Patent Publication Number: US-2021191135-A1

Title: Adhesive for middle far infra-red optical elements and beam splitter prism set where an adhesive is used

Description:
CROSS REFERENCE TO THE RELATED APPLICATION 
     This application is the national stage entry of International Application No. PCT/TR2019/050286, filed on Apr. 29, 2019, which is based upon and claims priority to Turkish Patent Application No. 2018/06798, filed on May 14, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is related to a transparent adhesive for middle and far infra-red beams, production method of mentioned adhesive and a beam splitter prism set which separates middle and far infra-red beam paths for electro-optical monitoring systems according to their wavelength&#39;s. 
     BACKGROUND 
     Electro-optical monitoring systems can have various features such as operating by using beams from different wavelengths, having different point of views, enabling thermal monitoring, having photon multiplexer, and enabling black-white or colored view. In electro-optical monitoring systems, the beams coming from the external environment to the system are fallen onto an optical sensor transmitting through various optical units (lens, prism etc.). Beams in mentioned wavelength transformed into a view by means of the optical sensors which is sensitive to the beams between selected wavelengths. In some electro-optical monitoring systems a plurality of sensors can be used. As an example, two optical sensors which are sensitive to the beams with different wavelengths for mentioned two views in systems where thermal monitoring and daytime monitoring are provided together, are used. In systems which have a plurality of sensors, instead of using a separate optical unit for each optical sensor, in some cases a single optical unit can be jointly used (for example jointly using tele-zoom lens). After the beam transmits through a single optical unit, it is separated into wavelengths according to the sensitivity of the optical sensor, and directed to the optical sensors at the relevant wavelength. Jointly using a single optical unit provides significant improvements in the total weight and cost of the electro-optical monitoring system. 
     In general, two different beam splitter units are uses for the separation of the beams transmitting from the optical unit and orientation of the beams to the optical sensors. One of these units are a beam splitter prism sets and the other are beam splitter plates. If the beams coming to the beam splitter units are parallel, there will not be significant differences between the advantages of the beam splitter prism sets and beam splitter plates. However if the beams coming are not parallel (in optical units in general the beams converge towards each other or move away from each other while they are transmitted through the lens), the important advantages of beam splitter prism sets are occurred, in eliminating the defects (particularly aberration) formed on the image at the focal plane . The ability of the systems where beam splitter plate is used, in respect to improving defects formed on the image is lower than the systems in which beam splitter prism set is used. In order to correct the defects in systems where the beam splitter prism set is used, a less number of and less complex optical components are required. Due to mentioned advantages, in visible and close infra-red wave bands, beam splitter prism sets are commonly used for a long time. Beam splitter prism sets are basically obtained by joining two triangular prisms optically by means of a transparent adhesive and coating prism surfaces with thin films having various features. In order to use mentioned adhesive in beam splitter prism sets, it has to fulfill some characteristics. The adhesive shall have the following characteristics;
         The refractive index shall have the refractive index value of at least one of the triangle prisms to be joined or shall be close to its refractive index,   It shall have a good adhesive characteristic,   It shall be optically transparent within the relevant wavelength band,   It shall be dried within an industrially acceptable duration after the joining process,   It shall protect cold, warm, moisture etc. integrity within selected ambient condition values, and continue its function,   It shall have a low gas release,   It shall not harm human health during assembly and operation processes, shall be non-poisonous.       

