US2885561A - Luminescent indicating device - Google Patents

Description

May 5, 1959 K. w. LANGE 2,885,561

LUMINESCENT INDICATING DEVICE 1 Filed May 29, 1956 `United States Patent O 2,885,561 LUMINESCENT INDICATING DEVICE Karl W. Lange, Zurich, Switzerland Application May 29, 1956, Serial No. 588,136 Claims priority, application Switzerland June 3, 1955 13 Claims. (Cl. Z50-77) The present invention relates to a luminescent indicating device, and more particularly it relates to an indicating device such as a watch or the like, the indicating portions of which are Visible in the dark while the luminescent material is covered from sight in daylight.

Most watches, with the exception of military and sport watches, are usuallyprovided with dials on which indicia are shown which have exactly determined geometric shapes and smooth surface structure. These indicia might showa metal coloring or might be executed in a strong and pleasing color. The use of luminescent color indicia is limited, except for special purpose watches because the luminescent colors which may be applied to watches and thelike show in daylight an unsightly yellowish or yellowfgreen color and also show an uneven surface structure. Consequently, primarily for esthetic reasons, the use of luminescent colors is frequently avoided although the advantage of being able to read the indicating device in the dark is lost by not using luminescent colorssfor the indicia.

lt is therefore an object of the present invention to ,provide a luminescent indicating device which is free of the above-described disadvantages.

-It is another object of the present invention to provide an indicating device having luminescent indicia which in daytime have the appearance of a pleasing and strong color.

It is still another object of the rpresent invention to provide an indicating device such as a watch or the like which can be read in the dark while showing in daylight indicia of a pleasing color and smooth surface.

Other objects and advantages of thepresent invention kwill become apparent from a further reading ofthe description ofthe appended claims.

With the above and other objects in view, thepresent invention comprises a device of the character described,

in combination, a luminescent body, and aypartly translucent reflecting layer superposed'upon the luminescent body, whereby the partly translucent reflecting layer is visible when the intensity of outside light reflected by the translucent reflecting layer exceeds the intensity of light rays emitted by the luminescent body and passing through the partly translucent reflecting layer.

The presentinvention also contemplates in an indicating device, in combination, a transparent support having an inwardly directed face formed with an indentation and having'an outwardly directed face, a luminescent body of predetermined configuration located inthe indentation,

the .luminescent body being adapted to emit'rays ofvisible light, and a reflecting layer ,being at least partially vtranslucent for the visible light'raysemitted by the luminescent body covering at least a portion of'one of the faces of the transparent support, whereby thepredetermined configuration of the luminescent body is visible when the intensity of light rays emitted by the luminescent body and passing through the transparent' support andthe at least partially translucent'reecting layer thereon extce'eds the intensity -of outside' light reflected 'by the fsame.

fface of the-luminescent body ICC According to the present invention, the portions of an indicating device which are to be visible in the dark, such asindicia, comprise a luminescent body and at least partly translucent reflecting layer which is superposed upon the luminescent body. The reflecting layer will partly absorb and partly reflect the light and thus will appear when seen in daylight in its natural color. It will also be made of a material which permits to give to the indicia and the like the pleasing smooth surface structure which enhances the esthetic appeal of the watch or the like. Since the reflecting layer according to the present invention is to `be at least partly translucent for the light rays emitted by the luminescent body, the light emitted by the luminescent body will become visible through the reflecting layerl whenever the intensity of the'portion of the emitted light which passes through the at least partly translucent reflecting layer is greater than the intensity of light around the indicating device. Thus, the indicia will appear in lighted surroundings in the natural color of the reflecting layer, will show light rays emitted from the luminescent body when looked at in the dark, and at no time will the yellowishegreen color of the luminescent body as such become visible.

Aspartly translucent reflecting layers may be used for instance semi-translucent metal mirror layers of gold, chromium, rhodium or other metal, or also non-metallic semi-,translucent colorings, especially interference mirror layers of cryolite, fluoride and the like, the selective translucency of which corresponds to the main portion of the emission spectrum of the luminescent body.

