High index brown photochromic glasses
FIELD OF THE INVENTION 
     The present invention concerns the preparation of transparent photochromic glasses, which are clear and essentially colorless in their virgin state (faded state), present improved darkening characteristics when exposed to an actinic radiation (generally a ultraviolet source), and display rapid transitions from the darkened to the clear state. The glasses are adequate for the fabrication of ophthalmic lenses and are characterized by a refractive index between 1.585 and 1.610. The possibility exists as well to confer a brown coloration to these glasses in their darkened state. 
     BACKGROUND OF THE INVENTION 
     Photochromic glasses were invented some thirty years ago, as described in U.S. Pat. No. 3,208,860 (Armistead et al). Because the main application for photochromic glasses is the fabrication of ophthalmic lenses, extensive research and development work was performed in order to design glass compositions which, not only show fast darkening and fading, but also have a refractive index of 1.523, the industrial standard for common lenses. 
     However, during the past few years, extensive research was devoted to the development of photochromic glasses of higher refractive index. The very significant benefit resulting from the use of higher refractive index glasses bears on the possibility to reduce the lens thickness, while keeping the same lens corrective power. Several types of compositions were developed, as summarized by the following works: 
     U.S. Pat. No. 3,703,388 (Araujo) describes the use lanthanum borate compositions containing silver halide; and 
     U.S. Pat. No. 3,999,996 (Faulstich et al) concerns the use of high lead oxide concentrations in aluminoborosilicate base compositions containing silver halide. 
     For various reasons, the glasses that came out of the first research efforts did not fulfil all commercial expectations. More recent investigations were focused on the use of two or several oxides of the group TiO 2 , Nb 2  O 5  and ZrO 2 , with additions of alkaline earths, in order to raise the refractive index. The addition of TiO 2  yields a rapid increase of the refractive index without increasing the glass density as much as BaO, Nb 2  O 5  and ZrO 2  do. The resulting lens is lighter in weight. The most widely sold photochromic ophthalmic lenses having a refractive index of about 1.6 contain over 2% TiO 2  in their composition, and are brown in the darkened state. Unfortunately, the presence of TiO 2  in these glasses confers a yellowish tint to the glass, which should be essentially colorless in the virgin/faded state. In other words, present commercial photochromic glasses exhibiting refractive indices close to 1.6 and darkening to a brown colour when they are exposed to actinic radiation are not as colorless in their virgin and faded states as commercial photochromic glasses exhibiting a refractive index of 1.523 which darken to a brown colour when they are exposed to actinic radiation. 
     Consequently, one of the objectives of the present invention was to provide transparent photochromic glasses with refractive indices close to 1.6, which darken to a desired colour when exposed to an actinic radiation, and which fade to an essentially colorless state (at least equivalent to that presented by photochromic glasses exhibiting a refractive index of 1.523}, while demonstrating equivalent or improved photochromic performances. 
     Another objective was to provide glasses of the above described type which display a brown colour in their darkened state and exhibit a virgin/faded coloration less than that of 1.523 index glasses. 
     SUMMARY OF THE INVENTION 
     The above objectives can be reached with glasses having base compositions, expressed in weight percent of oxides, consisting essentially of: 
     
         ______________________________________                                    
SiO.sub.2                                                                 
         43-52    Li.sub.2 O                                              
                           1.5-3.5                                        
                                  CaO    0-5                              
B.sub.2 O.sub.3                                                           
         12.5-18  Na.sub.2 O                                              
                           0-3    SrO    0-9                              
Al.sub.2 O.sub.3                                                          
         0-3      K.sub.2 O                                               
                           2-9    BaO    0-9                              
ZrO.sub.2                                                                 
         6-14     MgO      0-5    Nb.sub.2 O.sub.5                        
                                         6-16                             
TiO.sub.2                                                                 
         0-&lt;2                                                             
______________________________________                                    
 
    
     with following additional conditions: 
     
         ______________________________________                                    
Li.sub.2 O + Na.sub.2 O + K.sub.2 O(X.sub.2 O)                            
                         7-12                                             
MgO + CaO + SrO + BaO(XO)                                                 
                         2-12                                             
X.sub.2 O + XO           12-20                                            
ZrO.sub.2 + Nb.sub.2 O.sub.5 + TiO.sub.2                                  
                         15-24                                            
ZrO.sub.2 + Al.sub.2 O.sub.3                                              
                         6-&lt;12                                            
Nb.sub.2 O.sub.5 + Al.sub.2 O.sub.3                                       
                         6-&lt;14                                            
Li.sub.2 O/X.sub.2 O     0.15-0.4                                         
______________________________________                                    
 
    
     and of photochromic elements in the following proportions, expressed as weight percentages: 
     
         ______________________________________                                    
Ag       0.100-0.175  Br       0.093-0.200                                
Cl       0.140-0.350  CuO      0.0080-0.0300                              
______________________________________                                    
 
    
     with the following conditions: 
     Ag+Br&gt;0.21 
     Br+Cl&gt;0.24. 
