Abstract:
A low-glare motor-vehicle includes a color reproduction index R a  of at least 70 and a reduced reflection for each of the illuminants A and C in scotopic vision (at night) as compared with photopic vision (in the daylight) by at least 3%. In one embodiment, the mirror consists of a transparent substrate, a thin transparent metallic layer, an adapted dielectric layer and the reflector properly speaking.

Description:
The present invention relates to a rearview mirror for vehicles, in particular for motor vehicles or the like, with a transparent layer carrier, preferably consisting of soda-lime glass, and a coating on the front and/or rear side made from dielectric and metallic materials, the mirror having a low glaring effect due to its reflectance, in particular at night. 
     BACKGROUND 
     Typical known coatings for automobile mirrors consist of the highly reflecting metals silver and aluminium. The reflection values achievable amount to more than 85% in the visible light spectrum. Although, however, high reflection values are desirable in the daylight, they lead at night to a glaring of the driver through the headlights of the following vehicles. Therefore, mirror coatings were developed in the past, which are destined to reduce the glaring effect when driving at night. 
     One possibility of reducing the glare is the use of alternative reflector materials, such as chromium or titanium chromium (double-layer system of chromium on titanium, see DE 197 390 46). With this, mirrors with a neutral reflection and a reduced reflectance of approx. 45% to 60% can be realized. A further reduction is impossible at present, due to statutory provisions, for example, ECE regulation No. 46, page 20, 6.1.2.2.5 (40%) and “FMVS111, page 329, S 11  (35%),which prescribe a minimum reflection of 40% in the daylight. 
     Afterwards, it was tried to improve the behavior of the mirrors through spectrally selective reflection coatings. EP 0176935 B1, for example, describes a blue rearview mirror. Blue mirrors reflect wavelengths in the red or green spectrum to a lesser extent than wavelength in the blue spectrum. Because the spectral composition of the light of the headlights usual at the moment of application of the patent EP 0 176 935 B1 had a high red contribution and a low blue contribution, the blue mirrors additionally diminished the (glaring) light of these headlights. 
     To evaluate the spectral properties of the mirror coatings, one must in general take into account that the response of the human eye differs greatly within the visible spectral region. This is represented by the so-called response characteristic V(λ), λ designating the wavelength of the light. V(λ) is valid, however, only for the bright adaptation of the eye. With decreasing brightness of the environment, this characteristic shifts towards shorter wavelengths up to the dark-adapted response characteristic V′(λ). 
     Therefore, another approach for the development of low-glare mirrors were mirrors which, at a relatively high level of spectral reflectance, reduced in particular the region of the highest eye responsiveness at wavelengths of approx. 550 nm (U.S. Pat. Nos. 4,921,331, 4,805,989 and 4,955,705). 
     With an alternative technology to be mentioned here, one tried to reduce the risk of glaring through the headlights of following vehicles by means of the fact that the mirror structure contains parts of variable transmission, switched, e.g., electrically (e.g. electrochromic mirrors or LCD mirrors). The before-mentioned technologies involve, however, a high expenditure. They require in particular measures of circuit technology to ensure that the low reflection values achievable with them are only effective in concrete cases of acute glare. Accordingly, only vehicles from the top segment are equipped with them at present. 
     Another important variable for evaluation is the so-called color rendering index Ra, following the standard DIN EN 410, which is a measure for the manner in which colors can be reproduced or distinguished in the reflected mirror image. The color reproduction index is relevant in particular for visual recognition of the road traffic happenings. Briefly, systems with neutral reflection have a good color rendering (Ra almost 100), while the color reproduction index of intensely colored mirrors is clearly reduced. This limits in practice the influencing of the glare behavior by means of the mirror spectrum. 
     The optimum reduction of glare is made even more difficult through the fact that meanwhile other light sources to be used in motor-vehicle headlights were developed, which spectrally differ clearly from the known light sources (halogen headlights) and which complicate the relations. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved rearview mirror with a clear reduction of the glaring through following vehicles (with different headlight structures), in particular in the darkness, and a color reproduction index R a  on at least the order of the known blue-shade mirrors (R a =74). 