     The thermal monitoring systems among the popular electro-optical monitoring systems are operated in the direction of different aims with beams within two main wavelengths such as in general middle (3.0-5.0 μm) and far (8.0-12.0 μm) infra-red beams. Therefore two separate optical sensors which are sensitive against middle (3.0-5.0 μm)-far (8.0-12.0 μm) infrared wavelength bands are used in these systems. Due to abovementioned advantages, in thermal monitoring systems, beam splitter prism sets are also desired to be used. However refractive indexes of the optical unit materials, elements in electro-optical systems where middle-far infra-red beams are used and triangle prisms to be used in beam splitter sets are significantly high. The refractive index value of mentioned optical units is generally more than 2.0. The refractive index of an adhesive to be used for joining the optical units must be close to the refractive index value of the optical units. If there is significant difference between mentioned refractive indexes, critical values determined according to Snell law will be very small. Therefore due to mentioned small critical angle, the coming beam will follow a different path instead of transmitting over the optical units. In a similar manner in case the refractive index of the adhesive to be used for joining the prisms in the beam splitter prism sets is significantly differently from the refractive indexes of the prisms, the critical angles will be very small. Therefore due to mentioned small critical value, coming beam will follow a different path by being reflected instead of transmitting over prisms. For this reason the refractive index of the adhesive to be used for joining the prisms in the beam splitter prism sets shall have a similar value with the refractive index of the prisms. Due to lack of a transparent adhesive in middle-far infra-red wavelength bands having particularly high refractive index and previously mentioned features, beam splitter prisms as far it is known are not used within the thermal monitoring systems operating in middle-far infra-red wavelength intervals. 
     SUMMARY 
     In the present disclosure, a transparent adhesive having high refractive index, for middle (3.0-5.0 μm) and far (8.0-12.0 μm) infra-red beams, for the production method of the adhesive and a beam splitter prism set is described which separates middle and far infra-red beam paths, where mentioned adhesive is used in electro-optical monitoring systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand the present invention, its exemplary embodiment is shown in the attached figures. The details of the present disclosure shall be evaluated by taking the whole description into consideration. These figures are as the following; 
         FIG. 1  is a schematic view of the usage of the prism set in the electro-optical monitoring systems in one embodiment of the invention. 
         FIG. 2  is a schematic view of the beam splitter prism set in one embodiment of the invention. 
         FIG. 3  is a reflection graph of antireflection optical coating which transmits only middle and far infra-red beams in one embodiment of the invention. 
         FIG. 4  is a reflection graph of antireflection optical coating which transmits only far infra-red beams in one embodiment of the invention. 
         FIG. 5  is a reflection graph of antireflection optical coating which transmits only middle infra-red beams in one embodiment of the invention. 
         FIG. 6  is a transmission graph of a coating which reflects far infra-red beams and transmits middle infra-red beams in one embodiment of the invention. 
         FIG. 7  is a beam distribution graph according to the wavelength showing total theoretical performance of the beam splitter prism set in one embodiment of the invention. 
         FIG. 8  temperature-time graph of the thermal treatment applied in the combination method of C surface (C) or D surface (D) with the adhesive in an exemplary embodiment of the invention wherein Ti(IV) butox solution is used. 
         FIG. 9  X-ray diffraction graph of the adhesives obtained by using Ti(IV) butox solution and nano TiO 2  solution. 
     
    
    
