Abstract:
The present invention relates to an optical window-filter including a thermochromic material and a light absorbing material. An absorption of light by the light absorbing material generates heat that causes phase transformation of the thermochromic material. The present invention further relates to a filter for an infrared imaging system having detectors sensitive to radiation in an infrared transmission spectrum. The filter includes a thermochromic material and a light-absorbing material. An absorption of high-power radiation in the infrared transmission spectrum by the light-absorbing material generates heat that causes phase transformation of the thermochromic material to attenuate the high-power radiation while transmitting substantially unaffected low-power radiation in the infrared transmission spectrum.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/706,208, filed Sep. 27, 2012, Israeli Application No. 218364, filed Feb. 28, 2012 and U.S. Provisional Application No. 61/601,715, filed Feb. 22, 2012, each of which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to sun light or visible light power-limiting devices, and more particularly, to a solar power-limiting passive device and to a method for limiting solar power transmission in windows. Employing absorption changes in a novel thermochromic composition, the responsivity of the composition is enhanced and the reaction time of the composition is shortened when exposed to solar light in the visible, near IR (infrared) region, i.e., at wavelengths of about 0.3 to 2 micrometer (μm) wavelength region of the optical spectrum and when exposed to infrared light in the about 3 to 5 μm and about 8 to 14 μm region of the optical spectrum. These additional response enhancing features enable the use of the thermochromic composition in places where efficient sunlight power limiting is essential and where low infrared emitting or “cold” objects are in the presence, in close vicinity, of high infrared emitting “hot” targets or background. 
     The present invention further concerns, but is not limited to, the production of windows, lenses, and other optical articles. The present invention further relates to protecting dedicated optical elements against bright emitting sources, like the sun, that have most of its radiation in the visible and infrared region of the optical spectrum: at wavelengths of between about 0.3 μm to 14 μm. 
     BACKGROUND OF THE INVENTION 
     IR imaging systems employ focal plane arrays in various cameras, having focal plane detectors that are sensitive to light at the 3-5 and 8-12 μm ranges of the optical spectrum. The two ranges cover the two main windows in the IR transmission spectrum of the atmosphere. These systems need to be protected from dazzling and temporary or permanent damage caused by intense radiation sources in the field of view, yet the optical systems should at the same time be completely transparent at these wavelengths for low light power. The intense radiation sources may be of two kinds; first, coherent lasers in the spectral region, continuous or pulsed, and second, non-coherent sources like explosions, field-fire or hot substances in the field of view. 
     SUMMARY OF THE INVENTION 
     Since the spectral emission of the hot and cold targets reaching the IR sensor are in the same spectral wavelength region, enhancing the ability to see cold targets can only be achieved by selective control of transparency in which bright light sources are attenuated while weak light sources are not affected. This can be done using a non-linear spatial filter, positioned, e.g., at the focal plane of an imaging system, which attenuates the hot areas in the picture and transmits unaffected the cold parts of the picture, thus expanding its dynamic range. Some aspects of the present invention enable better visibility of low infrared emitting cold targets in the presence, in close vicinity, of high infrared emitting hot targets or background. 
     Some similar problems for visible light cameras are solved using image processing of the stored picture. Unfortunately, this approach does not work effectively in the IR windows, since saturation effects hinder the operation of the whole focal plane array sensor. Thus, a novel approach is needed for the visible and IR regions combined together in a single window, or when separated into two windows, one for the visible and the other for the IR. 
     In the phenomenon of thermochromism the optical properties of a material change reversibly as a function of temperature. There are various types of thermochromism. Of special relevance is thermochromism exhibited in the semiconductor-to-metal transition by solid-state materials, such as vanadium and titanium oxides. These can be used in the extreme environmental conditions needed. 
     A particular case of interest is vanadium dioxide since its transition temperature Tc is close to room temperature. Observations and current understanding of the nature of the semiconductor-to-metal transition exhibited by these materials is known and reported in the literature as seen for example in Pragna Kiri et al., “Solid state thermochromic materials,” Adv. Mat. Lett. 2010, 1(2), 86-105, the disclosure of which is incorporated entirely herein by reference. The possibility of fine-tuning the transition temperature by introducing various dopants into the, e.g., vanadium dioxide lattice is a phenomenon which was extensively studied and examined as seen for example in P. Jin et al., “Tungsten doping into vanadium dioxide thermochromic films by high-energy ion implantation and thermal annealing,” Thin Solid Films 324, 1998, 151-158, the disclosure of which is incorporated entirely herein by reference. 
