for a better understanding of the present invention , together with other and further objects , advantages and capabilities thereof , reference is made to the following disclosure and appended claims taken in conjunction with the above - described drawings . the long - afterglow electroluminescent lamps of the present invention are based on a thick - film el lamp structure . in a preferred embodiment , the electroluminescent phosphor layer contains a blend of electroluminescent phosphor particles and particles of a long - afterglow phosphor . a lamp according to this embodiment is illustrated in cross section in fig2 . a long - afterglow phosphor 25 is mixed with the el phosphor 8 and then formed into the layer 30 in the el lamp 21 . the el phosphor 8 is excited by the electrical energy from an external power supply ( not shown ). at the same time , the long - afterglow phosphor 25 is excited by a portion of the light emitted by the el phosphor 8 . the light from both the el phosphor and the long - afterglow phosphor are transmitted through the transparent electrode 6 and the plastic film 12 a that constitute the light - emitting side of the lamp . in an alternate embodiment illustrated in fig3 , it is possible to apply a long - afterglow phosphor as a layer ( or multiple layers ) to the surface of an electroluminescent lamp . here , the el lamp 40 is of a conventional thick - film construction except that the long - afterglow phosphor 25 is coated in a layer 43 on the light - emitting side of the lamp . the afterglow phosphor can be applied in a solid or patterned layer using the same equipment that is used to fabricate the el lamp . in another alternate embodiment illustrated in fig4 , the long - afterglow phosphor 25 is applied as a layer ( or multiple layers ) 53 to a separate transparent film 52 to form an overlay 60 . the long - afterglow phosphor can be applied in a solid or patterned layer using the same equipment that is used to fabricate the el lamp . after fabrication , the long - afterglow overlay 60 is affixed to the light - emitting side of the el lamp 50 , preferably with the transparent film 52 facing outward in order to protect the long - afterglow layer 53 . the advantage of a separate overlay is that the long - afterglow feature can be added retroactively to electroluminescent lamps that did not have this feature originally . the preferred method for applying the layers to the electroluminescent lamp and for applying the layers of long - afterglow phosphor to transparent films is screen printing , also referred to as “ silk - screening .” however , other coating techniques such as draw blade coating and roll - to - roll coating may also be used . the present invention will be described in further detail with reference to the following examples . however , it should be understood that the present invention is not restricted to such specific examples . in the examples given below , the electroluminescent lamps are constructed in the following general manner . electroluminescent phosphors are mixed with a binder ( dupont microcircuit materials luxprint ® 8155 electroluminescent medium ). the electroluminescent phosphors were blue , blue - green , green and white - emitting zns - based el phosphors . in particular , osram sylvania glacierglo ® types gg25 ( blue - green ), gg45 ( green ), gg64 ( blue ), and gg73 ( white ) encapsulated phosphors were used . the percentage of phosphor in the liquid binder is 60 weight percent ( wt . %). the phosphor suspension is screen - printed onto a 0 . 0065 - 0 . 0075 in .- thick pet film having a transparent , conductive layer of indium - tin oxide ( e . g ., oc - 200 from cp films ). the polyester screen has 137 or 140 threads per inch . after drying , a barium titanate - filled dielectric layer ( dupont microcircuit materials luxprint ® 8153 electroluminescent dielectric insulator ) is applied over the phosphor layer in the same way . after drying , a second dielectric layer is applied in the same way and dried . finally a rear carbon electrode ( dupont microcircuit materials luxprint ® 7144 carbon conductor ) is applied over the dielectric layer and dried . the long - afterglow phosphors used in the examples were nemoto & amp ; co . luminova ® types g - 300 ( green - emitting sral 2 o 4 : eu , dy ) and bg - 300 ( blue - emitting sr 4 al 14 o 25 : eu , dy ) phosphors . an electroluminescent lamp was constructed as described previously with the following exception . the phosphor used in this lamp was a mixture of 87 wt . % type gg25 electroluminescent phosphor and 13 wt . % type g - 300m long - afterglow phosphor . this mixture was achieved by dry blending the two powders . a percentage of 60 wt . % mixed phosphor was then combined with the binder to make the phosphor suspension . an electroluminescent lamp was constructed as described in example 1 except that the phosphor mixture used in this lamp was 77 wt . % type gg25 electroluminescent phosphor and 23 wt . % type g - 300m long - afterglow phosphor . an electroluminescent lamp was constructed as described in example 1 except that the phosphor used in this lamp was a mixture of 87 wt . % type gg45 electroluminescent phosphor and 13 wt . % type g - 300m long - afterglow phosphor . a percentage of 60 wt . % mixed phosphor was then combined with the binder to make the phosphor suspension . an electroluminescent lamp was constructed as described