     The present invention provides a rearview mirror wherein the reflection layer has a spectral reflection behavior showing at night a relative reduction of the reflection values of illuminants A and C by at least 3%, preferably by at least 5%, particularly preferably by at least 10%, as compared with the bright-adapted. The color rendering index is in this case at least 70. 
     Advantageous embodiments of the invention are the subject matter of the claims. 
     In particular, the coating is located advantageously on the side of the layer carrier which is facing away from the viewer. In an alternative or additional advantageous development, the coating comprises, in the order of their deposition, a transparent metallic layer, a dielectric layer and a metallic reflecting layer, the transparent metallic layer preferably consisting of NiCr and having a thickness of 1 to 21 nm. The dielectric layer preferably consists of TiO 2 , SiO 2  or SnO 2 . 
     Particularly good results can be achieved by designing the optical properties of the rearview mirror according to a so-called design wavelength, which is particularly well adapted to the response characteristic of the eye, both for photopic and for scotopic vision. A wavelength of approx. 530 nm was chosen as a particularly well suited design wavelength, corresponding to an intermediate value between the maxima of the response characteristic for photopic vision, on the one hand, and for scotopic vision, on the other hand. To guarantee a particularly well suited adaptation of the layer system to this design wavelength, the layer thicknesses, in particular the so-called optical-layer thickness, i.e. the product of actual layer thickness and refractive index, of the dielectric layer are preferably chosen in a suitable manner. Preferably, the optical thickness (do) of the dielectric layer is chosen as a function of its refractive index n530, with the design wavelength being 530 nm, in such a way that it lies within a range which is considered as particularly well suited. As used herein, refractive index n530 is used to refer to the refractive index (n) of a material when exposed to light at a wavelength of 530 nm (n530). This range is limited for a refractive index n530 of maximally 1.99, as the lower limit, by the straight line given in the (n530, do) plane, which is defined as an orthogonal plane having the reflective index (n530) plotted along one orthogonal axis and the optical thickness of a material (do) plotted along a second orthogonal axis, and running through the points (n530=1.46, do=235 nm) and (n530=1.99, do=216 nm), preferably by the straight line given in the (n530, do) plane and running through the points (n530=1.46, do=244 nm) and(n530=1.99, do=233 nm), and as the upper limit, by the straight line given in the (n530, do) plane and running through the points (n530 =1.46, do=362 nm) and (n530=1.99, do=353 nm), preferably by the straight line given in the (n530, do) plane and running through the points (n530=1.46, do=335 nm) and (n530=1.99, do=330 nm). For a refractive index n530 of at least 1.99, however, the range considered as preferable is limited, as the lower limit, by the straight line given in the (n530, do) plane and running through the points (n530=1.99, do=216 nm) and (n530=2.48, do=205 nm), preferably by the straight line given in the (n530, do) plane and running through the points (n530=1.99, do=233 nm) and (n530=2.48, do=225 nm), and as the upper limit by the straight line given in the (n530, do) plane and running through the points (n530=1.99, do=353 nm) and (n530=2.48, do=340 nm), preferably by the straight line given in the (n530, do) plane and running through the points (n530=1.99, do=330 nm) and (n530=2.48, do=319 nm). 
     By choosing these parameters, it is guaranteed that the dielectric layer has an optical thickness of 235 nm to 362 nm, preferably of 244 to 335 nm, if its refractive index amounts to 1.45 at 530 nm, whereas the dielectric layer has an optical thickness of 216 nm to 353 nm, preferably of 233 to 330 nm, if its refractive index amounts to 1.99 at 530 nm, and an optical thickness of 205 nm to 340 nm, preferably of 225 to 319 nm, if its refractive index amounts to 2.45 at 530 nm. In the intermediate range between these refractive indices, the dielectric layer has an optical thickness in a range which results from linear interpolation due to its refractive index at 530 nm, lying in the interval of 1.46 to 1.99 or in the interval of 1.99 to 2.48. 