     Typically, the thermochromic effect occurs over a range of temperatures and it is observed as a gradual color change, i.e., continuous thermochromism. Discontinuous thermochromism involves a structural phase change at the transition temperature. This phase change can be first or second order in nature, and may be reversible or irreversible, as governed by the thermodynamics of the system as explained for example in N. F. Mott, Metal-Insulator Transitions, second edition (Taylor &amp; Francis, London, 1990). 
     Transition-metal oxides such as Ti 2 O 3 , V 2 O 3 , VO 2 , and VO are all semi-conducting at low temperatures and show a transition into a metallic state at the Tc temperature. The electrical properties of these oxides were thoroughly studied using thermo-conductive studies by Morin, F. J. Phys. Rev. Lett. 1959, 3, 34-36. It was discovered that all lower oxides of titanium and vanadium exhibit this behavior except for TiO, which is metallic over the entire temperature range. 
     Vanadium dioxide is by far the most studied solid state thermochromic material. It shows great promise for applications such as “intelligent” architectural glazing. A single pure crystal of VO 2  has a semiconductor-to-metal transition temperature of 341K (68° C.). There is a corresponding structural phase change upon passing Tc, from the low temperature monoclinic crystal structure to the high temperature rutile, tetragonal-type lattice. The phase change alters the optical properties of VO 2 . In the semiconductor phase it is mostly transparent, in the relevant part of the spectrum, while the transition to metal makes it opaque as explained in S. M. Babulanam et al., “Thermochromic VO 2  films for energy efficient windows,” Solar Energy Materials 16 (1987) 347-363. 
     Critical temperature Tc of 68° C. is too high to be effective in many applications. For example, the ideal transition temperature for “intelligent” glazing is in the region of about 18-25° C. Dopants can be incorporated into VO 2  to increase or decrease its Tc, in order to make the VO 2  more commercially viable as explained in Pierce, J. W.; Goodenough, J. B. Physical Review B 1972, 5(10), 4104. Tungsten reduces Tc of VO 2  and there are a number of other dopants that can be incorporated into vanadium oxide to reduce its Tc. 
     Rare earth nicklates with the general formula RNiO 3 , where R is a rare earth element, exhibit a metal-insulator transition at temperature of 130K (−140° C.), 200K (−70° C.), 400K (130° C.) and 560K (290° C.) for R═Pr, Nd, Sm and Gd, respectively. The transition temperature decreases with increasing size of the rare earth ion. The nicklates are good candidates for non-linear filters. 
     Some prior patents include VO 2  solid layers as a filter. U.S. Patent Application No. 2011/0233404A1, the disclosure of which is incorporated herein by reference in its entirety, relates to an optical switch-window for an uncooled focal plane array camera for the IR region, using thermochromic-optical-switch-window coated with crystalline thin layer of vanadium oxide. Previously, it was shown in U.S. Pat. No. 7,259,925, the disclosure of which is incorporated herein by reference in its entirety, that a layer of VO 2  would generally protect infrared sensors in a Forward Looking Infrared imager (FLIR) against radiation from high power lasers. However, the patent does not disclose how the layer should be incorporated in an uncooled sensor, or how it can be designed to block radiation that is less intense than lasers, such as sunlight, explosion or fire. 
     The relation between the size of thermochromic particle and the transition properties of individual nano-crystals has been demonstrated and is currently extensively studied. The collective response of VO 2  nanoparticles can be related to small size effects, where most of the atoms are in close proximity to the external matrix and not to VO 2  lattice, and the properties of opto-electronic devices can be tuned by the size and the arrangements of single domains or single nano-particles as explained in J. Y. Suh et al., “Semiconductor to metal phase transition in the nucleation and growth of VO 2  nanoparticles and thin films,” J. Appl. Phys., Vol. 96, No. 2, 15 Jul. 2004, the disclosure of which is incorporated entirely herein by reference. The optical properties of the oxide are also greatly affected by the transition. The properties of the transition in such systems are partially dictated by the size of the individual domains, e.g. small size VO 2  particles are expected to have a wider hysteresis. Indeed, optical hysteresis loops with a width as large as 50° C. have been observed for isolated VO 2  nanoparticles. The collective properties of nano-crystals can be tuned by controlling their size and their arrangement. Properties of individual nano-crystals open the possibility to design devices which have unique characteristics. 