     Parts in the figures are numbered one by one, the parts correspond to these numbers are given below. 
       4 . Beam splitter prism set 
       5 . First prism 
       6 . Second prism 
     A. A surface 
     B. B surface 
     C. C surface 
     D. D surface 
     E. E surface 
     I 1 . Middle (3-5 μm) infra-red beam 
     I 2 . Far (8-12 μm) infra-red beam 
     S 1 . Optical sensor sensitive to the middle (3-5 μm) infra-red beams 
     S 2 . Optical sensor sensitive to the far (8-12 μm) infra-red beams 
     O. Optical unit 
     GS. Electro-optical monitoring system 
     NTE. Nano TiO 2  solution curve 
     TBE. Ti(IV) butox solution curve 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An Adhesive for Middle-Far Infra-Red Optical Elements 
     A metallic-nonmetallic composition for optical elements (lens, prism etc.) used for middle-far infra-red beams of the present disclosure which comprises all metal oxide and sulfides within organic or inorganic MyAx salt formulation in which x, y is any number, preferably between 1-5, adhesive M is one of the Hf, Zn, Ni, Ti, Y elements and A is one of the O, S elements. 
     In a preferred embodiment of the present invention, Ti(IV)Ax is used by selecting Ti for M and IV for y. 
     The adhesive can be used for joining the prisms to be used for optical units (O) for middle (3.0-5.0 μm) and far (8.0-12.0 μm) infra-red beams, for example lenses and particularly prisms for middle-far beam splitting. The refractive index of the adhesive is 2.0 and above that value, it is transparent in the infra-red band. In a preferred embodiment of the present invention, the refractive index of the adhesive is 2.25. However, the adhesive has required features such as good adhesion, ability of drying within an industrially acceptable duration, ability of protecting its integrity and continue its function under predetermined ambient conditions (cold −40°, hot +70°, moisture etc.), having low gas release, not being harmful to human health during assembly and operation processes. As a result an adhesive with high refractive index and transparent at the infra-red wave band is created. The adhesive allows for joining optical elements which have high refractive indexes. Due to protecting its integrity in difficult conditions and continuing its functionality, the adhesive allows for using the devices (for example electro-optical monitoring system (GS)) where optical elements and/or beam splitter prism sets ( 1 ) are used in said conditions. 
     In a preferred embodiment of the present invention an adhesive in a solid phase is used. An adhesive in a liquid form causes the formation of gaps where loss of light occurs. The refractive index of the adhesive in liquid form changes at high temperatures and it can freeze in cold environments. Also the adhesive in liquid form has a risk of leakage at high altitudes. The technical problems encountered with the adhesives at liquid phase are eliminated by using the adhesive at solid phase. 
     Production Method of the Adhesive in Ti(IV)Ax Formulation in a Preferred Embodiment of the Present Invention 
     The adhesive for optical elements used for middle (3.0-5.0 μm)-far (8.0-12.0 μm) infra-red beams of the present invention is produced from sol-gel based solution. In the adhesive production method, an adhesive with anatase structure is obtained as a result of dissolving a component (starter material) of the Ti(IV)R formulation where for organic R isopropoxide, butoxide are used or for inorganic R chloride salts are used. Together with dissolving the starter material within the solution, a transparent adhesive is obtained for middle-far infra-red beams. 
     In a preferred embodiment of the present invention, the adhesive with an anatase structure is transformed into a gel solution by means of the sol-gel method. Mentioned gel solution formation process is performed by dissolving Ti(IV) butoxide (preferably 97% isopropanol solution of Ti(IV) butoxide) or nano TiO 2  solution in isopropanol alcohol and by mixing the obtained mixture preferably for 20 minutes. 
     Middle-Far Infra-Red Beam Splitter Prism Set ( 1 ) 
     The subject of the invention is a beam splitter prism set ( 1 ) for electro-optical monitoring systems (GS) using middle and far infra-red beams. Beam splitter prism set ( 1 ) only transmits middle (3.0-5.0 μm) and far (8.0-12.0 μm) infra-red beams through an optical unit (O) and it separates middle-far infra-red beam paths. Thus, middle and far infra-red beams which their beam paths were separated are oriented to the relevant sensors in the electro-optical monitoring systems (GS). [Optical sensor (S 1 ) sensitive to the near (3-5 μm) infra-red beams and optical sensor (S 2 ) sensitive to the far (8-12 μm) infra-red beams, see  FIG. 1 ]. As a result of separating the middle-far infra-red beam paths by means of the beam splitter prism set ( 1 ), the faults (aberration) are eliminated from the views created in the electro-optical monitoring systems (GS). 
     Beam splitter prism set ( 1 ) comprises two optical right triangular prisms such as a first prism ( 2 ) and a second prism ( 3 ) whose hypotenuses are positioned in a manner facing each other. The first prism ( 2 ) and second prism ( 3 ) consists of a material in ZnX formulation wherein X is Se or S element (Zn=zinc). In a preferred embodiment of the present invention, the first prism ( 2 ) is manufactured from ZnSe material and the second prism ( 3 ) is manufactured from ZnS material. In another embodiment of the present invention first prism ( 2 ) is ZnSe and second prism ( 3 ) is ZnSe. 
     The section which faces the coming beam first is a vertical surface (A) of the first prism ( 2 ). The A surface (A) is coated with an antireflection optical coating which only transmits middle and far infra-red beams. Therefore the beam coming to the A surface (A) is filtered and only middle and far infra-red beams can penetrate into the first prism ( 2 ). 
     In one embodiment of the present invention another vertical B surface (B) of the first prism ( 2 ) is coated with an antireflection optical coat which only transmits far (8-12 μm) infra-red beams ( 12 ). Thus the transmission of only far (8-12 μm) infra-red beam (I 2  ) through the B surface (B) to the ambient is guaranteed. As a result only far (8-12 μm) infra-red beams (I 2  ) are transmitted to the optical sensors (S 2 ) which are sensitive to the far (8-12 μm) infra-red beams to be located against the B surface (B). A vertical E surface (E) of the second prism ( 3 ) which is parallel with the A surface (A) is coated with an antireflection optical coating which only transmits middle (3-5 μm) infra-red beams (I 1 ). Thus the transmission of only middle (3-5 μm) infra-red beams (I 1 ) through the E surface (E) to the ambient is guaranteed. As a result only middle (3-5 μm) infra-red beams (I 1 ) are transmitted to the optical sensors (S 1 ) which are sensitive to the middle (3-5 μm) infra-red beams to be located against the E surface (E). The first prism ( 2 ) and second prism ( 3 ) are joined in a manner such that a hypotenuse C surface (C) of the first prism ( 2 ) and a hypotenuse D surface (D) of the second prism ( 3 ) face each other. The angle between the C surface (C) and the B surface (B) are determined previously and in a preferred embodiment of the present invention said angle is approximately 45°. There is an optical coat between the C surface (C) and D surface (D) which reflects far infra-red beams and transmits middle infra-red beams. Mentioned optical coating can be coated onto the C surface (C) or D surface (D). In case mentioned optical coating in layers is used, the layers can be allotted and coated on the C surface (C) and D surface (D). Thus far infra-red beams are reflected from the joint face and reaches to the B surface (B) and middle infra-red beams transmits through the joint face and reaches to the E surface (E) (see  FIG. 1  and  FIG. 2 ). 
     The coatings within the embodiments of the invention are preferably in a thin film form. C surface (C) and D surface (D) are joined with an adhesive which has same or near refractive index of the refractive indexes of the first prism ( 2 ) or second prism ( 3 ). Preferably the refractive index of the uncoated surface is taken as a reference and the adhesive is performed. 
     Joining C surface (C) and D surface (D) with each others is performed by means of the adhesive in the previously described MyAx formulation. In a preferred embodiment of the present invention, Ti is selected for M and IV is selected for y and Ti(IV)Ax is used. In order to adhere C surface (C) and D surface (D) to each other, an adhesive in any embodiment described under the heading “An Adhesive for Middle-Far Infra-red Optical Elements” can be used. 
     In one embodiment of the present invention, when the first prism ( 2 ) and the second prism ( 3 ) is joined, in order to strengthen the joint between the C surface (C) and D surface (D), for increasing the adhesion force, there is a TiO 2  interlayer between the C surface (C) and D surface (D) with an anatase structure. 
     Antireflection optical coating layers which only transmit middle and far infra-red beams in one embodiment of the present invention are given in the following table 1. This exemplary embodiment is an optical coating to be applied on the A surface (A) of the first prism in ZnSe formulation. In the example reference wavelength is 10000 nm and its coming angle is 0°. Also in mentioned exemplary embodiment, the refractive graph of the optical coating is shown in the following table 4. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Optical 
                 Physical 
               
