fig2 is a diagram of a uv detection according to the present invention . the body of the uv solar blind sensor 11 contains an optical array 14 which directs uv light 13 from a source 12 onto crystal filter 15 . the optical array 14 can include any collection of optical elements ( e . g ., lenses ) and filters which in most embodiments can absorb in the visible range ( i . e ., above 350 nm ). the ultraviolet light then strikes the fmt crystal filter 15 and only the desired wavelengths are passed onto a light detector 16 . light detector 16 then passes a signal to an alarm or display device 17 . light detection 16 can include any detection device for detecting radiation in the desired wavelength range , e . g . about 200 - 350 nm . detection devices for detector 16 are well known in the art and include , for example , photo - multiplier tubes , charge coupled devices ( ccds ), and solid state detector devices such as gallium nitride detectors . light detector 16 also includes detection electronics associated with the detection device ( e . g ., electronics for signal processing , data displays and data acquisition ). alarm or display device 17 can be any device which , upon detection of a threshold level ( which can be set as low as desired ), alerts a user of the presence of uv radiation . in the case of a fire detector , for example , alarm or display device 17 may alert a conventional alarm system to evacuate a building and alert a fire department and / or fire suppression system . in most embodiments , crystal filter 15 is an alkaline earth fluoride which is codoped with a lanthanide or actinide fluoride and a lanthanide or actinide nitride , boride , carbide , oxide or hydroxide . the fluoride host crystal can be an alkaline earth fluoride , zinc fluoride ( znf 2 ) or cadmium fluoride ( cdf 2 ). the alkaline earth fluorides include magnesium fluoride ( mgf 2 ), calcium fluoride ( caf 2 ), strontium fluoride ( srf 2 ), and barium fluoride ( baf 2 ). these single - crystal host materials transmit in the 200 - 350 nm region , each having a uv absorption edge of around 150 nm . the lanthanide and actinide elements include ce , pr , nd , sm , eu , gd , tb , dy , ho , er , tm , yb , lu , and u . the combination of lanthanide or actinide fluoride dopant , and lanthanide or actinide nitride , boride , oxide , carbide or hydroxide doping , provides optical absorption bands which limits the transmissions of the crystal filters to the useful range of wavelengths and in wavelengths above the absorption band , away from the useful range of wavelength , i . e . creates a pass - band having the desired characteristics . fig3 , for example , shows an optical absorption spectrum of one embodiment of the invention , fmt - 1 ( caf 2 codoped with cef 3 and cen ). the crystal filter transmits in the pass - band region of around 220 nm to about 275 nm and above about 350 nm . fig4 through 6 show a similar optical absorption spectrum for other examples of embodiment crystal filters : fmt - 2 ( caf 2 codoped with cef 3 , cen , euf 3 and eun ), fmt - 3 ( caf 2 codoped with cef 3 and cec 2 ) and fmt - 4 ( caf 2 codoped with cef 3 and ce ( oh ) 2 ), respectively . the optical absorption spectra shown in fig3 through 6 were taken using a perkin elmer lamda 19 uv - vis - nir spectrophotometer . fig3 through 6 each show two spectra , one taken before and one after a 2 hours 450 ° c . anneal in air , however the two spectra overlap on the scale shown in the figures and are indistinguishable . fig3 through 6 , therefore , also show that embodiments of the invention are stable at temperatures of at least 450 ° c . filter crystal 15 ( fig2 ) can be formed from a number of growth techniques , including czochralski and bridgeman methods of crystal growth . in general , crystals are grown from a melt , i . e ., liquid solution , of the constituent elements of a mixture . the crystal formed from the melt of the mixture will include the constituent elements of the mixture in proportion related to the proportion of those elements in the mixture . several examples of mixtures for growing filter crystals according to the present invention are discussed below . in one embodiment of the invention , denoted fmt - 1 , the mixture from which a filter crystal is grown is primarily comprised of caf 2 , which forms the host crystal . the mixture further includes codopants including about 0 . 01 to about 5 . 0 mole percent of cerium fluoride ( cef 3 ) and about 0 . 001 to about 1 . 0 mole percent of cen . other constituents may also be added to the mixture . for example , about 0 . 01 to about 5 . 0 mole percent of manganese fluoride ( mnf 2 ), about 0 . 