     It was shown that nanotechnology techniques can be used to adjust the threshold value of VO 2  networks by controlling the size and the arrangement of the nanocrystals. This is a perfect example to show how the properties at the nano scale can lead to devices with unique characteristics that are not found in the bulk material as explained in F. Guinneton et al., “Comparative study between nanocrystalline powder and thin film of vanadium dioxide VO 2 : electrical and infrared properties,” Journal of Physics and Chemistry of Solids 62 (2001) 1229-1238. 
     Some aspects of the present invention relate to a novel approach, based on a passive, non-linear filter for the solar radiation spectrum, visible and the IR region, attenuating the solar light when it is hot and transmitting the solar light when it is cold and attenuating the hot areas in the picture and transmitting unaffected the cold parts of the picture. The filter is adapted to block lasers and lower power light sources, like sun light and fire or explosive light burst as well as hot background. Reactions in the filter are improved by employing methods to enhance the light intensity modulation and shorten reaction time of the thermochromic materials. 
     One aspect of the present invention relates to an optical power-limiting window, and more particularly, to an optical power-limiting passive device and to a method for limiting optical power transmission in windows. Using absorption changes, in a novel thermo chromic composition having enhanced response, the nonlinearity is enhanced and the reaction time is shortened when exposed to solar light in the visible and near IR (infrared) region, 0.3 to 2 micrometer wavelength region of the optical spectrum and when exposed to infrared light in the 3 to 5 and 8 to 14 micrometer wavelength regions of the optical spectrum. These additional response enhancing features enable the use of the thermo-chromic composition in solar light limiting windows and in places where low infrared emitting or “cold” objects are in the presence, in close vicinity, of high infrared emitting “hot” targets or background. 
     A further aspect of the present invention relates to an optical window-filter including a thermochromic material and a light absorbing material. An absorption of light by the light absorbing material generates heat that causes phase transformation of the thermochromic material. 
     Another aspect of the present invention relates to a filter for an infrared imaging system having detectors sensitive to radiation in an infrared transmission spectrum. The filter includes a thermochromic material and a light-absorbing material. Absorption of high-power radiation, e.g., higher than about 0.2 Joule/cm 2  in the infrared transmission spectrum by the light-absorbing material generates heat that causes phase transformation of the thermochromic material to attenuate the high-power radiation while transmitting substantially unaffected low-power radiation, e.g., lower than about 0.02 Joule/cm 2  in the infrared transmission spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more understood. With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. 
         FIG. 1  depicts a cross-sectional view of a thermo-chromic bulk material window-filter. 
         FIG. 2  depicts a cross-sectional view of a thermo-chromic nanoparticle matrix window-filter. 
         FIG. 3  depicts a cross-sectional view of a thermo-chromic and light absorbing nanoparticle matrix window-filter. 
         FIG. 4  depicts a cross-sectional view of a thermo-chromic, thermal conductivity and light absorbing nanoparticle matrix window-filter. 
         FIG. 5  depicts a cross-sectional view of a thermo-chromic, thermal conductivity and light absorbing nanoparticle matrix window-filter where each kind of particle is in a separate layer. 
         FIG. 6  shows an infrared transmission spectrum of VO 2  thin film below and above Tc. 
         FIG. 7  shows a thermo chromic transition curve having a hysteresis. 
         FIG. 8A  depicts an example place for filter placement. 
         FIG. 8B  depicts another example place for filter placement. 