               
                   
                   
                   
                 Refractive 
                 Thickness 
                 Thickness 
               
               
                   
                 Layer 
                 Material 
                 Index 
                 (QWOT) 
                 (nm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Ambient 
                 Air 
                 1 
                   
                   
               
               
                   
                 1 
                 ZnS 
                 2.24 
                 0.02 
                 25.46 
               
               
                   
                 2 
                 YbF 3   
                 1.53 
                 0.57 
                 939.10 
               
               
                   
                 3 
                 Ge 
                 4.20 
                 0.08 
                 45.03 
               
               
                   
                 4 
                 YbF 3   
                 1.53 
                 0.29 
                 480.17 
               
               
                   
                 5 
                 Ge 
                 4.20 
                 0.09 
                 53.11 
               
               
                   
                 6 
                 ZnS 
                 2.24 
                 0.42 
                 476.46 
               
               
                   
                 7 
                 Ge 
                 4.20 
                 0.11 
                 68.05 
               
               
                   
                 8 
                 ZnS 
                 2.24 
                 0.30 
                 329.45 
               
               
                   
                 9 
                 Ge 
                 4.20 
                 0.05 
                 27.40 
               
               
                   
                 Substrate  
                 ZnSe 
                 2.39 
                   
                   
               
               
                   
                 Total 
                   
                   
                 1.93 
                 2444.23 
               
               
                   
                 thickness 
               
               
                   
                   
               
            
           
         
       
     
     The antireflection optical coating layers which only transmits far infra-red beams in one exemplary embodiment of the present invention is given in the following Table 2. This exemplary embodiment is the optical coating to be coated onto the B surface (B) of the first prism ( 2 ). In the example reference wavelength is 10500 nm and its coming angle is 0°. Also in mentioned exemplary embodiment, the refractive graph of the optical coating is shown in the following table 5. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Optical 
                 Physical 
               
               
                   
                   
                   
                 Refractive 
                 Thickness 
                 Thickness 
               
               
                   