The novel features which are considered as characteristic for the invention are set forth in particular in the appendedclaims. The invention itself, however, both-as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following decsription of specific embodiments when read in connection with the 'accompanying drawings, in which:

Fig. l is a schematic elevational view in cross section of an embodiment of the present invention;

Fig. 2 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. 3 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. 4 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. 5 is a schematic elevational View in crosssection of another embodiment of the present invention;

Fig. 6 is a schematic elevational view in cross section 4of another embodiment of the present invention;

Fig. 7 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. 8 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. 9 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. l0 is aschematic elevational view in cross section of another embodiment of the present invention;

Fig. 1l is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. l2 is a schematic elevational view in cross section of another embodiment of the present invention;

Fig. 13 is a schematic elevationalview in cross section of another embodiment of thepresent invention; and

Fig. 14 is a-schematc elevational view in cross section of yet anotherembodiment of the present invention.

Referring now to the drawings and particularly to Fig. l, a cross section through a portion of a dial is shown consisting of a metal plate 10 'forming a support and having on its face an indentation which is filled with a luminescent body 20, such as luminous paint. The free which has sharply defined edge portions giving the luminescent body a predetermined configuration, is ground or polished so as to be smooth and is covered with a semitranslucent composite layer 30 which consists of a translucent brilliant varnish base layer directly on the free face of luminescent body 20, and of a semi-translucent metal mirror layer or interference mirror layer covering the lacquer layer. The metal mirror layer or interference mirror layer causes the sharply defined configuration of the luminescent body to have in daylight a metallic or brilliantly colored appearance, while in the dark the light rays emitted by luminescent body 20 at least partly pass through the semitranslucent reflecting cover layer 3@ thus making the predetermined configuration of the face of luminescent body 20 visible in the dark. The dial face area surrounding luminescent body 26 and translucent reflecting layer 30 is covered with a layer 40 which is of a color contrasting with the daylight color of layer 30, for instance a layer of paint or a dull finished metal layer.

Fig. 2 shows an enlarged portion of the dial fragment shown in Fig. l in order to show in more detail the several layers of the at least partly translucent reflecting layer which covers the face of the luminescent body. The face of luminescent body 20 which has been polished or ground so as to be smooth is first covered with a transparent brilliant lacquer layer 31. Lacquer layer 31 is covered with a semi-translucent metal mirror layer 32, for instance a layer of gold, rhodium or chromium which has been applied in vapor form. The metal mirror layer 32 is then covered and protected on its free outer face by a translucent protecting layer 33 which may for instance consist of thorium fluoride. The semi-translucent reflecting layer 32 may also be instead of a metal mirror layer f an interference mirror layer or it may comprise superimposed upon each other a semi-translucent metal mirror layer and an interference mirror layer, whereby either the metal mirror layer or the interference mirror layer may be farther distant from luminescent body 20.

ln the embodiment shown in Fig. 3 a metallic relief body 11 forming the support for luminescent body 20 is placed on the opaque metal dial face 10. Relief body 11 is formed with an indentation in its outer visible face which indentation is filled with luminous paint 20. The visible face of relief body 11 and of the luminous paint 20 filling the indentation therein is smoothly polished and is covered with a semi-translucent shining reflecting cover layer 30 which may consist of a simple metal mirror layer or interference mirror layer or of a composite layer as described in connection with Fig. 2. In daylight, the relief body 11 forms a for instance metallic appearing relief indicia, as is especially desired in which dials, while in the dark the light emanating from luminescent body 20 glows through cover layer 30.

In the embodiment illustrated in Fig. 4, a metal dial plate 10 has on its face a translucent relief body 12 made for instance of polystyrene, the outer face of which is covered with a shining reflecting semi-translucent cover layer 30. In the lower face of relief body 12 an indentation is formed and filled with a body 20 of luminous paint. The light emanating from luminous body 20 will glow in the dark through the translucent support 12 and the semitranslucent cover layer 30. In daylight, however, the relief body herein described has the appearance of a metallic body.

While according to the embodiment illustrated in Figure 4 a translucent relief body 12 is superposed upon the metal dial plate 10, and the outer face of relief body 12 is covered with a shining reflecting semi-translucent cover layer 30, and a body 20 of luminous paint fills an indentation in the lower face of the relief body 12, it is also within the scope of the present invention that a luminous paint mass to which an adhesive substance has been admixed is arranged as a connecting layer between dial plate 10 and relief body 12. It is also possible to vform a dial plate 10 with indentatious in the portions of its outer face which will be located below relief body 12, and to fill these indentations, prior to positioning of relief body 12, with a body of luminous paint. In all these cases, it will be possible to see in the dark the light emanating from the luminous paint located underneath the lower face of transparent relief body 12, which light will in part penetrate through semi-translucent cover layer 30 covering the outer face of translucent relief body 12.