     
    
    
     DETAILED DESCRIPTION 
     Despite the remarkable utility potential of thermochromic materials, their practical uses have been extremely limited. The limitations are largely attributed to the difficulties in providing thermochromic materials that react in low temperatures in the region of 18-25° C. Here we introduce a novel Thermochromic Composition (TCC) containing two kinds of nanoparticles (about 1 to 100 nanometers in size), embedded in a transparent matrix, where one kind of nanoparticle is thermochromic, e.g., VO 2  nanoparticles, and the second kind is light absorbing nanoparticles (1 to 100 nanometers in size) that exhibit strong absorption of the impinging light, e.g., carbon nanoparticles. This combination enhances and accelerates the heating of the neighboring thermochromic particles by the sun-light absorbing nanoparticles and lead to a phase transformation in the TCC at a lower impinging sun-light intensity than needed for matrices including only thermochromic materials, thus enabling the limiting of the solar light intensity by absorption in the hot thermochromic particles. When the environmental temperature is high, the TCC will start limiting at low solar power, and when the environmental temperature is low, the TCC will start limiting at higher solar power, thus enabling to preserve passively the optimal room temperature when windows of this kind are used in directions exposed to the sun. 
     The matrix in the TCC can be organic-based, e.g., a polymer film, a polymerizable composition, a transparent adhesive, or inorganic-based, e.g., mineral glass, sol-gel, and any other window materials, and also an inorganic-organic composite. Since the whole thickness of the composition is much less than the light wavelength, even a composition that is opaque in bulk materials is transparent in the sub wavelength thickness. 
     Specific embodiments utilize various TC nanoparticles and combinations of TC nanoparticles in the TCC, such as rare earth nicklates with the general formula RNiO 3 , where R═Pr, Nd, Sm or Gd, or any combination thereof, or transition-metal oxides such as Ti 2 O 3 , V2O 3 , VO 2 , and VO, or any combination thereof. 
     Various absorption enhancing materials can be used in the TCC to enhance light absorption from the impinging external light. Examples include, but are not limited to, carbon nanoparticles, metallic or organic nanoparticles, hollow-shell nanoparticles, rice-like nanoparticles, nonconcentric-nanoshell nanoparticles, crescent-moon-structured nanoparticles, nanoshells composed of layers of metal or carbon. 
     Addition of a third kind of particles to the TCC matrix, nanoparticles that are thermal conductivity enhancers, can be used to enhance the thermal conductivity of the matrix. The three component TCC compositions effectively achieve two purposes; first, heat that builds up in the optical element during the absorption of light can dissipate more easily to other elements in the system, effectively reducing the thermal degradation of both the organic material and the TCC matrix. Second, most TC materials have a hysteresis behavior with a temperature range of up to tens of degrees between Tc of heating up and Tc of cooling down, thus removing the heat fast, by using thermal conductivity enhancers, will reduce the recovery time to transparency after exposure to strong sun light radiation. 
     In one specific embodiment, the thermal conductivity of matrices is achieved by the addition of nanoparticles, nanorods, nanowires, hollow nanoparticles, core-shell nanoparticles, spiked particles, and nanoparticles with various shapes. These may include nanoparticles of metal, metal oxide, metal nitrides, metal carbides, metal sulfides, and carbon-based nanomaterials, such as nanodiamond, diamond-like carbon (DLC), single-wall carbon nanotubes, double-wall carbon nanotubes, multiwall carbon nanotubes, and their functionalized forms like graphene. The various compositions can be polymerized, cured or fabricated in the form of nanoparticles and/or microparticles. The nanoparticles and/or the microparticles can be further dispersed in a new matrix, appropriate for forming a window. 
     In one specific embodiment, the light absorbing, the TC and the thermal conductivity particles can be placed each in adjacent nano-layers, each containing one or more kinds of particles. 
       FIG. 1  depicts a cross-sectional view of a thermochromic bulk material window-filter  2 . Solar light  4  enters the filter  2  through a transparent plate of light transmitting material  8 , e.g., glass slab, and impinges on a layer of thermo-chromic solid material  10 . In case the material  10  is, e.g., VO 2 , when the impingement spot is cold, less than Tc, the light passes through nearly unaffected in direction  6 . When the impingement spot is hot, from Tc and up, the light is strongly affected, most of it, (e.g., more than about 60%) is absorbed in the layer  10 . The filter  2  stays opaque until the temperature Tc gets back to cold, and then returns to transparency. 