     Because TiO 2  gives the glass an undesirable yellow colour, the glasses described in the invention are essentially free from this oxide. In other words, although small quantities of TiO 2  can be tolerated, its total concentration should not exceed 2% and preferred glasses should be free of this oxide, that is; not more than amounts resulting from impurities. 
     Although the colour of the invention glasses is naturally aesthetically pleasant, classical colorants, such as CoO, Er 2  O 3  and Nd 2  O 3 , may be added to the base composition in appropriate amounts that confer the desired colour to the glass. 
     Beyond the fact that they have a refractive index of about 1.6, the inventive glasses are also characterized by an Abbe number ranging from 42 to 47, a density less than 2.82, excellent chemical durability, and a liquidus viscosity compatible with traditional melting and forming technologies. 
     The glasses described in the invention can withstand heat treatments required for the deposition of coatings such as anti-reflective coatings. Typically, these treatments require that the glass be heated up to about 280° C., without causing substantial modification of the photochromic performances of the glass. The glasses can also be chemically strengthened (chemtempered) without significant alteration of their photochromic characteristics. Furthermore, the glasses can be heated to a temperature allowing their sagging inside molds, this temperature being higher than the characteristic annealing temperature of the glass. In this case, the photochromic performances are as well not affected. 
     As observed above, one advantage brought by the addition of TiO 2  to the base composition of high index commercial glasses was to raise their refractive index without substantially increasing their density, as BaO, Nb 2  O 5  and ZrO 2  (and indeed PbO) do. The compositions of the inventive glasses are designed in such a way that their density is less than 2.9 and preferably below 2.82 g/cm 3 . 
     U.S. Pat. No. 5,023,209 (Grateau et al) describes the fabrication of photochromic glasses containing from 2 to 8% TiO 2 , which demonstrate a brown colour upon darkening when exposed to actinic radiation. The darkened colour is due to the presence of up to 6 ppm Pd and/or Au, or oxides such as As 2  O 3 , Sb 2  O 3  and SnO 2  in quantities not exceeding 0.15% in weight. These components are useable as well in the present glasses. 
     The glasses of the invention exhibit the following photochromic properties, when their thickness is 2 mm: 
     a) a luminous transmittance in their clear state (or faded), designated as T 0 , greater than or equal to 85%, preferably greater than 87%; 
     b) a luminous transmittance in their darkened state after exposure for 15 minutes to actinic radiation at room temperature [T D15  (25° C.)] between 20 and 35%; 
     c) a difference in luminous transmittance in the darkened state after exposure for 15 minutes to actinic radiation over the temperature range from 25° to 40° C. [T D15  (40°-25° C.)] of less than 30 points; 
     d) a fading rate at ambient temperature (25° C.) such that, five minutes after its removal from actinic radiation, the glass presents a luminous transmittance (T F5 ) of at least 60%; and 
     e) a difference in the absolute value of the luminous transmittance (T) in the darkened state, before and after a thermal treatment of one hour at 275° C., of less than 5 points, preferably less than 4 points; this heat treatment simulating the application of an anti-reflective coating on the glass surface. 
     Because that the darkened colour of glasses described in this invention is naturally grey, one can add to the necessary raw materials small amounts of at least one of the components Pd, Au, As 2  O 3 , Sb 2  O 3  and SnO 2  to assure the development of brown tint, which is commercially more desirable in ophthalmic lenses. To prevent any unwanted coloration of the glass in its clear state, the amount of Pd and/or Au must not exceed 15 ppm (as weighed in the raw material batch). Similarly, the total concentration of As 2  O 3 , Sb 2  O 3  and SnO 2  must not exceed 0.3% (as analyzed in the glass). 
     The preferred compositions are described below, taking into account for the base compositions and the photochromic elements the same conditions as those mentioned for the broader composition range 
     
         ______________________________________                                    
SiO.sub.2                                                                 
         43-50    Li.sub.2 O                                              
                           1.5-3  CaO   0-3                               
B.sub.2 O.sub.3                                                           
         12.5-17  Na.sub.2 O                                              
                           0.3-2.5                                        
                                  SrO   1-7                               
Al.sub.2 O.sub.3                                                          
         0-2      K.sub.2 O                                               
                           3-8    BaO   1-7                               
ZrO.sub.2                                                                 
         8-12     Nb.sub.2 O.sub.5                                        
                           8-13                                           
Ag       0.110-0.140  Br        0.150-0.185                               
Cl       0.200-0.300  CuO       0.011-0.014                               
______________________________________                                    
 
    
     At a thickness of 2 mm the preferred glasses with the addition of at least one of the colorant additives aforementioned exhibit: 
     f) a colour in their clear state, as defined by chromatic coordinates x 0  and y 0 , so that x 0  &lt;0.3150 and y 0  &lt;0.3250; 
     g) a brown colour in the darkened state, as defined by chromatic coordinates x 2  and y 20  measured after 20 minutes of exposure to actinic radiation, so that x 20  &gt;0.3300 and y 20  &gt;0.3250. 
     PRIOR ART 
     The aforementioned U.S. Pat. No. 5,023,209 is representative of commercial photochromic glasses having refractive indices close to 1.6. This patent proposes base glass compositions containing large concentrations in TiO 2  ; the glass compositions of present invention demonstrating, therefore, a significant improvement over those compositions. The glasses of U.S. Pat. No. 5,023,209 had densities lower than 2.8 g/cm 3 , not noticeably less than those described in present invention. 
     U.S. Pat. No. 4,891,336 (Prassas) also proposes photochromic glasses with refractive indices close to 1.6 and densities lower than 2.8. The base glass compositions contain large amounts of TiO 2 . 
     U.S. Pat. No. 4,980,318 (Araujo) also describes photochromic glasses having refractive indices close to 1.6. The high index is obtained either with a combination of ZrO 2  and high amounts of K 2  O, optionally with Li 2  O, or with a combination of ZrO 2  with lower amounts of K 2  O and with an alkaline-earth oxide. TiO 2  is considered as a useful optional ingredient and appears in most examples at concentrations ranging from 3 to 10%. As a consequence, although an overlap exists between the ranges claimed by Araujo and those described in the present invention (except for Br content), none of the examples provided in Araujo&#39;s patent has a base composition included in the glass composition ranges covered by the present invention. 
     U.S. Pat. No. 5,104,831 (Behr et al) and U.S. Pat. No. 5,217,927, a continuation of U.S. Pat. No. 5,104,831, describe as well the production of photochromic glasses with refractive indices close to 1.6. However, here again, TiO 2  is a necessary component and is present in large quantities. 
     European Patent No. 63,790 (Schott) proposes a very broad range of photochromic glass compositions with refractive indices larger than 1.59. The ranges of glass components described in this patent overlap the base compositions of the present invention, but the approach used to obtain the high refractive index is totally different from the one described in present invention. For instance, PbO is present in substantial amounts in all examples but one. TiO 2  is mentioned as a useful component in concentrations up to 14%. Furthermore, this patent does not require the combination of photochromic elements in quantities such as those precisely limited and defined as critical in present invention. 
     U.S. Pat. No. 4,486,541 (Gliemeroth et al) uses a combination of PbO, TiO 2  and ZrO 2  to obtain high refractive index photochromic glasses and U.S. Pat. No. 4,686,196 (Gliemeroth et al) is based on a combination of TiO 2 , ZrO 2  and Li 2  O to reach the same objective. Therefore, no patent suggests the TiO 2  -free glasses described in the present invention. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Table I contains glass compositions, expressed as weight percents of oxides, which are good illustrations of the inventive products. Because the sum of individual components totals 100 or is close to this figure, the values indicated in this Table can be taken for all practical purposes as representative of weight percentages. In addition, since the identity of the cation(s) which are combined with the halide elements is unknown and inasmuch as the proportions of these elements are very small, Cl and Br are simply expressed as chlorine and bromine. Finally, because silver is also present in small quantities, it is also expressed, in the same way as Cl and Br, in its elemental form. The values indicated for Cl, Br and Ag are determined by analysis. The amount of CuO is expressed as a batch quantity, based on experience that retention of this oxide is on the order of 98 to 100%. 
     The actual ingredients of the batch can consist of any type of raw material, oxides or other components, which, when melted together, are converted into the desired oxides and in proper proportions. Cl and Br are generally incorporated into the batch as alkali metal halides. Silver or other components used for coloration purposes are usually added as oxides or appropriate salts. 
     The batch ingredients are mixed together to provide homogeneity, placed inside a platinum crucible, and Joule-heated at about 1250° C. When melting is completed, the temperature is raised to between 1350° and 1430° C. to obtain glass homogeneity and fining. The melt is then cooled and simultaneously formed into the desired shape, and finally transferred into an annealing furnace operating at about 450° C. 
     The above description is only relevant to melting and forming in the laboratory, but glasses of the invention can indeed be melted and formed at a larger industrial scale, using classical glass technology processes. 
     
                                           TABLE I                                 
__________________________________________________________________________
     1   2   3   4   5   6   7   8                                        
__________________________________________________________________________
SiO.sub.2                                                                 
     46.1                                                                 
         46.1                                                             
             46.1                                                         
                 46.1                                                     
                     46.1                                                 
                         46.1                                             
                             46.1                                         
                                 46.1                                     
B.sub.2 O.sub.3                                                           
     14.5                                                                 
         14.5                                                             
             14.5                                                         
                 14.5                                                     
                     14.5                                                 
                         14.5                                             
                             14.5                                         
                                 14.5                                     
ZrO.sub.2                                                                 
     11.5                                                                 
         11.5                                                             
             11.5                                                         
                 11.5                                                     
                     11.5                                                 
                         11.5                                             
                             11.5                                         
                                 11.5                                     
Li.sub.2 O                                                                
     1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9                                      
Na.sub.2 O                                                                
     1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1                                      
K.sub.2 O                                                                 
     5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2                                      
BaO  5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2                                      
SrO  1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8                                      
Nb.sub.2 O.sub.5                                                          
     12.4                                                                 
         12.4                                                             
             12.4                                                         
                 12.4                                                     
                     12.4                                                 
                         12.4                                             
                             12.4                                         
                                 12.4                                     
Ag   0.120                                                                
         0.128                                                            
             0.123                                                        
                 0.129                                                    
                     0.129                                                
                         0.127                                            
                             0.131                                        
                                 0.125                                    
Cl   0.262                                                                
         0.260                                                            
             0.253                                                        
                 0.222                                                    
                     0.244                                                
                         0.261                                            
                             0.259                                        
                                 0.263                                    
Br   0.155                                                                
         0.161                                                            
             0.162                                                        
                 0.165                                                    
                     0.159                                                
                         0.178                                            
                             0.173                                        
                                 0.163                                    
CuO  0.011                                                                
         0.011                                                            
             0.011                                                        
                 0.011                                                    
                     0.011                                                
                         0.011                                            
                             0.011                                        
                                 0.011                                    
Sb.sub.2 O.sub.3                                                          
     0.150                                                                
         0.175                                                            
             0.214                                                        
                 --  --  --  --  0.150                                    
Au   --  --  --  9   15  --  --  6                                        
(ppm)                                                                     
Pd   --  --  --  --  --  2   3   --                                       
(ppm)                                                                     
Ag + Cl                                                                   
     0.28                                                                 
         0.29                                                             
             0.28                                                         
                 0.29                                                     
                     0.29                                                 
                         0.30                                             
                             0.30                                         
                                 0.29                                     
Cl + Br                                                                   
     0.42                                                                 
         0.42                                                             
             0.41                                                         
                 0.39                                                     
                     0.40                                                 
                         0.44                                             
                             0.43                                         
                                 0.43                                     
__________________________________________________________________________
     9   10   11  12   13  14   15                                        
__________________________________________________________________________
SiO.sub.2                                                                 
     46.1                                                                 
         46.1 46.1                                                        
                  46.1 46.1                                               
                           46.1 46.5                                      
B.sub.2 O.sub.3                                                           
     14.5                                                                 
         14.5 14.5                                                        
                  14.5 14.5                                               
                           14.5 14.6                                      
ZrO.sub.2                                                                 
     11.5                                                                 
         11.5 11.5                                                        
                  11.5 11.5                                               
                           11.5 9.5                                       
Li.sub.2 O                                                                
     1.9 1.9  1.9 1.9  1.9 1.9  1.9                                       
Na.sub.2 O                                                                
     1.1 1.1  1.1 1.1  1.1 1.1  1.1                                       
K.sub.2 O                                                                 
     5.2 5.2  5.2 5.2  5.2 5.2  5.2                                       
BaO  5.2 5.2  5.2 5.2  5.2 5.2  5.2                                       
SrO  1.8 1.8  1.8 1.8  1.8 1.8  1.8                                       
TiO.sub.2                                                                 
     --  --   --  --   --  --   1.4                                       
Nb.sub.2 O.sub.5                                                          
     12.4                                                                 
         12.4 12.4                                                        
                  12.4 12.4                                               
                           12.4 12.5                                      
Ag   0.125                                                                
         0.130                                                            
              0.131                                                       
                  0.127                                                   
                       0.122                                              
                           0.123                                          
                                0.134                                     
Cl   0.258                                                                
         0.273                                                            
              0.260                                                       
                  0.250                                                   
                       0.265                                              
                           0.261                                          
                                0.257                                     
Br   0.158                                                                
         0.160                                                            
              0.165                                                       
                  0.166                                                   
                       0.170                                              
                           0.176                                          
                                0.159                                     
CuO  0.011                                                                
         0.011                                                            
              0.011                                                       
                  0.011                                                   
                       0.011                                              
                           0.011                                          
                                0.011                                     
Sb.sub.2 O.sub.3                                                          
     0.151                                                                
         0.151                                                            
              0.075                                                       
                  0.34 0.072                                              
                           0.186                                          
                                0.079                                     
Au   9   12   15  --   --  --   15                                        
(ppm)                                                                     
Pd   --  --   --  1    2   3                                              
(ppm)                                                                     
Ag + Cl                                                                   
     0.28                                                                 
         0.29 0.30                                                        
                  0.29 0.29                                               
                           0.30 0.39                                      
Cr + Br                                                                   
     0.42                                                                 
         0.43 0.42                                                        
                  0.42 0.43                                               
                           0.44 0.42                                      
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     Samples cut from the annealed glass articles were introduced into an electrically heated furnace and exposed for the durations in minutes and at the temperatures in °C. recorded in Table II, in order to develop photochromic properties. Generally, temperatures between 670° and 700° C. proved to be satisfactory to develop the desired properties. 
     Table II also shows measurements of photochromic behaviour, as well as colour of the glasses, their refractive index (I.R.), their Abbe number (Abbe) and their density (Dens.) in g/cm 3 , when these properties have been measured. 
     Colour of the glasses is defined according to the CIE trichromatic system (1931), using White C as light source; this colorimetric system and the light source are described by A. C. Hardy in Handbook of Colorimetry, Technology Press, M.I.T., Cambridge, Mass., USA (1936). 
     The colour of the glass in the darkened state (x 20 ,y 20 ) is determined after exposure of ground and polished samples during 20 minutes at 25° C. to a UV light source (BLB). The corresponding transmittance is designed by T D20 . The colour of the glass in the clear state (T 0 ) is determined in the absence of the light source. 
     Luminous transmittance most representative of the photochromic behaviour of the glass under the action of an actinic radiation similar to the solar spectrum was measured with a solar simulator, the principle of which was described in U.S. Pat. No. 4,190,451 (Hares et al). These results are listed in Table II under the title &#34;Solar simulator&#34;. 
     For the sake of comparison, Table II also contains an example of commercial photochromic glass (labelled PBX) having a refractive index of 1.523 and darkening into a brown colour when exposed to an actinic radiation, as well as an example of commercial photochromic glass with refractive index 1.6 (labelled PB16), which demonstrates a brown colour upon darkening in the presence of an actinic radiation. 
     In Table II: 
     T 0  denotes the luminous transmittance of a glass in its clear state (not darkened); 
     DT denotes the absolute difference in luminous transmittance in the darkened state before and after a thermal treatment simulating vacuum deposition of a coating (one hour at 280° C.); 
     T D15  (25° C.) denotes the luminous transmittance in the darkened state of a glass after an exposure for 15 minutes at 25° C. to actinic radiation representative of the solar spectrum; 
     T D15  (40° C.) denotes the luminous transmittance in the darkened state of a glass after an exposure for 15 minutes at 40° C. to actinic radiation representative of the solar spectrum; 
     T F5  (25° C.) denotes the luminous transmittance after fading of a glass five minutes after removal of the actinic radiation simulating solar radiation at 25° C.; and 
     DT D15  (40-25) denotes the difference in luminous transmittance in the darkened state of a glass over the temperature interval 25° to 40° C. 
     Measurements of refractive index and Abbe number were performed on annealed samples according to classical methods (yellow He ray was used for n D ). 
     Density was measured by immersion. 
    
    
     Examples 1 to 3 are representative of photochromic glass compositions containing Sb 2  O 3  as only colorant. 
     Examples 4 and 5 are representative of photochromic glass compositions containing Au as only colorant. 
     Examples 6 and 7 are representative of photochromic glass compositions containing Pd as only colorant. 
     Examples 8 to 11 are representative of photochromic glass compositions containing both Sb 2  O 3  and Pd as colorants. 
     Example 15 shows that TiO 2  can be tolerated in small amount, but should preferably be absent. 
     Examples 1 to 15 illustrate preferred embodiments of present invention, not only for their photochromic properties, but also because they can be formed as lens blanks by sagging for ophthalmic applications. 
     The behaviour of photochromic glasses is essentially unchanged after the sagging process. 
     
                       TABLE II                                                    
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Heat                                                                      
Treatment                                                                 
         1       2       3     4     5     6                              
______________________________________                                    
Temperature                                                               
         690     690     690   690   690   690                            
Time     15      15      15    15    15    15                             
T.sub.0 (BLB)                                                             
         88.6    88.2    88.0  88.6  89.2  88.9                           
x.sub.0  0.3133  0.3139  0.3138                                           
                               0.3126                                     
                                     0.3126                               
                                           0.3118                         
y.sub.0  0.3031  0.3230  0.3232                                           
                               0.3228                                     
                                     0.3226                               
                                           0.3226                         
T.sub.D20                                                                 
         41.5    38.3    40.2  37.1  38.5  39.3                           
x.sub.20 0.3389  0.3391  0.3377                                           
                               0.3419                                     
                                     0.3416                               
                                           0.3308                         
y.sub.20 0.3313  0.3306  0.3299                                           
                               0.3319                                     
                                     0.3329                               
                                           0.3271                         
DT       -0.9    +1.8    -1.2  +0.5  -2.0  -2.0                           
T.sub.0  87.1    87.7    87.6  88.0  87.1  87.8                           
T.sub.D15 (25° C.)                                                 
         23.1    25.3    28.3  24.8  24.9  27.5                           
T.sub.D15 (40° C.)                                                 
         45.1    --      --    --    --    --                             
T.sub.F5 (25° C.)                                                  
         62.0    62.9    64.7  61.5  60.1  61.7                           
ΔT.sub.D15 (40-25)                                                  
         --      --      --    --    --    --                             
R.I.     1.599   --      --    --    --    --                             
Abbe     45.3    --      --    --    --    --                             
Dens.    2.81    --      --    --    --    --                             
______________________________________                                    
Heat                                                                      
Treatment                                                                 
         7       8       9     10    11    12                             
______________________________________                                    
Temperature                                                               
         690     700     690   690   690   690                            
Time     15      15      15    15    15    15                             
T.sub.0 (BLB)                                                             
         88.9    88.8    88.6  88.5  87.0  88.7                           
x.sub.0  0.3125  0.3127  0.3134                                           
                               0.3134                                     
                                     0.3149                               
                                           0.3129                         
y.sub.0  0.3226  0.3230  0.3232                                           
                               0.3234                                     
                                     0.3245                               
                                           0.3227                         
T.sub.D20                                                                 
         37.2    41.8    39.1  37.8  35.9  38.7                           
x.sub.20 0.3319  0.3400  0.3473                                           
                               0.3493                                     
                                     0.3428                               
                                           0.3401                         
y.sub.20 0.3282  0.3317  0.3353                                           
                               0.3362                                     
                                     0.3333                               
                                           0.3318                         
DT       -2.2    +0.2    +1.3  -2.4  -1.9  +0.3                           
T.sub.0  89.3    87.4    87.3  86.7  87.4  88.2                           
T.sub.D15 (25° C.)                                                 
         25.9    26.8    25.8  24.7  26.2  26.0                           
T.sub.D15 (40° C.)                                                 
         --      --      --    --    --    --                             
T.sub.F5 (25° C.)                                                  
         60.1    64.3    63.6  61.8  62.3  63.5                           
ΔT.sub.D15 40-25)                                                   
         --      --      --    --    --    --                             
R.I.     --      --      --    --    --    --                             
Abbe     --      --      --    --    --    --                             
Dens.    --      --      --    --    --    --                             
______________________________________                                    
Heat                                                                      
Treatment                                                                 
         13       14       15    PBX    PB16                              
______________________________________                                    
Temperature                                                               
         690      690      690   690    690                               
Time     15       15       15    --     --                                
T.sub.0 (BLB)                                                             
         88.0     87.2     87.4  88.4   86.6                              
x.sub.0  0.3125   0.3143   0.3145                                         
                                 0.3155 0.3164                            
y.sub.0  0.3237   0.3239   0.3246                                         
                                 0.3230 --/3258                           
T.sub.D20                                                                 
         41.5     42.7     41.0  37.1   48.2                              
x.sub.20 0.3380   0.3344   0.3380                                         
                                 0.3443 0.3302                            
y.sub.20 0.3309   0.3290   0.3297                                         
                                 0.3383 0.3278                            
DT       +1.2     -0.8     --    --     --                                
T.sub.0  87.9     86.9     87.8  87.9   85.7                              
T.sub.D15 (25° C.)                                                 
         27.2     31.5     27.5  28.1   33.0                              
T.sub.D15 (40° C.)                                                 
         --       --       --    47.8   50.1                              
T.sub.F5 (40-25)                                                          
         63.9     67.3     62.7  67.0   66.7                              
ΔT.sub.15 (40-25)                                                   
         --       --       --    38.9   33.7                              
R.I.     --       --       --    1.523  1.600                             
Abbe     --       --       --    56.4   42.2                              
Dens.    --       --       --    2.41   2.70                              
______________________________________                                    
 
    
     As one can see from Table II, glasses from the present invention have photochromic properties at least equivalent to, and in certain cases better than those of a commercial photochromic glass with refractive index 1.523. When compared to the commercial 1.6 photochromic glasses, not only do glasses of the present invention have a higher transmittance in the clear state, but they also have better darkening and fading performances. 
     The most preferred embodiment currently is Example 12.