     In conventional rearview mirrors, usually silver (Ag) is used as reflector material. This usually requires, however, expensive anticorrosive measures, for example the application of protective varnish or the like, in order to guarantee a reliable reflection effect under all conditions of application. On the one hand, for reasons of cost and, on the other hand, for reasons of environmental protection, in particular in view of the materials used in such protective varnishes, it is, therefore, desirable to provide alternative materials for the reflecting layer. This is, however, not easily possible, in view of the optical properties provided by the design, in particular for safeguarding the required reflection values. Surprisingly, it turned out, however, that these design targets, i.e. observation of particularly advantageous optical properties when using alternative reflector materials, can be achieved by arranging, in a particularly advantageous embodiment, a further dielectric layer in the manner of an additional layer between the dielectric layer provided anyhow and the reflecting layer. This additional layer has in a further advantageous embodiment a reduced refractive index as compared with the dielectric layer. 
     Advantageously, the additional dielectric layer consists of SiO 2  or MgF 2 , or, if the refraction of the dielectric layer is high enough, of SnO 2 . 
     In this multilayer system, too, the set of characteristic parameters, i.e. the combination of layer thicknesses, is preferably chosen in such a suitable way that an adaptation to a design wavelength, preferably of approx. 530 nm, is effected. This is achieved by the fact that the sum of the optical-layer thicknesses of the dielectric layer and the additional layer is at least 250 nm, preferably at least 275 nm, and maximally 390 nm, preferably maximally 375 nm. With that, the total optical-layer thickness given through the dielectric double layer lies within a band of a certain bandwidth above and around half the design wavelength. In an alternative advantageous embodiment, the sum of the optical-layer thicknesses of the dielectric layer and the additional layer is at least 530 nm, preferably at least 560 nm, and maximally 670 nm, preferably maximally 635 nm. 
     The multilayer structure of the dielectric enables the use of alternative reflector materials. Advantageously, the reflecting layer consists of aluminium (Al), nickel (Ni), titanium (Ti), chromium (Cr) or of an alloy of these materials, but preferably of Cr. 
     The coating is advantageously applied on the rear side of the layer carrier, so that it is protected by the layer carrier against damage or other detrimental effects from the exterior. In another advantageous embodiment, the front of the layer carrier is provided with a hydrophilic and/or a photocatalytically active coating, which prevents an undesired fogging-up in a manner known in itself. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated by the following drawings, in which: 
         FIG. 1  shows the spectral course of a halogen lamp (illuminant A), of daylight (illuminant C), of a high-pressure discharge lamp and of a white LED; 
         FIG. 2  shows the photopic and scotopic response characteristic; 
         FIG. 3  shows the reflection curves of known blue-shade or neutral-shade mirrors; 
         FIG. 4  and  FIG. 5  show the reflection curves of known color mirrors (U.S. Pat. No. 4,955,705 and WO0241049); 
         FIG. 6  shows the layer structure of the color mirror according to the exemplary embodiments; and 
         FIG. 7  to  FIG. 10  show the reflection curves of embodiments according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the development of the spectra of different light sources in the past. While up to recent times, only the illuminants designated as C (daylight) and A (at night) in accordance with the International Commission on Illumination (CIE) were relevant, the spectral conditions for driving at night have become more complex due to the development of new light sources (discharge lamp HID and white light-emitting diodes LED). In  FIG. 1 , the spectra are designated with (a) illuminant A, (b) illuminant C, (c) discharge lamp, and (d) white light-emitting diode. 
       FIG. 2  illustrates that the human eye is first of all highly responsive within a narrow range only and that, therefore, this range is mainly relevant for examinations concerning the glare. It can also be seen that the response characteristic of scotopic vision (at night) shifts towards shorter wavelengths. The response characteristics (e) “photopic vision” and (f) “scotopic vision” correspond to the standard CIE 1951. 
     Table 1 represents the characteristic optical data of known mirror systems and systems according to the present invention. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Layer system 
                 Photopic 
                 Scotopic 
                   