       FIG. 2  depicts a cross-sectional view of a thermochromic nanoparticle matrix window-filter  18 . Solar light  4  enters the filter  18  through a transparent plate of light transmitting material  8 , e.g., glass slab, and impinges on a layer of TCC  14  containing nanoparticles of TC material  16  in a matrix  12 . In case the material  16  is, e.g., VO 2 , when the impingement spot is cold, less than Tc, the light passes through nearly unaffected in direction  6 . When the impingement spot is hot, from Tc and up, the light is strongly affected; most of it is absorbed in the layer  14 . The filter  18  stays opaque until the temperature gets back to cold, and then returns to transparency. Since Tc is affected by the size and volumetric arrangement of particles  16 , this Tc temperature can be tuned in some narrow temperature range using various sizes, e.g., between about 1 to 100 nm, of nanoparticles and arranging them in the desired geometry. 
       FIG. 3  depicts a cross-sectional view of a thermochromic and light absorbing nanoparticle matrix window-filter  20 . Solar light  4  enters the filter  20  through a transparent plate of light transmitting material  8 , e.g., glass slab, and impinges on a layer of TCC solid material  22 . The TCC  22  contains two kinds of nanoparticles, embedded in a transparent matrix  12 , where one kind of nanoparticle is thermochromic  16 , e.g., VO 2  nanoparticles and the second kind  24  is light absorbing nanoparticles (about 1 to 100 nanometers in size) that exhibit strong absorption of the impinging light, e.g., carbon nanoparticles (about 1 to 100 nanometers in size). This combination will result in enhanced heating of the neighboring thermochromic particles  16  by the light absorbed by absorbing nanoparticles  24 . The enhanced absorption, in the transparent case of the TCC will cause phase transformation in the TCC in lower impinging light intensity than needed for matrices including only thermochromic materials  16 , thus enhancing the ability of the material to limit the amount of sun light transmitted through. In case the material  16  is, e.g., VO 2 , when the impingement spot is cold, less than Tc, the light passes through nearly unaffected in direction  6 . When the impingement spot is hot, from Tc and up, the light is strongly affected and most of it is absorbed in the layer  22 . The filter  20  stays opaque until the temperature gets back to cold, and then returns to transparency. 
       FIG. 4  depicts a cross-sectional view of a thermochromic, thermal conductivity and light absorbing nanoparticle matrix window-filter  26 . Solar light  4  enters the filter  26  through a transparent plate of IR transmitting material  8 , e.g., glass slab, and impinges on a layer of TCC solid material  30 . The TCC  30  contains three kinds of nanoparticles, embedded in a transparent matrix  12 , where one kind of nanoparticle is thermo-chromic  16 , e.g., VO 2  nanoparticles (1 to 100 nanometers in size) and the second kind  24  is light absorbing nanoparticles (1 to 100 nanometers in size) that exhibit strong absorption of the impinging light, e.g., carbon nanoparticles. The third kind is nanoparticles that are thermal conductivity enhancers  28  (about 1 to 100 nanometers in size) intended to enhance the thermal conductivity of the matrix. The three component TCC compositions effectively achieve two purposes; one, heat that builds up in the optical element during the absorption of light can dissipate more easily to other elements in the system, effectively reducing the thermal degradation of both the matrix and the TCC matrix. Second, since most TC materials have a hysteresis behavior with a temperature range of up to tens of degrees between Tc of heating up and Tc of cooling down, removing the heat fast, by using thermal conductivity enhancers will reduce the recovery time to transparency after exposure to strong light. This combination will result in enhanced heating of the neighboring thermochromic particles  16  by the light absorbed by absorbing nanoparticles  24 . The enhanced absorption, in the transparent case of the TCC  30  will cause phase transformation in the TCC  30  in lower impinging light intensity than needed for matrices including only thermo-chromic materials  16 , thus enhancing the ability of the material to limit the amount of sun light transmitted through. In case the material  16  is, e.g., VO 2 , when the impingement spot is cold, less than Tc, the light passes through nearly unaffected in direction  6 . When the impingement spot is hot, from Tc and up, the light is strongly affected and most of it is absorbed in the layer  30 . The filter  26  stays opaque until the temperature gets back to cold, and then returns to transparency. Particles  28 , being thermal conductivity enhancers intended to enhance the thermal conductivity of the matrix, are bringing the TCC  30  back to transparency faster than without them. By tuning the TCC  30  composition between the three kinds of particles  16 ,  24 ,  28 , an optimal design can be reached. 