                 Layer 
                 Material 
                 Index 
                 (QWOT) 
                 (nm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Ambient 
                 Air 
                 1 
                   
                   
               
               
                   
                 1 
                 Zns 
                 2.24 
                 0.05 
                 58 
               
               
                   
                 2 
                 YbF 3   
                 1.48 
                 0.87 
                 1535 
               
               
                   
                 3 
                 Zns 
                 2.24 
                 0.32 
                 380 
               
               
                   
                 4 
                 YbF 3   
                 1.48 
                 0.16 
                 287 
               
               
                   
                 Substrate 
                 ZnSe 
                 2.40 
                   
                   
               
               
                   
                 Total 
                   
                   
                 1.40 
                 2260 
               
               
                   
                 thickness 
               
               
                   
                   
               
            
           
         
       
     
     The antireflection optical coating layers which only transmit near infra-red beams in one exemplary embodiment of the present invention are given in the following Table 3. This exemplary embodiment is an optical coating to be coated on E surface (E) of the second prism ( 3 ) in ZnSe formulation. In the example reference wavelength is 4000 nm and its coming angle is 0°. Also in mentioned exemplary embodiment, the refractive graph of the optical coating is shown in the following table 6. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Optical 
                 Physical 
               
               
                   
                   
                   
                 Refractive 
                 Thickness 
                 Thickness 
               
               
                   
                 Layer 
                 Material 
                 Index 
                 (QWOT) 
                 (nm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Ambient 
                 Air 
                 1 
                   
                   
               
               
                   
                 1 
                 SiO 2   
                 1.39 
                 0.96 
                 690.39 
               
               
                   
                 2 
                 Ge 
                 4.24 
                 0.13 
                 30.72 
               
               
                   
                 3 
                 SiO 2   
                 1.39 
                 0.28 
                 202.96 
               
               
                   
                 Substrate 
                 ZnSe 
                 2.43 
                   
                   
               
               
                   
                 Total 
                   
                   
                 1.37 
                 924.07 
               
               
                   
                 thickness 
               
               
                   
                   
               
            
           
         
       
     
     The optical coating layers which reflect far infra-red beams and transmit middle infra-red beams in one exemplary embodiment of the present invention are given in the following Table 4. This exemplary embodiment is an optical coating which is coated onto C surface (C) of the first prism ( 2 ) or onto D surface (D) of the second prism ( 3 ) in the ZnSe formulation or to be coated onto the C surface (C) and D surface (D) of the layers mentioned in the table in a shared manner. In the example reference wavelength is 8000 nm and its coming angle is 45°. Also in mentioned exemplary embodiment, the refractive graph of the optical coating is shown in the following table 7. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Optical 
                 Physical 
               
               
                   
                   
                   
                 Refractive 
                 Thickness 
                 Thickness 
               
               
                   
                 Layer 
                 Material 
                 Index 
                 (QWOT) 
                 (nm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Ambient 
                 ZnSe 
                 2.42 
                   
                   
               
               
                   
                 1 
                 Ge 
                 4.20 
                 1.19 
                 567.46 
               
               
                   
                 2 
                 ZnS 
                 2.24 
                 1.69 
                 1507.81 
               
               
                   
                 3 
                 Ge 
                 4.20 
                 1.20 
                 572.28 
               
               
                   
                 4 
                 ZnS 
                 2.24 
                 1.68 
                 1503.61 
               
               
                   
                 5 
                 Ge 
                 4.20 
                 1.19 
                 565.08 
               
               
                   
                 6 
                 ZnS 
                 2.24 
                 1.70 
                 1514.6 
               
               
                   
                 7 
                 Ge 
                 4.2 
                 1.18 
                 559.76 
               
               
                   
                 Substrate 
                 TiO 2   
                 2.25 
                   
                   
               
               
                   
                 Total 
                   
                   
                 9.83 
                 6790.61 
               
               
                   
                 thickness 
               
               
                   
                   
               
            
           
         
       
     
     The coating structure of the beam splitter prism set ( 1 ) in the transmission direction in the exemplary embodiment of the present invention is given in the following table 5. Also the total theoretical performance of the beam splitter prism set ( 1 ) in said exemplary embodiment is given in  FIG. 7 . 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Pre 
                 Subsequent 
                 Surface 
                 Transmission 
                 Coating  
               