01 to about 5 . 0 mole percent of cobalt fluoride ( cof 2 ), and / or about 0 . 01 to about 5 . 0 mole percent of lead fluoride ( pbf 2 ) may be added in order to remove unwanted oxides . in addition , about 0 . 01 to about 20 . 0 mole percent of sodium fluoride ( naf ) may be added in order to maximize distribution of active components through the resulting crystal . in one particular embodiment , an fmt - 1 filter crystal is grown from a calcium fluoride ( caf 2 ) mixture having about 0 . 3 mole percent of cerium fluoride ( cef 3 ), about 0 . 05 mole percent of cerium nitride ( cen ), about 0 . 3 mole percent of manganese fluoride ( mnf 2 ) and about 2 . 0 mole percent of sodium fluoride ( naf ). in one particular example of a mixture for forming an fmt - 1 filter , about 12 . 8074 moles of calcium fluoride ( caf 2 ) powder or broken crystals are mixed and the mixture is transferred to an argon dry box . then , about 0 . 3 mole percent of cerium fluoride ( cef 3 ), about 0 . 05 mole percent of cerium nitride ( cen ), about 0 . 3 mole percent of manganese fluoride ( mnf 2 ) and about 2 . 0 mole percent of sodium fluoride ( naf ) is added to the mixture . the mixture is blended to ensure homogeneity . an fmt - 1 crystal filter is then grown from a melt of this mixture . in a second embodiment of the invention , fmt - 2 , the mixture includes primarily calcium fluoride ( caf 2 ), which is the host material . the mixture also includes codopants of about 0 . 01 to about 5 . 0 mole percent of cerium fluoride ( cef 3 ), about 0 . 001 to about 1 . 0 mole percent of cerium nitride ( cen ); about 0 . 01 to about 5 . 0 mole percent of europium fluoride ( euf 3 ), and about 0 . 001 to about 1 . 0 mole percent of europium nitride ( eun ). as in fmt - 1 , the mixture can include inactive components . the inactive components may be included about 0 . 01 to about 5 . 0 mole percent of manganese fluoride ( mnf 2 ), cobalt fluoride ( cof 2 ), or lead fluoride ( pbf 2 ) and about 0 . 01 to about 20 . 0 mole percent of sodium fluoride ( naf ). the inactive components serve to remove unwanted oxides from the melt and to maximize distribution of the active dopants . in one particular embodiment of a mixture to form the fmt - 2 crystal filter , the crystal filter is grown from a mixture having primarily , about 12 . 8075 moles , calcium fluoride ( caf 2 ) with about 0 . 2 mole percent of cerium fluoride ( cef 3 ), about 0 . 05 mole percent of cerium nitride ( cen ), about 0 . 1 mole percent of europium fluoride ( euf 3 ), about 0 . 05 mole percent of europium nitride ( eun ), about 0 . 3 mole percent of lead fluoride ( pbf 2 ), and about 2 . 0 mole percent of sodium fluoride ( naf ). in a third embodiment of a crystal according to the invention , fmt - 3 , a host material of about 12 . 8074 moles of calcium fluoride ( caf 2 ) powder or broken crystals is mixed with codopants of about 0 . 25 mole percent cerium fluoride ( cef 3 ) powder and about 0 . 01 mole percent of cerium carbide ( cec 2 ) powder . in order to remove any unwanted oxides from the starting materials , one or more of about 0 . 01 to about 5 . 0 mole percent manganese fluoride ( mnf 2 ), about 0 . 01 to 5 . 0 mole percent cobalt fluoride ( cof 2 ), and about 0 . 01 to about 5 . 0 mole percent of lead fluoride ( pbf 2 ) is added to the mixture . about 1 . 6 mole percent of sodium fluoride ( naf ) can also be added to the mixture . in general , the caf 2 and cef 3 are mixed and transferred to an argon dry box where the remainder of the components are added and blended to ensure homogeneity . in a fourth embodiment of a filter crystal according to the present invention , fmt - 4 , a host material of about 12 . 8074 moles of calcium fluoride ( caf 2 ) powder or broken crystals is mixed with codopants of about 0 . 30 mole percent cerium fluoride ( cef 3 ). the mixture is transferred into an argon dry box and about 0 . 6 mole percent sodium hydroxide ( naoh ) is added in order to form the codopant cerium hydroxide ( ce ( oh ) 2 ) during crystal growth . in order to remove any unwanted oxides from the starting materials , about 0 . 2 mole percent of lead fluoride ( pbf 2 ) is added to the mixture . additionally , about 1 . 