       FIG. 5  depicts a cross-sectional view of a thermochromic, light absorbing and thermal conductivity nanoparticles matrix window-filter  32  where each kind of particle is located in a separate layer  34 ,  36  and  38 , respectively. The layers are thin, in the order of 100 micrometers, and heat conduction between them is carried out through their common facets. Solar light  4  enters the filter  32  through a transparent plate of IR transmitting material  8 , e.g., ZnSe or ZnS slab, and impinges on a layer of TCC solid materials  34 ,  36 ,  38 . The TCC solid materials  34 ,  36 ,  38  contain three kinds of nanoparticles, each one embedded in its own transparent matrix  12 , where one kind of nanoparticle, in layer  34 , is thermochromic  16 , e.g., VO 2  nanoparticles (1 to 100 nanometers in size). The second kind in layer  36  is light absorbing nanoparticles  24  (1 to 100 nanometers in size) that exhibit strong absorption of the impinging light, e.g., carbon nanoparticles, the third kind, in layer  38  is nanoparticles that are thermal conductivity enhancers  28  (1 to 100 nanometers in size) intended to enhance the thermal conductivity of the matrix. The three TCC layer composition effectively achieves two purposes; one, heat that builds up in the optical element during the absorption of light can dissipate more easily to other elements in the system, effectively reducing the thermal degradation of both the matrix and the TCC matrix layers. Second, since most TC materials have a hysteresis behavior with a temperature range of up to tens of degrees between Tc of heating up and Tc of cooling down, removing the heat fast, by using thermal conductivity enhancers will reduce the recovery time to transparency after exposure to strong light. This combination will result in enhanced heating of the neighboring thermochromic particles  16  by the light absorbed by layer  36  of absorbing nanoparticles. The enhanced absorption, in the transparent case of the TCC  32  will cause phase transformation in the TCC  32  in lower impinging light intensity than needed for matrices including only thermochromic materials  16 , thus enhancing low light emitting objects in the field of view. In case the material  16  is, e.g., VO 2 , when the impingement spot is cold, less than Tc, the light passes through nearly unaffected in direction  6 . When the impingement spot is hot, from Tc and up, the light is strongly affected since most of it is absorbed by the filter  32 , which stays opaque until the temperature reduces, and then returns to transparency. Particles  28 , being thermal conductivity enhancers intended to enhance the thermal conductivity of the matrix, are bringing the TCC layer back to transparency faster than without them. By tuning the TCC  34 ,  36 ,  38  composition between the three kinds of particles  16 ,  24 ,  28  an optimal design can be reached. Additional cooling can be achieved by making the back plate  39  of heat conducting material, e.g., Sapphire or diamond. We note that the order in which layers  34 ,  36  and  38  are presented in  FIG. 5  is only for the sake of example and other permutations (arrangements) of the layers  34 ,  36 ,  38  are also considered. 
       FIGS. 6 and 7  show the thermochromic behavior of VO 2 . 
       FIG. 6  shows an infrared transmission spectrum of VO 2  thin film below and above Tc. The curves show Fourier transform infrared transmittance spectrum for VO 2  thin film on amorphous silica in the 2.5 to 5 micrometer infrared transmittance spectrum for VO 2  thin film on amorphous silica after computational correction to remove the effects of substrate absorption. The curves depict the difference between the transparent and opaque states.  FIG. 6  is cited from F. Guinneton et al., “Comparative study between nanocrystalline powder and thin film of vanadium dioxide VO 2 : Electrical and infrared properties,” Journal of Physics and Chemistry of solids 62, (201) 1229-1238. 
       FIG. 7  shows a thermochromic transition curve having a hysteresis.  FIG. 7  is cited from F. Guinneton et al., “Comparative study between nanocrystalline powder and thin film of vanadium dioxide VO 2 : electrical and infrared properties,” Journal of Physics and Chemistry of Solids 62 (2001) 1229-1238. The curves on the right side show the transition when heating the sample, and on the left side when cooling the sample. There are two wavelengths at which measurements are carried out as depicted in the figure. 
       FIG. 8  depicts example locations for filter placement. In  FIG. 8A , the placement geometry  40  shows the filter  42  near the focal location of lens or telescope  41 , in front of the focal plane detector  44 . In  FIG. 8B , the placement geometry  46  shows the filter  48  at the back side of lens  41 . In both cases the filters  42  and  48  shield the focal plane detector  44 . 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.