               
                 Surface 
                 Material 
                 Material 
                 Angle 
                 Mode 
                 Direction 
               
               
                   
               
             
            
               
                 A 
                 Air 
                 ZnSe 
                  0° 
                 Transmit 
                 Forward  
               
               
                 Surface 
                   
                   
                   
                   
                 direction 
               
               
                 (A) 
                   
                   
                   
                   
                   
               
               
                 C 
                 ZnSe 
                 TiO 2   
                 45° 
                 Transmit 
                 Forward  
               
               
                 Surface 
                   
                   
                   
                   
                 direction 
               
               
                 (C) 
                   
                   
                   
                   
                   
               
               
                 D 
                 TiO 2   
                 ZnSe 
                 45° 
                 Transmit 
                 Opposite  
               
               
                 Surface 
                   
                   
                   
                   
                 direction 
               
               
                 (D) 
                   
                   
                   
                   
                   
               
               
                 E 
                 ZnSe 
                 Air 
                  0° 
                 Transmit 
                 Opposite  
               
               
                 Surface 
                   
                   
                   
                   
                 direction 
               
               
                 (E) 
               
               
                   
               
            
           
         
       
     
     Method for Joining Surface (c) and D Surface (d) with Each Other 
     Previously in a preferred embodiment of the present invention, sol-gel based solution described under “The Production Method of the Adhesive in Ti(IV)Ax Formulation” heading is first of all applied on the C surface (C) or D surface (D) by means of utilizing from the methods such as spin coating, spray coating, dip coating and the like. The thin film coating is gently compressed between the surfaces and then the joining process of the two surfaces is realized. Subsequently the joining process is completed by means of drying the joined layer in a film form by subjecting it to a thermal process. At the end of the thermal treatment, the adhesive is transformed into a solid phased nonorganic adhesive form consisting of a single metal oxide (A Component in MyAx salt formulation described under the title “An Adhesive for Middle-Far Infra-red Optical Elements”). Thus a single phased adhesive is positioned between the C surface (C) and D surface (D). As a result of creating a single phased adhesive, the absorption of the light within the adhesive and affecting the optical features of the prisms in a negative manner are eliminated. 
     The prisms in mentioned ZnX formulation are very sensitive against temperature. The critical temperature for drying process of the adhesive without damaging the crystal structures of the prisms is at most 150° C. Open or controlled atmosphere ovens are not preferred due to the possible serious damages that it can give to the performances of the prisms. For this reason in a preferred embodiment of the present invention, microwave method is used for the thermal process. 
     In one embodiment of the present invention when the first prism ( 2 ) and the second prism ( 3 ) is joined, in order to strengthen the joint between C surface (C) and D surface (D), for increasing the adhesion force, a TiO 2  interlayer with an anatase structure is added. In one embodiment of the present invention the Physical Vapor Deposition—PVD method which is one of the physical vaporization methods is used and TiO 2  interlayer is enlarged. While TiO 2  interlayer is enlarged by means of the sol-gel method, in order not to damage the crystal structures of the prisms mentioned previously, TiO 2  interlayer with anatase structure is enlarged under 150° C. Preferably it is produced with the microwave source which can be changed between 1 W (watt) and 1000 W and under reaction conditions between 5° C. and 150° C. 
     In an exemplary embodiment of the present invention [Ti(IV) butox solution] the method for joining C surface (C) or D surface (D) with the adhesive is as the following: 
     Materials 
     
         
         
           
             [Preferably 3 μml] Ti(IV) butoxide (97%)-isopropanol 
             [Preferably 25 μml] acetyl acetone 
           
         
       
    
     Mixing Process 
     Preferably for 15 minutes at 200 rpm at approximately room temperature in a magnetic stirrer 
     Coating 
     Spin coating
         Approximately 5 seconds, 250 rpm   Approximately 90 seconds, 1500 rpm   Coated substrate (C surface (C) or D surface (D)) is adhered for 5 minutes at approximately room temperature       

     Thermal Process 
     In a microwave oven preferably under 800 W power, at a temperature under 150° C. (preferably 80° C.′de) and for approximately 20 minutes. 
     The thermal process application with the temperature-time curve shown in  FIG. 8  used in the microwave oven is given in the following.
         20° C.-80° C. (heating step), 5 minutes   80° C., 20 minutes   Cooling to the room temperature in 5 minutes       

