layers of different refractive index will have an optical power. In most implementations, the external surface of the outermost layer can be coated with a partially reflective coating to enhance the performance of the retroreflector. Preferably, the partially reflective coating has a reflectivity of approximately one third for the electromagnetic radiation wavelength normally used with the retroreflector.
Such spherical retroreflectors do not require the use of expensive components/materials (e.g. components or materials having a refractive index of about two); it is therefore possible to construct devices having a relatively high performance at a reasonable cost using conventional optical materials and fabrication techniques.
The performance of a spherical reflector can be characterised by the scattering cross-section (SCS). The reference book by Weik, M. H., Communications Standard Dictionary, 1997, Chapman & Hall, defines the SCS as “the area of an incident wave front, at a reflecting surface or medium, such as an object in space, through which will pass radiant energy, that, if isotropically scattered from that point, would produce the same power at a given receiver as is actually provided by the entire reflecting surface”. The SCS is also referred to as the optical cross-section and the radar cross-section e.g. see Peebles P. Z., “Radar Principles”, John Wiley & Sons, New York, 1998.
The value of the SCS for retroreflectors at optical wavelengths is typically relatively large, because of the high directionality of the return beam. The SCS for a perfect circular mirror at normal incidence can be calculated using the Fraunhofer model of diffraction, as for instance described on page 396 of Born M. and Wolf E. “Principles of Optics (4th ed.)”, Pergamon Press, Oxford, 1970, as:
where d is the diameter of the mirror and λ is the wavelength. For convenience d may be expressed in mm and λ in microns to give SCS in m2. For example, a circular mirror of diameter 25 mm with a reflectivity of 100% provides an SCS of 7.57 km2 at 632 nm.
The SCS of non-planar reflecting devices, such as the spherical reflectors described herein, can be predicted using numerical techniques. For example, an optical software design package can be utilised to compute the efficiency (Strehl ratio) of light emitted from a pre-defined entrance/exit pupil in the case of planar elimination. The Strehl ratio in such a context is the ratio between the actual intensity of the optical access and the intensity that would be produced by a mirror of the same aperture with a reflectivity of 100%. The SCS of a non-planar reflector can be estimated from the limiting value in equation (2) below.
where D is the diameter of the sphere and S(d) is the Strehl ratio computed for an exit pupil of diameter d. S(d) must be evaluated at a number of values for d in order to find the limit in equation (2). For small values of d, S(d) will have a value close to the overall reflective efficiency of the sphere (15%). As d becomes larger, S(d) will fall as 1/d4 due to optical aberration. The SCS is the limit of equation (2) for large d. The computation of S(d) must be sufficiently accurate to give convergence, and this typically requires that a relatively large number of rays must be traced (i.e. typically 104).
Values of SCS as quoted herein have been calculated using the optical software design package Zemax™, a software package produced by the ZEMAX Development Corporation of Bellevue, Wash., USA.
In most implementations, to allow easy manufacturing of the retroreflector, the outer layer 102 will be formed in two or more segments. In the example shown in
It will be observed that the structure of the retroreflector 100 in
However, the retroreflector in
In the retroreflector 100, the relative radii r1, r2 and refractive indices of the materials forming the layers 102, 104 are selected such that light from a distant source is brought to a focus at a point 114 on the outer wall or surface of the device 100.
The radii and the refractive indices are the correct values to bring the collimated beam to a focus on the outer wall if the below equation (3) is satisfied.
Ideally, the refractive indices and radii are also such that the retroreflector is fully corrected for spherical aberration. The third order spherical aberration is given by the expression −1/8*S1h4, where S1 is the first Seidel sum and h is the ray height. Further detail regarding the first Seidel sum can be found within the book by Welford W. T. “Abberations of the Symmetrical Optical System”, Academic Press, London, 1974.
For a unit ray height, the third order spherical aberration is given by equation (4) below. Ideally, values of radii and refractive index are selected such that the equation is zero i.e. so as to provide zero spherical aberration.
Ideal solutions to both equations (3) and (4) are rarely found in real devices, as real optical materials (i.e. transparent materials of sufficient homogeneity and transparency to be utilised in optical devices) have discrete values of refractive index. The selection of materials (and hence the values of n1 and n2) is therefore limited, and in many instances it will be impossible to utilise materials for the layers of the retroreflector that have exactly the desired refractive indices to provide zero spherical aberration. Hence during the design process, taking into account the desired function of the retroreflector and the acceptable cost, materials will be selected that minimise the value of expression (4). Rarely will the relevant materials lead to the expression (4) being equal to zero. Table 1 below provides examples of suitable pairs of materials for the different layers 102, 104.