6 mole percent of sodium fluoride ( naf ) is added to the mixture to maximize the distribution coefficients of the constituents . one skilled in the art will recognize that other mixtures for forming filter crystals according to the present invention can be formed . example mixtures 1 through 4 are presented for demonstrative purposes only and are not to be considered limiting . mixtures for forming filter crystals according to the present invention can , for example , include other host materials , including any of the alkali fluorides or znf 2 or cdf 2 and any number of codopants . embodiments of the invention include any host material having dopants such that a pass band in a desired spectral region is created . crystal filters according to the present invention can be grown in many ways . as examples , several methods of growing these crystals are discussed below . the crystals can be grown using any number of known methods , including the czochralski method and the bridgeman method . in the czochralski method , the growth mixture is melted in a crucible . crucibles can be formed from many materials , including platinum , carbon , gold and ceramic . the crucible itself can serve as the heating element to melt the mixture to form a melt or the crucible can be placed in a furnace . the mixture can be any mixture of components for producing a crystal filter according to the present invention , examples of which are discussed above as example mixtures 1 through 4 . a seed crystal mounted on the end of a rod is inserted into the melt . the seed crystal , around which crystal growth is initiated , is slowly rotated and slowly lifted out of the melt . the crystal forms around the seed crystal . in the bridgeman crystal growth method , the mixture is placed in an insulating crucible , most often formed from a ceramic material . the crucible is then placed into an furnace and heated to form a melt . a temperature gradient is formed across the crucible ( usually along the longest dimension of the crucible ) either by increasing the temperature of the furnace at one end of the crucible relative to the opposite end or by lowering the crucible out of a hot side of the furnace to a cooler side of the furnace . a crystal is formed as the melt cools . even though the various common methods of crystal growth are generally well known , the particular parameters of the growth method are unique to growth of each type of crystal . several examples of particular methods of growing crystal filters according to the present invention are described below . a mixture is loaded into a crucible for use in a bridgeman growth system . the mixture can be any mixture of components from growing a filter crystal according to the present invention , including the mixtures described in mixture examples 1 through 4 above . in one example , a carbon crucible of about 90 mm by 150 mm , is shaped at one end with a 45 ° bridgeman taper . the crucible containing the host material is often loaded into an argon dry box where the codopant material and other components are added . the filled crucible is then sealed from atmospheric contamination with a lid ( often of carbon ) and removed from the argon dry box . crucible and mixture can then be placed into a two - zone vacuum furnace capable of obtaining about 1500 ° c . temperatures . the vacuum furnace is sequentially evacuated and back filled a number of times with an inert gas in order to remove any water or oxygen . flushing the furnace about three times with argon gas is typically sufficient . the furnace is heated in a controlled fashion above the melting temperature of the mixture in order to create a melt . for the mixtures described in mixture examples 1 through 4 , the following temperature profile can be implemented : after the temperature has reached the melt temperature , e . g . about 1450 ° c ., the crucible is slowly lowered out of the heated zone . in most embodiments , the lowering rate is between about 0 . 5 and about 2 . 0 mm / hr . in one particular embodiment , the rate is about 1 . 0 mm per hour . after about 150 mm of lowering , when the crucible is completely lowered out of the heated zone , the crucible has been “ accomplished ,” i . e . the melt has been solidified , and the furnace is slow cooled to room temperature . in most embodiments , the furnace is cooled in about 24 hours . the crystal can then be removed from the furnace and fabricated into optical filter elements . fabrication of crystal materials ( e . g ., cutting and polishing ) into crystal elements is well known to one skilled in the art . other mixtures for forming filter crystals according to the present invention may utilize different temperature profiles . in a second example , a mixture , which can be one of example mixtures 1 - 4 described above , is loaded into a crucible , as was described in crystal growth example 1 . the crucible is then placed into a two - zone vacuum furnace , which is then repeatedly backflushed and evacuated with an inert gas ( e . g ., about 3 times with