     In the exemplary embodiment of the present invention in which the above summarized Ti(IV) butox solution is used, the method for joining the C surface (C) or D surface (D) with the adhesive comprises the following steps. 
     First of all [preferably 3 μml] Ti(IV) butoxide solution [preferably 97% isopropanol solution] and [preferably 25 μml] acetyl acetone mixture is stirred in a magnetic stirrer at room temperature at approximately 200 rpm speed for approximately 15 minutes. Then C surface (C) or D surface (D) are coated with said mixture by means of spin coating method for approximately 5 seconds duration at 250 rpm, subsequently for approximately 90 seconds duration at 1500 rpm. Then C surface (C) and D surface (D) is joined and the first prism ( 2 ) and second prism ( 3 ) are bonded by applying pressure on surfaces with a predetermined value during approximately 5 minutes. Subsequently preferably under power of 800 W in a microwave oven, thermal process is applied for 5 minutes by starting with approximately 20° C. up to 80° C. Immediately after 5 minute thermal process, thermal process is continued at 80° C. for 20 minutes (see  FIG. 8 ). After 20 minutes thermal process at 80° C., it is cooled to the room temperature in 5 minutes. 
     In another exemplary embodiment of the present invention [nano TiO 2  solution] the method for joining the C surface (C) or D surface (D) with the adhesive comprises the following steps. 
     Materials 
     
         
         
           
             [Preferably 3 μml] Ti(IV) butoxide (%97)-isopropanol 
             [Preferably 20 μml] nano TiO 2  solution 
             [Preferably 5 μml] isopropanol 
           
         
       
    
     Mixing 
     
         
         
           
             For 15 minutes at approximately 200 rpm at room temperature in a magnetic stirrer 
           
         
       
    
     Coating 
     Spin coating
         Approximately 5 seconds, 250 rpm   Approximately 90 seconds, 1500 rpm   Coated substrate (C surface (C) or D surface (D)) is adhered for 5 minutes at approximately room temperature       

     Thermal Process 
     It is realized preferably under power of 800 W in a microwave oven, at a temperature under 150° C. (preferably at 80° C.) and for approximately 20 minutes. 
     In an exemplary embodiment of the present invention wherein the above summarized nano TiO 2  solution is used, the method for joining the C surface (C) or D surface (D) with the adhesive comprises the following steps. 
     First of all [preferably 3 μml] 97% isopropanol solution of Ti(IV) butoxide, [preferably 20 μml] nano TiO 2  solution and [preferably 5 μml] isopropanol mixture is stirred with a magnetic stirrer at room temperature at approximately 200 rpm speed and for approximately 15 minutes. Subsequently C surface (C) or D surface (D) are coated with said mixture by means of the spin coating method for approximately 5 seconds at approximately 250 rpm, then for approximately 90 seconds at approximately 1500 rpm. Then C surface (C) and D surface (D) is joined and the first prism ( 2 ) and second prism ( 3 ) are bonded by applying pressure on surfaces with a predetermined value for approximately 5 minutes. Subsequently preferably under power of 800 W in a microwave oven, thermal process is applied approximately for 20 minutes at a temperature under 150° C. (preferably 80° C.). 
     X-rays diffraction curves which are obtained by means of using two different methods mentioned above between C surface (C) and D surface (D) are shown in the  FIG. 9 . In  FIG. 9 , X-rays diffraction curve of the adhesive created by using nano TiO 2  solution is named as nano TiO 2  solution curve (NTE) and X-rays diffraction curve of the adhesive created by means of using Ti(IV) butox solution is named as Ti(IV) butox solution curve (TBE). In the adhesive obtained by using nano TiO 2  solution, although a crystal structure belong to titanium dioxide (TiO 2 ) is observed, in the adhesive obtained by means of Ti(IV) butox solution, only an amorphous structure is observed. 
     The crystal structure belonging to the titanium dioxide (TiO 2 ) in the adhesive, obtained by means of using nano TiO 2  solution, does not cause a shift or interference on the quality of display received from the electro-optical monitoring systems (GS). 
     The following elements which are shown in figures;
         Optical sensor (S 1 ) sensitive to the near (3-5 μm) infra-red beams   Optical sensor (S 2 ) sensitive to the far (8-12 μm) infra-red beams   Optical unit (O)   Electro-optical monitoring system (GS)
 
are not an integral part of the present invention, their aim is to illustrate an exemplary embodiment of the middle far infra-red beam splitter prism set ( 1 ).