               
             
          
           
               
                 Variant/l 
                 Absorber 
                 Intermediate layer 
                 Additional layer 
                 Reflection 
                 Reflection 
                   
               
             
          
           
               
                 Reflector 
                   
                 ds 
                   
                 ds 
                 Do 
                   
                 ds 
                 do 
                 A 
                 C 
                 LED 
                 HID 
                 A 
                 C 
                 LED 
                 HID 
                 Ra 
                 M* 
               
               
                 material 
                 Mat. 
                 [nm] 
                 Mat. 
                 [nm] 
                 [nm] 
                 Mat 
                 [nm] 
                 [nm] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
                 [%] 
               
               
                   
               
             
          
           
               
                 Chromium 2 
                 — 
                 — 
                 — 
                 — 
                 — 
                   
                   
                   
                 57 
                 57 
                 57 
                 57 
                 58 
                 58 
                 58 
                 58 
                 97 
                 102 
               
               
                 Titanium 
                 — 
                 — 
                 — 
                 — 
                 — 
                   
                   
                   
                 46 
                 46 
                 46 
                 46 
                 45 
                 45 
                 45 
                 45 
                 99 
                 98 
               
               
                 chromium 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 TEREF 
                 NiCr 
                 3.4 
                 SnO2 
                 82 
                 164 
                   
                   
                   
                 49 
                 53 
                 52 
                 51 
                 64 
                 68 
                 66 
                 66 
                 74 
                 130 
               
               
                 Example 1/Ag 
                 NiCr 
                 3.2 
                 SiO2 
                 248 
                 362 
                   
                   
                   
                 40 
                 37 
                 37 
                 38 
                 34 
                 36 
                 37 
                 37 
                 75 
                 97 
               
               
                 Example 2/Ag 
                 NiCr 
                 2.8 
                 SiO2 
                 240 
                 351 
                   
                   
                   
                 47 
                 44 
                 44 
                 45 
                 38 
                 39 
                 40 
                 40 
                 75 
                 88 
               
               
                 Example 3/Ag 
                 NiCr 
                 2.4 
                 SiO2 
                 226 
                 330 
                   
                   
                   
                 59 
                 55 
                 55 
                 56 
                 46 
                 44 
                 45 
                 45 
                 75 
                 80 
               
               
                 Example 4/Ag 
                 NiCr 
                 3.15 
                 SiO2 
                 197 
                 288 
                   
                   
                   
                 74 
                 70 
                 71 
                 72 
                 55 
                 49 
                 52 
                 51 
                 75 
                 74 
               
               
                 Example 5/Ag 
                 NiCr 
                 10.6 
                 SiO2 
                 172 
                 251 
                   
                   
                   
                 76 
                 73 
                 77 
                 77 
                 58 
                 49 
                 54 
                 52 
                 79 
                 76 
               
               
                 Example 6/Ag 
                 NiCr 
                 13.6 
                 SiO2 
                 165 
                 240 
                   
                   
                   
                 73 
                 73 
                 76 
                 75 
                 63 
                 54 
                 58 
                 57 
                 75 
                 86 
               
               
                 Example 7/Ag 
                 NiCr 
                 9.2 
                 SiO2 
                 161 
                 235 
                   
                   
                   
                 74 
                 76 
                 79 
                 77 
                 72 
                 65 
                 68 
                 67 
                 75 
                 97 
               
               
                 Example 8/Ag 
                 NiCr 
                 2.3 
                 SnO2 
                 177 
                 353 
                   
                   
                   
                 51 
                 48 
                 48 
                 49 
                 44 
                 46 
                 47 
                 46 
                 76 
                 97 
               
               
                 Example 9/Ag 
                 NiCr 
                 2 
                 SnO2 
                 169 
                 337 
                   
                   
                   
                 59 
                 55 
                 56 
                 57 
                 47 
                 47 
                 49 
                 48 
                 76 
                 85 
               
               
                 Example 10/Ag 
                 NiCr 
                 1.9 
                 SnO2 
                 161 
                 321 
                   
                   
                   
                 66 
                 62 
                 63 
                 64 
                 51 
                 48 
                 51 
                 50 
                 76 
                 77 
               
               
                 Example 11/Ag 
                 NiCr 
                 3.3 
                 SnO2 
                 140 
                 279 
                   
                   
                   
                 77 
                 72 
                 74 
                 75 
                 57 
                 49 
                 53 
                 52 
                 75 
                 74 
               
               
                 Example 12/Ag 
                 NiCr 
                 14 
                 SnO2 
                 119 
                 237 
                   
                   
                   
                 76 
                 74 
                 77 
                 77 
                 60 
                 52 
                 55 
                 54 
                 81 
                 79 
               
               
                 Example 13/Ag 
                 NiCr 
                 16 
                 SnO2 
                 114 
                 227 
                   
                   
                   