argon ). the furnace is then heated to a temperature above the melting temperature of the mixture for a sufficiently long time to insure uniformity of the melt . in one embodiment , the temperature is raised to about 1450 ° c . and held for about 2 hours . a temperature gradient is then created along the vertical axis of the crucible ( i . e ., along the long dimension of the crucible ). in most embodiments , the temperature gradient is between about 1 - 20 ° c . per centimeter . in one embodiment , it is about 20 ° c . per centimeter . the furnace is then slowly cooled to grow the crystal . in most embodiments , the cooling rate is between about 5 - 20 ° c . per hour . in one embodiment , it is about 10 ° c . per hour . when the temperature reaches a temperature below which the crystal is formed , the furnace is slowly cooled to room temperature . in one embodiment , when the furnace temperature reaches about 1250 ° c . the furnace is then cooled to room temperature at a rate of about 50 ° c . per hour . the crystal is then removed from the furnace at room temperature and fabricated into optical filters . a third example of crystal growth is applicable to growth of a filter crystal where one of the codopants is a nitride , oxide or hydroxide compound . a mixture , which can be one of example mixtures 1 through 4 described above without nitride , oxide or hydroxide compounds , is loaded into a crucible as described in crystal growth example 1 . the crucible is then placed into a two - zone vacuum furnace , which is repeatedly backflushed and evacuated with an inert gas ( e . g ., about 3 times with argon ). a tubing , for example an about 5 mm diameter carbon tube , is inserted into the top of the vacuum furnace and extends down into the crucible , ending above the surface of the melt . crystals are grown with similar growth technique as was described in examples 1 or 2 , but a gas mixture ( such as nitrogen , oxygen , or water vapor ( for forming hydroxide compounds ) and hydrogen gas ) is forced down the tube during growth . in most embodiments , about 5 to about 100 cubic centimeters per minute ( cc / min ) of nitrogen , oxygen or water vapor and about 2 to about 100 cc / min of hydrogen is forced down the tube . in one embodiment , about 100 cc / min of nitrogen , oxygen or water vapor and 20 cc / min of hydrogen is used . the introduction of the gas mixture above the liquid melt causes a small percentage of the lanthanide or actinide fluoride dopant to form the necessary nitride , oxide or hydroxide compound needed for the codoping . after growth , the crystal can be removed from the furnace and fabricated into optical filters . another example a mixture , which can be one of example mixtures 1 - 4 described above , is placed in a flat bottom crucible and then into a vacuum furnace . in most embodiments , a 90 mm diameter flat bottom carbon crucible is utilized . the furnace is repeatedly evacuated and back filled with an inert gas ( e . g ., about three times with argon gas ) prior to the heating cycle . the furnace temperature is raised to a temperature above the melting temperature of the mixture ( e . g ., about 1450 ° c .) and homogenized for a period of time ( e . g ., about two hours ). czochralski growth is then performed , starting with the introduction of a seed crystal at the top of the melt . the seed crystal can be rotated , in most embodiments at about 2 - 20 rpm , and the temperature lowered until the onset of nucleation on the seed crystal . in one embodiment , the seed crystal is rotated at about 25 rpm and the onset of nucleation occurs at about 1415 ° c . after nucleation on the seed , the crystal can be grown by pulling the seed out of the top of the melt . the pull rate is typically about 0 . 1 - 5 . 0 mm per hour . in one example , the pull rate is about 4 mm per hour . when a sufficient length of the crystal is achieved ( e . g ., about 10 cm ), the furnace can be cooled to room temperature . cooling usually occurs over about 24 hours . the crystal can be removed from the furnace and fabricated into optical filter elements . in another example of crystal growth is applicable to growth of a filter crystal where one of the codopants is a nitride , oxide or hydroxide compound . a mixture , which can be one of example mixtures 1 through 4 described above without nitride , oxide or hydroxide compounds , is loaded into a crucible as described in crystal growth example 4 . a czochralski growth technique as in example 4 is then performed , with the addition of one of a nitrogen gas atmosphere , on oxygen atmosphere , or a water vapor ( hydroxide ) atmosphere in the furnace . the introduction of the appropriate atmosphere above the liquid melt causes a small percentage of the codopant fluoride compound to form the nitride , oxide or hydroxide compound necessary for the codoping . the resulting crystal filter can be removed from the furnace and fabricated into optical filter elements . in another example , a crystal filter of the type described above can be made by growing a crystalline thin film on an undoped substrate . a suitable substrate , such as for example calcium fluoride , is placed in a high vacuum deposition chamber . the substrate is heated to a sufficiently high temperature , e . g . 400 - 600 ° c . prior to film growth . a crystalline thin film can be deposited on the substrate by common thermal evaporation or sputtering techniques utilizing a mixture such as example mixtures 1 - 4 described above . the crystalline thin film will include the constituents of the mixture . the substrate and grown crystalline thin film , which can be of thickness from about 0 . 01 - 10 , 000 microns , can be cooled to room temperature , removed from the high vacuum deposition chamber , and used as a crystal filter . a crystal filter having a nitride , oxide or hydroxide compound codopant can also be produced by in - diffusion of the corresponding one of nitrogen gas ( nitration ), oxide gas , or hydroxide into a crystal grown with the nitride , oxide or hydroxide compound in the mixture . a crystalline filter material is made , for example , by any of growth examples 1 - 6 above where the nitride , oxide , or hydroxide compound has been excluded from the mixture and not added to the melt during growth . a filter blank can be made of the crystalline filter material by cutting and polishing the crystal . the crystalline material blank is then placed into a vacuum furnace . the vacuum furnace is repeatedly evacuated and back filled with an inert gas , e . g . argon gas , a number of times , e . g . about three times , in order to remove the ambient atmosphere . the furnace temperature is raised to a temperature just below melting , e . g . about 800 - 1395 ° c ., and nitrogen gas , oxygen gas or water vapor is introduced into the furnace . the introduction of the gas above the crystalline blank reacts with a small percentage of the fluoride codopant to form the necessary nitride , oxide or hydroxide compound codoping . after a sufficient time of in - diffusion of the gas , e . g ., about 2 hours , the crystalline blank can be cooled to room temperature and removed from the furnace . the resulting crystal can then be fabricated into an optical filter element . the crystal filter fmt - 1 ( caf 2 host with cef 3 and cen codoping ) prepared by example 1 has absorption spectra , before and after a two hour thermal anneal at 450 ° c ., is shown in fig3 . to demonstrate the temperature stability of the fmt - 1 , the filter was heated in a box furnace to about 450 ° c . at a rate of about 25 ° c . per hour from room temperature , annealed at about 450 ° c . for about 2 hrs . and then the furnace was shut off and cooled to room temperature . as can be seen from fig3 , the two spectra are indistinguishable on the scale of fig3 . similarly , as can be seen from fig4 through 6 , the crystal filters fmt - 2 ( caf 2 host with cef 3 , cen , euf 3 and eun codopants ), fmt - 3 ( caf 2 host with cef 3 and cec 2 codopants ), and fmt - 4 ( caf 2 host with cef 3 and ce ( oh ) 2 codopants ) also exhibit little variation when treated similarly to the fmt - 1 crystal shown in fig3 . fig3 through 6 also show that the corresponding crystal filter embodiments of fmt - 1 through fmt - 4 have a pass band in the wavelength range appropriate for solar blind uv filters . fig3 shows an embodiment of fmt - 1 crystal filter having a pass band of between about 220 nm to about 280 nm . fig4 shows an embodiment of fmt - 2 crystal filter having a pass band of between about 250 nm and 280 nm . fig5 shows an embodiment of fmt - 3 crystal filter having a pass band of between about 250 nm and about 300 nm . fig6 shows an embodiment of fmt - 4 crystal filter having a pass band of between about 220 nm and about 280 nm . other embodiments of filter crystals according to the present invention can have optical absorption spectra from various codopants tailored to create pass bands of various widths . the above examples of embodiments of the invention are demonstrative only . one skilled in the art will recognize several variations from the examples specifically discussed . these variations are also intended to be included in this disclosure . as such , the invention is limited only by the following claims .