Referring now to Figs. 5, 6 and 7, luminescent bodies 21, 22 or 23, respectively, are positioned on metal plate supports 10 or 13, respectively. The luminescent bodies 21, 22 and 23 consist of a mass of synthetic plastic material, for instance polystyrene and of particles of luminescent material such as crystals of zinc sulfide finely distributed therein. The sharply defined visible planes of the luminescent relief bodies are covered with a smooth refleeting semitranslucent cover layer 30 of the type de scribed in connection with Figs. l and 2. Cover layer 30 covers in daylight the color of the luminescent body while it permits in the dark the passing through of light rays emitted by the luminescent body.

The luminescent body 21 shown in Fig. 5 contains in addition to particles of luminescent material also particles of a radioactive material which induces the luminescent material to the emission of light rays. According to Fig. 6, an adhesive layer is interposed between luminescent body 22 and support plate 10. Adhesive layer 51 contains a radioactive material which will induce the emission of light rays by the luminescent body 22. According to Fig. 7, the radioactive material 52 is placed below support plate 13. The rays emitted by the radioactive material, preferably beta-rays, penetrate through support plate 13 into luminescent relief body 23 and cause emission of light rays by the same.

According to Fig. 8, a luminescent body 20 is applied to the lower face of a transparent support disk 14 which for instance may be employed as a cover disk on a watch movement. The light rays emanating from luminescent body pass through a window 42 formed in an opaque paint layer 41 covering the outer face of translucent disk 14. Either between luminescent body 20 and translucent disk 14, or preferably on the outer face of translucent disk 14 in the area not covered by opaque paint layer 41, a semi-translucent reflecting mirror 3f) is applied, which covers the daylight color of luminescent body 2t) while it permits passing through of the light rays emitted by luminescent body 20, which thus becomes visible in the dark.

The embodiment of the present invention illustrated in Fig. 9 is in many respects similar to the embodiment illustrated in Fig. 8, however, with the exception that the window portion 44 of the outer face of translucent disk 15, which portion is not covered by opaque paint layer 43, is projecting outwardly from the remainder of the outer face of translucent support disk 15. The semitranslucent retiecting layer 30 may again be applied either on the projecting portion of the outer face of translucent support disk 15, or it may be applied between luminescent body 20 and the lower face of translucent support disk 15, whereby the luminescent body 20, the projecting portion 44 of disk 15 and the semitranslucent refiecting layer are arranged in alignment with each other. Y t

In the embodiment illustrated in Fig. 10, the outer rim portion of a curved watch crystal 16 made of transv lucent material is formed with an indentation in the lower face of its rim portion of watch crystal 16 is covered with a semi-translucent mirror layer 30 and filled with luminescent body 20. The indicia formed by the indentation in the rim portion of watch crystal 16 has in daylight the appearance of a metallic or colored brilv liant relief body, while in the dark the light emitted by luminescent body 20 becomes visible. The dial hands are indicated by reference numerals 61 and 62.

Fig. 11 is an enlarged illustration of an indentation in the lower face of a translucent support 17 made of a synthetic resin. The indentation is covered with a composite partially translucent reecting layer which consists of several individual layers 34, 35 and 36, and the cavity defined in the indentation is filled with luminescent body 2t).

Fig. 12 illustrates an embodiment somewhat similar to the embodiment illustrated in Fig. 11, however, as shown in Fig. 2 the entire lower face of transparent support plate 17 is covered with an opaque layer 45 having a color contrasting to the daylight color of reiiecting layer 30 so that support disk 17 may be used as dial plate.

' As illustrated in Figs. 13 and 14, it is also within the scope of the present invention to provide a dial hand with a groove in its visible outer face. Dial hand 18 is shown to have a groove lled with luminescent paint 20 and, similar to the embodiment illustrated in Figs. l, 2 and 3, the visible face of the luminescent paint body 20 is polished so as to be smooth and it is covered with a semi-translucent reflecting layer 30.