                 74 
                 73 
                 77 
                 76 
                 64 
                 56 
                 60 
                 58 
                 79 
                 86 
               
               
                 Example 14/Ag 
                 NiCr 
                 14.9 
                 SnO2 
                 110 
                 219 
                   
                   
                   
                 72 
                 74 
                 76 
                 74 
                 70 
                 63 
                 65 
                 64 
                 75 
                 97 
               
               
                 Example 15/Ag 
                 NiCr 
                 2.4 
                 TiO2 
                 137 
                 340 
                   
                   
                   
                 55 
                 52 
                 52 
                 53 
                 46 
                 50 
                 51 
                 50 
                 75 
                 97 
               
               
                 Example 16/Ag 
                 NiCr 
                 2.2 
                 TiO2 
                 131 
                 325 
                   
                   
                   
                 63 
                 59 
                 60 
                 61 
                 49 
                 49 
                 53 
                 50 
                 76 
                 84 
               
               
                 Example 17/Ag 
                 NiCr 
                 2.15 
                 TiO2 
                 124 
                 307 
                   
                   
                   
                 71 
                 66 
                 68 
                 69 
                 53 
                 50 
                 55 
                 52 
                 76 
                 76 
               
               
                 Example 18/Ag 
                 NiCr 
                 4.3 
                 TiO2 
                 108 
                 268 
                   
                   
                   
                 80 
                 75 
                 77 
                 78 
                 59 
                 50 
                 54 
                 53 
                 75 
                 74 
               
               
                 Example 19/Ag 
                 NiCr 
                 14.6 
                 TiO2 
                 92 
                 228 
                   
                   
                   
                 79 
                 76 
                 79 
                 79 
                 63 
                 55 
                 59 
                 57 
                 82 
                 80 
               
               
                 Example 20/Ag 
                 NiCr 
                 15.9 
                 TiO2 
                 84 
                 208 
                   
                   
                   
                 75 
                 77 
                 79 
                 77 
                 73 
                 66 
                 68 
                 67 
                 79 
                 97 
               
               
                 Example 21/Ag 
                 NiCr 
                 1.2 
                 SnO2 
                 177 
                 353 
                   
                   
                   
                 64 
                 62 
                 62 
                 63 
                 58 
                 59 
                 60 
                 59 
                 86 
                 96 
               
               
                 Example 22/Ag 
                 NiCr 
                 1.1 
                 SnO2 
                 170 
                 339 
                   
                   
                   
                 69 
                 66 
                 67 
                 67 
                 59 
                 59 
                 61 
                 60 
                 85 
                 89 
               
               
                 Example 23/Ag 
                 NiCr 
                 1 
                 SnO2 
                 165 
                 329 
                   
                   
                   
                 73 
                 70 
                 71 
                 71 
                 62 
                 60 
                 62 
                 61 
                 85 
                 86 
               
               
                 Example 24/Ag 
                 NiCr 
                 1.35 
                 SnO2 
                 143 
                 284 
                   
                   
                   
                 82 
                 79 
                 80 
                 81 
                 68 
                 63 
                 65 
                 65 
                 85 
                 84 
               
               
                 Example 25/Ag 
                 NiCr 
                 4 
                 SnO2 
                 125 
                 249 
                   
                   
                   
                 84 
                 82 
                 84 
                 84 
                 71 
                 64 
                 66 
                 65 
                 85 
                 85 
               
               
                 Example 26/Ag 
                 NiCr 
                 6 
                 SnO2 
                 113 
                 225 
                   
                   
                   
                 80 
                 81 
                 83 
                 82 
                 78 
                 72 
                 74 
                 73 
                 85 
                 97 
               
               
                 Ag 
                 NiCr 
                 20.8 
                 SnO2 
                 109 
                 216 
                 — 
                 0 
                 0 
                 69 
                 70 
                 73 
                 72 
                 66 
                 60 
                 63 
                 62 
                 72 
                 97 
               
               
                 Ag 
                 NiCr 
                 20.8 
                 SnO2 
                 117 
                 233 
                 — 
                 0 
                 0 
                 72 
                 69 
                 72 
                 73 
                 57 
                 51 
                 54 
                 53 
                 85 
                 80 
               
               
                 Ag 
                 NiCr 
                 20.9 
                 SnO2 
                 109 
                 216 
                 — 
                 0 
                 0 
                 68 
                 70 
                 73 
                 72 
                 66 
                 60 
                 63 
                 62 
                 72 
                 97 
               
               
                 Ag 
                 NiCr 
                 2.6 
                 SnO2 
                 174 
                 347 
                 — 
                 0 
                 0 
                 48 
                 44 
                 44 
                 45 
                 39 
                 42 
                 44 
                 42 
                 71 
                 96 
               
               
                 Ag 
                 NiCr 
                 19.8 
                 SiO2 
                 161 
                 235 
                 — 
                 0 
                 0 
                 68 
                 69 
                 72 
                 71 
                 61 
                 55 
                 58 
                 57 
                 70 
                 91 
               
               
                 Ag 
                 NiCr 
                 3.3 
                 SiO2 
                 246 
                 359 
                 — 
                 0 
                 0 
                 38 
                 35 
                 35 
                 36 
                 32 
                 34 
                 35 
                 34 
                 71 
                 96 
               
               
                 Ag 
                 NiCr 
                 19.5 
                 SiO2 
                 167 
                 244 
                 — 
                 0 
                 0 
                 70 
                 68 
                 71 
                 72 
                 56 
                 49 
                 53 
                 52 
                 80 
                 80 
               
               
                 Ag 
                 NiCr 
                 2.8 
                 SiO2 
                 230 
                 335 
                 — 
                 0 
                 0 
                 51 
                 47 
                 48 
                 49 
                 39 
                 38 
                 39 
                 39 
                 70 
                 80 
               
               
                 Ag 
                 NiCr 
                 20.5 
                 TiO2 
                 83 
                 205 
                 — 
                 0 
                 0 
                 73 
                 74 
                 77 
                 76 
                 70 
                 64 
                 67 
                 66 
                 75 
                 97 
               
               
                 Ag 
                 NiCr 
                 2.6 
                 TiO2 
                 135 
                 335 
                 — 
                 0 
                 0 
                 54 
                 50 
                 50 
                 51 
                 44 
                 48 
                 50 
                 48 
                 72 
                 97 
               
               
                 Ag 
                 NiCr 
                 20.5 
                 TiO2 
                 91 
                 225 
                 — 
                 0 
                 0 
                 75 
                 72 
                 75 
                 75 
                 60 
                 54 
                 57 
                 56 
                 85 
                 80 
               
               
                 Ag 
                 NiCr 
                 2.7 
                 TiO2 
                 129 
                 319 
                 — 
                 0 
                 0 
                 61 
                 56 
                 57 
                 58 
                 44 
                 45 
                 49 
                 46 
                 71 
                 80 
               
               
                 Cr 
                 NiCr 
                 3.0 
                 SnO2 
                 76 
                 150.91 
                 SiO2 
                 84 
                 123.15 
                 70 
                 69 
                 70 
                 70 
                 65 
                 62 
                 62 
                 62 
                 92 
                 94 
               
               
                 Cr 
                 NiCr 
                 4.0 
                 SnO2 
                 87 
                 172.47 
                 SiO2 
                 75 
                 109.47 
                 68 
                 66 
                 67 
                 67 
                 58 
                 54 
                 55 
                 55 
                 87 
                 87 
               
               
                 Cr 
                 NiCr 
                 2.0 
                 SnO2 
                 184 
                 366.49 
                 SiO2 
                 159 
                 232.64 
                 50 
                 46 
                 49 
                 49 
                 35 
                 33 
                 39 
                 35 
                 77 
                 71 
               
               
                 (SnO2 2. Ord.) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cr 
                 NiCr 
                 3.0 
                 SnO2 
                 206 
                 409.61 
                 SiO2 
                 131 
                 191.58 
                 55 
                 50 
                 53 
                 54 
                 35 
                 31 
                 38 
                 34 
                 71 
                 65 
               
               
                 (SnO2 2. Ord.) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cr 
                 NiCr 
                 12.0 
                 SnO2 
                 195 
                 388.05 
                 SiO2 
                 141 
                 205.26 
                 50 
                 46 
                 50 
                 50 
                 31 
                 27 
                 32 
                 30 
                 70 
                 62 
               
               
                 (SnO2 2. Ord.) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cr 
                 NiCr 
                 17.0 
                 SnO2 
                 54 
                 107.79 
                 SiO2 
                 112 
                 164.21 
                 65 
                 64 
                 66 
                 66 
                 57 
                 52 
                 54 
                 54 
                 88 
                 88 
               
               
                 Cr 
                 NiCr 
                 2.0 
                 SnO2 
                 108 
                 216 
                 SiO2 
                 94 
                 137 
                 49 
                 46 
                 46 
                 47 
                 36 
                 34 
                 36 
                 35 
                 74 
                 75 
               
               
                 Cr 
                 NiCr 
                 1.0 
                 SnO2 
                 87 
                 172 
                 SiO2 
                 131 
                 192 
                 50 
                 47 
                 48 
                 49 
                 39 
                 37 
                 39 
                 38 
                 77 
                 79 
               
               
                 U.S. Pat. No. 
                   
                   
                   
                   
                   
                   
                   
                   
                 54 
                 54 
                 53 
                 53 
                 56 
                 59 
                 58 
                 58 
                 91 
                 109 
               
               
                 4,955,705 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 FIG. 4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 U.S. Pat. No. 
                   
                   
                   
                   
                   
                   
                   
                   
                 46 
                 46 
                 45 
                 46 
                 51 
                 55 
                 53 
                 54 
                 88 
                 119 
               
               
                 4,955,705 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 FIG. 5 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 U.S. Pat. No. 
                   
                   
                   
                   
                   
                   
                   
                   
                 49 
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                 54 
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                 53 
                 77 
                 114 
               
               
                 4,955,705 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 FIG. 6 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 U.S. Pat. No. 
                   
                   
                   
                   
                   
                   
                   
                   
                 40 
                 40 
                 39 
                 40 
                 45 
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                 48 
                 48 
                 83 
                 123 
               
               
                 4,955,705 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 FIG. 7 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 U.S. Pat. No. 
                   
                   
                   
                   
                   
                   
                   
                   
                 38 
                 37 
                 36 
                 37 
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                 44 
                 42 
                 43 
                 74 
                 119 
               
               
                 4,955,705 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 FIG. 8 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 WO0241049 
                   
                   
                   
                   
                   
                   
                   
                   
                 50 
                 44 
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                 30 
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                 30 
                 65 
                 64 
               
               
                 FIG. 5 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 WO0241049 
                   
                   
                   
                   
                   
                   
                   
                   
                 33 
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                 21 
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                 FIG. 7 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 WO0241049 
                   
                   
                   
                   
                   
                   
                   
                   
                 43 
                 40 
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                 36 
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                 78 
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                 FIG. 10 
               
               
                   
               
             
          
         
       
     
     First of all, the terms used there will be explained. The executions “chromium 2”, “titanium chromium 2” and “TEREF”, belonging to the state of the art, are mirrors having the coating on the side of the substrate facing away from the viewer, “chromium 2” being an ordinary chromium mirror, while “titanium chromium 2” was manufactured according to DE 19739046 and “TEREF” (blue), according to EP 0176035 B1. “Photopic” and “scotopic” designate integral reflection values corresponding to the respective eye response characteristics according to CIE 1951. The designations A and C symbolize the associated illuminants (according to CIE 1971), HID and LED are weighted reflection values based on spectra of typical discharge lamps and white light-emitting diodes of the company Hella. The designations under the heading “Layer system” will be explained below. R a  is the general color rendering index according to DIN EN 410 and results from the reflection curve. M* is the larger of the two ratios reflection (illuminant A, scotopic)/reflection (illuminant A, photopic) and reflection (illuminant C, scotopic)/reflection (illuminant C, photopic) and indicates the low glare of the mirror. If the value is smaller than 1, a reduced reflection is perceptible with scotopic vision for the two standardized illuminants A and C. This value should be maximally 97%. 
     Table 1 shows that the experimental values of LED and HID are very similar to those of C, so that further considerations shall be based on the quantities of C, which are subject to a generally accessible standardization (like A). 
     As is evident from Table 1, the known systems do not fulfill the required criterion of low glare and, at the same time, sufficient color reproduction. The blue TEREF mirror (reflection spectrum, see  FIG. 3 ), for example, has for illuminant A a daylight reflection of 53%, but at night, of 68%. A possible and understandable explanation lies in the fact that the course of the eye response characteristic has the consequence that spectral reflection values contribute to the perceived integral reflectivity within a narrow range only. The fact that the eye response is shifted in the direction of “blue” at night results in a higher integral reflectivity. The course of the lamp spectrum, which must also be taken into account, leads, e.g. in the case of illuminant A, at shorter wavelengths to a lower illuminance, but obviously, this effect is of secondary importance. It is interesting and surprising in this connection that the low glare of such blue mirrors has been considered as recognized for almost twenty years. 
     Exemplary Embodiments 
     The examples mentioned in Table 1 can be manufactured, for example, as follows, as a rear-side coating of a glass carrier ( FIG. 6 ).
       1  glass     2  absorbing thin metallic layer (few nm)     3  dielectric     4  metallic reflector   

     The layer structure of the above-mentioned examples is listed in Table 1 and the reflection spectra are represented in  FIG. 6  to  FIG. 9 , ds designating the respective layer thickness of the thin metallic layer and of the dielectric layer, do designating the optical-layer thickness of the dielectric layer at a wavelength of 530 nm (approx. in the middle between photopic and scotopic eye response characteristic). The dielectric  3  can for its part have a multilayer structure and comprise in particular an additional layer additionally to the dielectric layer properly speaking, with a lower refractive index than that of the dielectric layer. 
     All exemplary embodiments show reflection values for CIE-designated illuminants A and C, which decrease by at least 3% (i.e. M* smaller than, or equal to, 97%) upon transition from photopic to scotopic vision and can, therefore, be qualified as low-glare mirrors. At the same time, the mirrors according to the exemplary embodiments have acceptable R a  values (larger than, or equal to, 70), while mirrors according to the state of the art do not reach these values in combination. 
     The layer system can be applied, for example, using common vacuum coating processes (e.g. vaporization, cathode sputtering, ion plating). Sol-gel processes are also suitable, in particular for the dielectric layer. 
     The layer structure of the above-mentioned examples is listed in Table 1 and the reflection spectra are represented in  FIG. 6  to  FIG. 9 , ds designating the respective layer thickness of the thin metallic layer and of the dielectric layer, do designating the optical-layer thickness of the dielectric layer at a wavelength of 530 nm (approx. in the middle between photopic and scotopic eye response characteristic). The dielectric 3 can for its part have a multilayer structure and comprise in particular an additional layer additionally to the dielectric layer properly speaking, with a lower refractive index than that of the dielectric layer. 
     All exemplary embodiments show reflection values for CIE-designated illuminants A and C, which decrease by at least 3% (i.e. M* smaller than, or equal to, 97%) upon transition from photopic to scotopic vision and can, therefore, be qualified as low-glare mirrors. At the same time, the mirrors according to the exemplary embodiments have acceptable Ra values (larger than, or equal to, 70), while mirrors according to the state of the art do not reach these values in combination. 
     As layer carrier  1  (substrate), in addition to soda-lime glass, other mineral or organic glasses having the necessary optical transparency can be used, too. 
     The dielectric layer  3  is of particular importance. For this layer, all transparent dielectrics which can be applied in an economic manner, e.g. SiO 2 , TiO 2 , ZnS, CeO 2 , Bi 2 O 3 , Ta 2 O 5 , can be used. The examples of Table 1 show that in dimensioning the layer system, one must see to it that the optical thickness of the dielectric is within a range leading to the desired properties. The thickness of the optical layer results from the product of thickness and refractive index at a certain wavelength. The materials SiO 2 , SnO 2  and TiO 2  have a refractive index of 1.46, 1.99 or 2.48, at 530 nm. For low-refractive materials like SiO 2 , the optical thickness according to the invention lies, according to Table 1, between 235 nm and 362 nm, towards higher-refractive materials, the range shifts a little (e.g. TiO 2 : 208 nm to 340 nm). Outside these ranges, no sufficiently low glare is achieved or color reproduction is insufficient. Inside the thickness intervals, variations result in reflection curves of different courses, which to a certain extent allow to influence the color shade of the mirror and are, therefore, accessible to design purposes. With the examples 15 to 20, for example, the colors violet, red, orange, yellow and yellowish green were achieved.