in example 1 except that the mixture used in this lamp was 77 wt . % type gg45 electroluminescent phosphor and 23 wt . % type g - 300m long - afterglow phosphor . an electroluminescent lamp was constructed as described in example 1 except that the phosphor used in this lamp was a mixture of 87 wt . % type gg64 electroluminescent phosphor and 13 wt . % type bg - 300m long - afterglow phosphor . an electroluminescent lamp was constructed as described in example 1 except that the mixture used in this lamp was 77 wt . % type gg64 electroluminescent phosphor and 23 wt . % type bg - 300m long - afterglow phosphor . an electroluminescent lamp was constructed as described in example 1 except that the phosphor used in this lamp was a mixture of 87 wt . % type gg73 electroluminescent phosphor and 13 wt . % type bg - 300m long - afterglow phosphor . an electroluminescent lamp was constructed as described in example 1 except that the mixture used in this lamp was 77 wt . % type gg73 electroluminescent phosphor and 23 wt . % type bg - 300m long - afterglow phosphor . the lamps from examples 1 - 8 and comparative control lamps without long - afterglow phosphor were each connected to a power supply operating at 125 v and 800 hz . the lamps were operated in a dark room at temperatures between 72 - 78 ° f . for 15 minutes , and then the power was removed . the light emitted by the lamps after the power was removed was read with a photometer . table 1 gives the brightness ( bright .) of each lamp in millicandela per square meter ( mcd / m 2 ) at increasing time intervals measured from the time the power was turned off . an electroluminescent lamp was constructed with type gg25 el phosphor . ( control lamp a ). the percentage of el phosphor in the liquid binder was 60 wt . %. separately , type g - 300m long - afterglow phosphor was combined with the binder ( dupont luxprint ® 8155 ) to make a suspension . the percentage of the long - afterglow phosphor in the liquid binder was 60 wt . %. the suspension of the long - afterglow phosphor was coated on another piece of pet film . after drying , the phosphor coverage on the overlay was 0 . 0168 g / cm 2 . the overlay with the long - afterglow phosphor was affixed with tape to the light - emitting side of the electroluminescent lamp . the electroluminescent lamp in this example was the same one as in example 9 ( without the overlay ). a new overlay was created in the same manner except that after drying , a second layer of the long - afterglow suspension was coated over the first layer . after drying , a third layer of long - afterglow suspension was coated over the previous two layers in the same way . the total phosphor coverage on the overlay was 0 . 0480 g / cm 2 . the overlay with the afterglow phosphor was then affixed to the light - emitting side of the electroluminescent lamp . an electroluminescent lamp was constructed with type gg73 el phosphor . ( control lamp d ). an overlay comprised of type bg - 300m long - afterglow phosphor on a pet film ( 0 . 0168 g / cm 2 ) was affixed to the light - emitting side of the electroluminescent lamp . the electroluminescent lamp in this example was the same one as in example 11 ( without the overlay ). three layers of the long - afterglow phosphor were applied to make a new overlay yielding a total phosphor coverage of 0 . 0494 g / cm 2 . this overlay with the afterglow phosphor was then affixed to the light - emitting side of the electroluminescent lamp . the lamps from examples 9 , 10 , 11 and 12 were each connected to a power supply operating at 125 v and 800 hz . the lamps were operated in a dark room at temperatures between 72 - 78 ° f . for 15 minutes , and then the power was removed . the brightness of examples 9 - 12 were read with a photometer . the brightness in millicandela per square meter ( mcd / m 2 ) corresponding to time in minutes after the power was removed are shown in table 2 . an electroluminescent lamp was constructed with type gg45 el phosphor . ( control lamp b ). the overlay film from example 9 was affixed to the light - emitting side of the electroluminescent lamp . the electroluminescent lamp in this example was the same one as in example 13 ( without the overlay ). the overlay film from example 10 was affixed to the light - emitting side of the electroluminescent lamp . an electroluminescent lamp was constructed with type gg64 el phosphor . ( control lamp c ). the overlay film from example 11 was affixed to the light - emitting side of the electroluminescent lamp . the electroluminescent lamp in this example was the same one as in example 15 ( without the overlay ). the overlay film with type bg - 300m long - afterglow phosphor from example 12 was affixed to the light - emitting side of the electroluminescent lamp . the lamps and overlays from examples 13 , 14 , 15 and 16 were each connected to a power supply operating at 125 v and 800 hz . the lamps were operated in a dark room at temperatures between 72 - 78 ° f . for 15 minutes , and then the power was removed . the brightness of examples 13 - 16 were read with a photometer . the brightness in millicandela per square meter ( mcd / m 2 ) corresponding to time in minutes after the power was removed are shown in table 3 . while there has been shown and described what are at the present considered the preferred embodiments of the invention , it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims .