a number of studies have been made with respect to the transmission characteristics of thallium doped potassium iodide ( ki : tl ) phosphors . articles in the field include &# 34 ; emission spectra of kcl : tl , kbr : tl , ki : tl at 300 , 80 and 12 ° kelvin &# 34 ;, edgerton , teegarden , phys . rev ., vol . 129 , no . 1 , p . 169 ( 1963 ) and &# 34 ; color centers in ki containing impurity electron acceptors : ki : tl &# 34 ;, hersh , j . chem . phys . vol . 30 , no . 3 , p . 790 ( 1959 ). the first mentioned article indicates that at room temperature ( i . e ., 300 ° k . ), the phosphors would interfere with a detector if used alone . they would produce their own signal and interfere with the one which is intended to be transmitted . tests performed upon a 3 mm thick sample of ki : tl resulted in a transmission curve shown in fig2 . the graph shows both a high transmission in the region of interest and a sharp cut - off edge between 2760 a and 2790 a . this edge results in an ideal long wavelength limit for definition of a solar blind region . the slope of the edge is so sharp that by 2820 a , the transmission is reduced by approximately nine orders of magnitude . the sample utilized was about 10 - 1 mole percent thallium . the invention is directed to an ultraviolet optical filter comprising a thallium doped potassium iodide element and a suspension of cation x . an example of such a filter is shown in fig1 in combination with a uv detector . the end of the filter assembly which is exposed to light includes an ultraviolet glass section a having a thickness of about 2 mm . it serves to filter wavelengths which are not in the ultraviolet range . ug - 5 glass , such as manufactured by schott , has been found to be acceptable in practice . the first element b within the filter assembly is a single crystal of thallium doped potassium iodide . the element is about 3 mm in thickness and is positioned between the ultraviolet glass described above and a second ultraviolet glass a as shown in the drawing . dow corning 200 fluid c is used as the coupling fluid of the ki : tl crystal to the continuous filter element surfaces . this is used rather than glycerol as the latter would tend to absorb water , thereby destroying the crystalline structure . the fluid c keeps surface reflection losses to a minimum and gives the hygroscopic crystal a protective coat against atmospheric moisture . dow corning 200 fluid is a silicon fluid having the advantageous properties of being highly transparent in the ultraviolet range and resists breaking down under operating conditions . a glass cell is defined by the ultraviolet glasses a to the right of the ki : tl crystal shown in fig1 . cation x in solution is contained in this cell . the cation x is designated in the drawing as element f . the second and third ultraviolet glasses a are both about 2 mm in thickness . a single crystal of niso 4 : 6h 2 o is coupled to the third ultraviolet glass by means of dow corning 200 fluid c . this fluid layer is less than about 0 . 025 mm in thickness as are the layers c discussed above . the kinematic viscosity of the fluid is about 300 , 000 centistokes . the niso 4 : 6h 2 o crystal is about 10 mm in thickness . the use of niso 4 : 6h 2 o crystals in filters is generally known , and the growth of such filters is well documented . see , for example , holder , a . and singer , p ., crystals and crystal growing , anchor books , doubleday and company , inc ., garden city , new york . 1960 , pages 112 - 114 . a 1 mm thick element e of ultraviolet grade quartz is provided to the right of the niso 4 : 6h 2 o crystal shown in fig1 . the element e utilized may be that sold under the tradename &# 34 ; suprasil &# 34 ; which is available from englehard industries of hillside , new jersey . a layer of dow corning 200 fluid couples the ultraviolet grade quartz to a uv detector . the ki : tl crystal is between 10 - 1 and 10 - 2 mole percent thallium ( known in the art as type ii ). when used in combination with the cation x in the arrangement described above , the phosphor will not interfere with the signal to be transmitted . the narrow absorption band is so situated in the ultraviolet spectrum relative to the cation x filter element to eliminate the residual spectral contribution of the sun . this function is performed even under the lowest occurring ozone conditions . fig2 illustrates the efficacy of the ki : tl crystal in the case of a 3 mm thick sample described above . it will be appreciated by those skilled in the art that modifications can be made in the structure described above without materially departing from the spirit of the invention . element d may be made from materials other than niso 4 : 6h 2 o and the coupling fluid may be varied . the scope of the invention should accordingly be determined by reference to the appended claims .