The material GLS (Gallium/Lanthanum Sulphide) within materials pair B(ii) is a new chalcogenide glass, recently produced by the spin-off company ChG Southampton Ltd from the University of Southampton's Optoelectronics Research Centre. GLS has many desirable properties for use in retroreflectors as described herein, including it being hard enough to be handled easily without damage, and the fact that it can be hot-pressed into any required shape. Potentially, the outer layer(s) of retroreflectors (i.e. any layer surrounding an inner sphere) could be formed in one step, without requiring such outer layer(s) to be split into two or more segments (e.g. hemispherical sections) for assembly.
Once the material types have been chosen, then n1 and n2 are generally fixed. The designer then has control over only one parameter, r2/r1, so as to satisfy equation (3), and hence give zero primary aberration (i.e. zero defocus). The graph shown in
Further analysis has indicated that it is generally better to choose material pairs that fall just below the curve within
For example, a device of external diameter (r1) 25.0 mm formed using the materials pair B(iii) would, according to equation (3), ideally have an inner radius (r2) of 9.087 mm. The predicted SCS from ray tracing for this configuration is 15,700 m2. However, if the inner radius (r2) is reduced slightly, to 8.970 mm, then the SCS is actually increased to 70,000 m2. The graph in
Similarly, taking an outer diameter of 25.0 mm as the starting point, for materials pair B(i) the inner radius calculated using equation (3) would be 8.5998 mm, giving an SCS of 10,300 m2. However, if the radius is increased to 8.6162 mm, then the SCS becomes 40,000 m2. Further, for materials pair B(ii), the inner radius calculated using equation (3) would be 5.9006 mm, giving an SCS of 2000 m2. However, if the radius r2 is increased to 5.9024 mm, then the SCS is increased to 2,800 m2. The relatively low values of SCS for pair B(ii) indicate that ideally any material pair should have refractive indices that are closer to the curve in
For comparison, it should be realised that the SCS of even the material pair B(ii) is better than the performance of known prior art spherical retroreflectors. For example, the SCS of the 50 mm spherical reflector described with reference to
As previously, a collimated incident radiation beam 210 will be refracted by the different layers (beam shown by arrows 212) to be focussed at a point on the outer wall 214 of the retroreflector 200. The beam will be reflected position 214, back along the incident path (212, 210). Spherical aberration of the reflected beam compared with the incident beam can be measured as a function of radial distance, in the reflected beam at the position indicated by Arrow 230.
If the (outer) radii of the layers 202, 204, 206 are denoted respectively by r1, r2 and r3, and the refractive indices by n1, n2 and n3, then the ray height at the reflected surface (position 214) should be zero i.e. the condition expressed by equation (5) should be met.
[It should be noted that, in three-layer devices within the claims, slightly different denotations are utilised than above, for consistent terminology between two layer and three layer devices; in particular, in the claims, the middle layer (204) is indicated as having radius r1 and refractive index n1, the inner layer (206) radius r2 and refractive index n2, and the outer layer (202) refractive index n3 and r3 i.e. a three layer device can be regarded as a two layer device, within the addition of an outer layer of relatively low refractive index]
An advantage of the three-layer configuration is that, with suitable choice of materials, more than one set of values of r2/r1 and r3/r1 will satisfy the requirement of equation (5). The values for r2/r1 and r3/r1 can be chosen to control the spherical aberration and hence maximise the SCS of the device using conventional iterative techniques of optical lens design. An iterative technique is best, as the expression for third-order spherical aberration of this three-layer type of retroreflector is relatively lengthy e.g. it can be given by the expression:
2/r3/r2̂2/n2̂2/n1+1/r3/r2/n2̂2/r1−2/r3̂2/n2̂2/n3/r2+1/r3̂2/n2/n3/r1
−1/r3/r2/n2̂2/r1/n1−1/r3̂2/r2/n1/n3̂2+1/r3/r2̂2/n1̂2/n3
−1/r3/r2/n1̂2/n3/r1+1/r3/r2/n1/n3/r1−1/r3/r2̂2/n1̂2/n2
+1/r3/r2/n1̂2/n2/r1−1/r3/r2/n1/n2/r1+1/2/r3̂2/r1/n1/n3̂2
+1/4/r3/r1̂2/n1̂2/n3−1/2/r3/r1̂2/n1/n3−1/4/r3/r1̂2/n1̂2/n2
+1/2/r3/r1̂2/n1/n2−1/r2̂2/n1̂2/n2/r1−1/r3̂2/n1/n2̂2/r2
+2/r3̂2/n2/n3/r2/n1−1/4/r2̂3/n1̂3−1/r3̂2/n2/n3/r1/n1
+1/r3̂2/r2/n2/n3̂2+1/r3/r2̂2/n2̂2/n3−2/r3/r2̂2/n2/n3/n1
+1/r3/r2/n2/n3/r1/n1−1/r3/r2/n2/n3/r1+1/4/r2̂3/n2̂3
−1/r2̂3/n1/n2̂2+1/r2̂3/n1̂2/n2+1/2/r2̂2/n1̂3/r1
−1/2/r2̂2/n1̂2/r1−1/4/r2/r1̂2/n1̂3+1/2/r2/r1̂2/n1̂2
+1/r2̂2/n1/n2/r1+1/2/r2̂2/r1/n1/n2̂2+1/4/r2/r1̂2/n1̂2/n2
−1/2/r2/r1̂2/n1/n2−1/2/r2̂2/r1/n2̂2+1/4/r2/r1̂2/n2
−1/4/r2/r1̂2/n1−1/4/r3̂3/n2̂3+1/4/r3̂3/n3̂3+1/2/r3̂2/r1/n1/n2̂2
+1/r3̂2/n2̂3/r2−1/r3̂3/n2/n3̂2+1/r3̂3/n2̂2/n3−1/r3/r2̂2/n2̂3
−1/2/r3̂2/r1/n3̂2+1/4/r3/r1̂2/n3−1/4/r3/r1̂2/n2−1/2/r3̂2/r1/n2̂2
+1/8/r1̂3/n1−1/8/r1̂3/n1̂2
As previously, the outer radius (r1) of the spherical retroreflector can be selected as desired. Using the above expression, the radii of the inner surfaces (r2 and r3) could be chosen to minimise third-order spherical aberration, but a design technique based on iterative ray-tracing is preferred. Such ray-tracing allows a non-zero value for primary and third-order spherical aberration to be used to balance higher order terms, thus minimising further the spherical aberration provided by the device, and maximising the SCS of the resulting retroreflector.
An example 3-layer device is now described in detail for use at a wavelength of 690 nm. Three glasses, BK6, STF2 and TF10, are selected from the GOST-RUS standard range (e.g. see “GlassBank” at www.ifmo.ru). The thermal expansion properties of these glasses are compatible. The nominal refractive indices of these glasses at 690 nm are computed using the catalog Sellmeier coefficients as 1.5361396, 1.9265330 and 1.7937143 respectively. Samples of the glasses were obtained and the actual refractive indices measured at wavelengths of 587, 643 and 706 nm. Refractive index interpolation using the linear model described in Langenbach E. “Melt Dependent Refractive Index Interpolation for Optical Glasses”, Paper number 3737-06, SPIE Proceedings: Design and Engineering of Optical Systems II, Berlin, 1999 gives the estimated refractive indices at 690 nm as: 1.5368, 1.9273 and 1.7926 respectively. The optical design shown in Table 2 was then produced using an iterative ray-tracing method.
The joints were glued with high-index optical cement, and the outer section (BK6) coated with a ¼ wave TiO2 part-reflective coating
The aberration performance for this design is shown in
This device was fabricated and tested in conjunction with a commercially available survey instrument (Sokkia SET3 230RM). A range of detection exceeding 1,500 metres was achieved. This represents adequate performance for many applications. The correction offset value for distance measurement (to obtain the centre of the sphere as if it were in free space) is −82.52 mm.
Following manufacture of the above sample, the inventor realised that the radii and tolerances could have been further optimised. Further, the part-reflective outer coating 208 should preferably have had a reflectivity of around 33%, rather than the 20% or so provided by the ¼ wavelength TiO2 coating. For example, better performance could have been provided by using a multilayer dielectric coating such as a three-layer ¼ wavelength TiO2/SiO2/TiO2 coating. Alternatively, a thin metal coating of appropriate thickness could have been utilised. Finally, performance of the three-layer device could have been further improved by utilising an anti-reflective coating on the surface of the middle layer 204 formed of STF2.
In the above example, the band width of the retroreflector is relatively limited, and hence it is desirable to only utilise the device in conjunction with a collimated radiation beam of wavelength 690 nm. In some applications, it may be preferable that the retroreflector is arranged for operation at more than one wavelength of incident radiation.
In many applications, it is preferable that the device is suitable for working over a reasonably wide temperature range.
For ease of manufacture, the two outer layers 302, 304 are each formed from hemispherical sections, which are joined together by optical cement along joint 322.
The materials forming the layers are chosen to give optimum performance at two wavelengths rather than one. In other words, as per the retroreflector indicated in
Although the device is described for operation at the wavelength pair of 690 nm and 380 nm, it will be appreciated that the same procedure could be used to design reflectors for other pairs of wavelengths of electromagnetic radiation e.g. in both the visible and infrared ranges.
The outer layer (302) is formed from S-TIL6 optical glass from Ohara. The middle layer (304) is Zinc Selenide sublimate ceramic (ZnSe) and the inner layer (306) is S-LAH79 optical glass from Ohara. The inner sphere (306) is formed by joining two hemispheres since glass of the required thickness to make a complete ball is not commercially available. This joining (320) is achieved using optical diffusion bonding rather than cement to avoid problems with internal reflection. The refractive index values for these materials at different temperatures are given below, in table 3, and the corresponding radii of each layer (including design tolerances) within table 4.
The SCS for this device is 0.427 km2 at 690 nm and 0.377 km2 at 830 nm. A scale ray-trace drawing for this device is shown in
From the above teachings, it will be apparent that various alternative embodiments will fall within the scope of the present invention. For example, although only two and three layer devices are described above, it should be appreciated that devices could be formed having other number of layers. A spherical retroreflector could be formed having four or more concentric spherical layers, allowing potentially improved device performance/greater selection of different materials but at the cost of increased manufacturing complexity.
Equally, the basic structure outlined herein could be modified. For example a retroreflector could incorporate a modulator structure, arranged to modulate the radiation beam. Such a modulator could be a passive modulation structure such as a fixed grating, or could be an active modulation structure e.g. incorporating a layer of nematic liquid crystal, with the orientation of the liquid crystal being controlled by applying a voltage to adjacent electrodes so as to modulate the incident radiation beam. The article “Large-aperture multiple quantum well modulating retroreflector for free-space optical data transfer on unmanned aerial vehicles”, by G. C. Gilbreath et al, Optical. Engineering, volume 40, number 7, pp 1348-1356, 2001, describes suitable modulator structures.
The possibility of providing a partially reflective coating to the outer surface of the outer layer has been described. However, if desired, a fully reflective coating (e.g. reflectivity 90% or greater, more preferably substantially 100%) could be provided as a covering a portion of the outer surface (e.g. as a hemispherical coating) so as to increase the reflectivity of the retroreflector, but decrease the acceptance angle.
Alternatively, a hemispherical mirror could be positioned at a predetermined distance from the outer layer.
Alternatively, instead of having a vacuum gap or an air gap between the hemispherical mirror, it will be appreciated that a material (i.e. a transparent material of uniform refractive index) could be placed between the two-layer spherical device and the hemispherical mirror.
In the embodiments illustrated in
The present inventor has appreciated that spherical aberration can be corrected for within retroreflectors, so as to improve the performance of the retroreflector. As indicated above, this spherical aberration correction can be performed by providing a curved surface having a negative optical power. Equally, the inventor has appreciated that spherical aberration can be at least partially corrected for by selecting the dimensions of the device such that spherical aberration is compensated for by primary defocus. Such a technique can be used in conjunction with the use of a negative curved surface (i.e. in two or more layer retroreflectors), or can be used on its own within a single layer optical device.
In other words, the radius of a device can be selected such that the spherical aberration is corrected for by primary defocus. This allows the provision of relatively efficient single layer retroreflectors (e.g. a ball lens, or retroreflectors in the form of a sphere of material) that are formed from relatively cheap optical materials e.g. materials that have a refractive index less than the previously thought “ideal” value of 2.0. In this way, the optical performance of the reflector is actually enhanced by spherical aberration.
If the device 600 is coated with a layer having a reflectivity of 33%, then the SCS increases to 13.2 m2. The wave aberration at a wavelength of 690 nm is illustrated in
Such small devices 600 are useful for surveying at distances of up to around 200 metres. In tests by the present inventor, uncoated devices of diameter 8.0 mm have produced consistent distance measurement results at distances in excess of 200 metres using commercially available instruments (Sokkia SET3 230RM). It is appreciated that similar ranges could be achieved using small pieces of reflective sheets, but the advantage of using a spherical retroreflector is that the return beam originates from a single point, leading to significantly better accuracy and measurement applications.
The useful range of this type of reflector will vary according to the fourth root of the SCS in most applications. The simplest way to increase the range is to scale up the device. The fourth root of SCS is shown plotted against the device radius in
Number | Date | Country | Kind |
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0613737.6 | Jul 2006 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB07/02253 | 6/15/2007 | WO | 00 | 1/6/2009 |