When indicia formed of commercial luminous paints are covered with semi-transparent cover layers, for instance metal layers formed by applying metal vapor, frequently a rough or at least dull surface structure is formed. ln order to obtain luminescent indicia with smooth and possibly shining surfaces, the present invention also provides for covering the luminescent body after drying of the same with a transparent lacquer layer, whereby the outer face of the luminescent body prior to being covered with a lacquer layer, or the lacquer layer. after the same has been applied may also be smoothly polished. Upon subsequent forming vof a semi-translucent reflecting metal mirror layer on the free lacquer surface, by applying metal vapor thereto, `a mirror-smooth surface is formed which in the dark glows due to the light rays emitted by the luminescent body and passing through the metal mirror layer. It is thereby possible to place the luminescent body in localized indentations of the dial disks or of the dial hands, or also n indentations formed in relief bodies which are to be adhered to the dial disks. These local indentations may be in the Shape of small surface, small square, or any other predetermined numerals or designs suitable to form indicia. After the indentation has been filled with the luminescent paint body and after the drying of the same, the free face of the luminescent body and the free face of the relief body around the free face of the luminescent body may be polished so that the surrounding face portion of the relief body will be free of luminescent material and at the same time the face of the luminescent body in the indentation of the relief body willV have a smooth surface structure. Subsequently, and if desired after application of a transparent lacquer layer in order to obtain high brilliancy, the free face of the luminescent body and the surrounding face of the relief body are covered with a semi-transparent reflecting metal mirror layer by application of metal vapor.

Since the local indentations into which the luminescent body is to be placed can be kept very small, for instance so as to have a width or a diameter of between 0.1 and 0.05 millimeter and a depth of 0.2 millimeter, it is possible to use the brightest or most highly activated luminescent paint without incurring the danger that the total radioactive radiation of the dial might become a health menace for the carrier of the watch or the like. This is possible'because luminescent areas of very small dimensions but specific high light intensity per square area are at least as clearly visible in the dark as larger luminescent areas of smaller specific light intensity.

As has been Stated further above, it is also within the scope of the present invention to employ relief bodies which can be glued to dial disks and which are provided with localized ndentations which can be lled with luminescent paint and covered with a mirror layer' as already described. If such relief bodies are made of transparent material for instance polystyrene, it is advantageous according to the present invention to provide the indentations which are to be covered with luminescent paint on the lower face of the relief body so that the emanated light becomes visible after passing through the relief body.

In this case, the semi-transparent reflecting metal layer, which Will cover the unsightly daylight color of the luminescent body will have to be steamed on either the outer face of the relief body which is closest to the viewer, or on the face of the indentation prior to filling of the same with the luminescent material.

The embodiments of the present invention illustrated in Figs. 5, 6 and 7 show luminescent bodies in the form of prismatic rods of small dimensions which may be formed by extrusion or by cutting from plates or by stamping with subsequent polishing. These rods may' for instance consist of a mixture of 50% polystyrene and 50% of very small crystals of luminescent, copper activated zinc sulfide. In place of polystyrene it is also within the scope of the present invention to use other transparent synthetic resins, and in place of small crystals of zinc sulfide also particles of different luminescent material may be used. Such a synthetic resin-containing luminescent material can easily be formed in known manner into rods, small circles, numerals or other indicia which show a smooth surface structure and a sharply dened geometric configuration.

When such rods are then covered on their upper face only, that is on their face which is closest to the viewer,` with a metal cover layer which is for instance applied in the form of metal vapor, while the side faces of the luminescent rod remain free of such metal layer, the rods which have a thickness of at most 1 millimeter will have for the viewer the color of the cover layer, that is, in the case gold is used as cover layer material, a shining gold color, or if chromium or rhodium is used as cover layer, a shining white color. lt is however also possible to obtain dull metal surfaces. ln place of metal vapor layers it is also possible to apply by vaporization interference layers of uorides or cryolite in which case the daylight color appearance depends on the type of material used for, and the thickness of, the at least partially transparent reflecting cover layer. Because of the interference effect which can be observed by using for instance any one of the last mentioned materials for the reflecting layer, a pleasing change in color and an iridescent effect can be observed by changing the angle under which the device is viewed. When the luminescent particles within the rods are then activated so as to emit visible light rays, the light emerges through the side faces of the rods and will be visible in the dark, while in daytime it will be unnoticeable due to the greater intensity of the daylight. In this case the thickness of the metallic or interference reflecting layer on the top face of the rod prevents passing of light through the top face of the rod.

It is easily possible to make a refiecting cover layer steamed upon the luminescent rods (either directly or steamed upon a clear lacquer layer covering the luminescent rods), of such small thickness that the light rays emanated by the luminescent rods can at least partly pass through the cover layer, i.e., that a suiiicient portion of the light rays will pass through the partially translucent reecting cover layers to make the rods visible in the darkness. When interference mirror cover layers are employed, the light rays emanating from the luminescent body can pass through the cover layer without diiiiculty provided that the selective suppression band of the covering material does not coincide with the principal spectrum of the rays emanated by the luminescent material. However, it is also possible to form in known manner metal cover layers which are so thin that they show an absorption rate of less than 20%, a reflection of about 40% f7 'and a transparency of about 40% so as to create in incident light the impression of a mirror face while in the dark, light from the luminescent rods can pass through the metal cover layer in a sufficient degree to make the rods visible. When cover layers are employed, which, as described above, are at least partialy transparent, it is of no harm, if also the side faces of the rod are covered by the same cover layers, whereby the forming of the layer is somewhat simplified.

In order to stimulate such rods or diiferently shaped luminescent bodies which consist of a dispersion of luminescent particles in a synthetic resin, preferably polystyrene, so as to cause emission of light rays, it is necessary to expose the luminescent particles to an eX- citer radiation. Luminescent particles are known which possess the quality of emitting light rays of longer wave lengths after being previously radiated with light of shorter wave lengths. Such subsequent emission of light might last up to six hours. If the relief bodies contain such after glowing luminescent particles, it is required that the cover layer must be suiciently transparent, or that the side faces of the luminescent body are exposed in order to guarantee that suflicient daylight can reach the luminescent particles in the relief body.

However, according to a preferred embodiment of the present invention the activation of the luminescent particles is accomplished by means of radioactive rays. This can be done for instance by adding to the polystyreneluminescent material mixtures of which the rods or the plates from which the rods are formed consist, radioactive materials. Thereby especially radioactive betaray emitters and primarily materials the radioactivity of which has been articially induced, namely isotopes of elements such as strontium 90 are suitable, because the electron emission of such isotopes into the luminescent body reaches much farther than proton (alpha) rays of the natural radioactive elements. For instance, beta rays of strontium 90 penetrate without considerable absorption 0.8 millimeter of polystyrene or 0.2 millimeter of brass sheet. Furthermore, beta rays have a less destructive effect on the adhesive materials and lacquer layers than alpha rays. Since the shaping (extruding, stamping, cutting, polishing) of materials which contain radioactive substances might cause harm to the worker unless special safeguards are employed, it is advantageous to introduce the radioactive light ray emission inciting materials only after the rods or the like have been formed. For instance, relief bodies which mainly consist of polystyrene may be heated under vacuum after having been formed and will then be degassed and completely dried. If one now immerses such dewatered and degassed luminescent materials containing polystyrene bodies which for instance may already be adhered to a dial disk, into water containing strontium 90, then together with water, atoms of strontium 90 will enter into the polystyrene body structure so that the beta-rays emitted by strontium 90 will subsequently incite the zinc sulfide crystals dispersed in the polystyrene to luminescence. The absorption of strontium 90 atoms or other beta-ray emanating atoms into the structure of the relief body can be considerably favored by admixing to the material of which the body is formed initially substances which have a special affinity to the element which subsequently is to be diffused into the body. By the use of such substances, one can also prevent an undesired loss of radioactive atoms or groups of atoms by diffusing out of the structure of the relief body.

A further problem, namely the brittleness of polystyrene or other synthetic resin plates with embedded luminescent particles, such as have been proposed further above as materials for making luminescent bodies, which brittleness becomes apparent especialy upon the stamping out of small rods or numerals or other indicia, can be solved by providing the somewhat brittle plates with a supporting foil of a cohesive material such as polyvinyl resins or metal foils. In the latter case, the stamping, cutting, grinding and polishing of the indicia forming relief bodies can be handled without difficulties since the supporting foil prevents breaking off of portions of the relief body.

In all the embodiments of the present invention, the semi-transparent metal layer may also consist of silver or aluminum; however, in such case in order to prevent dulling it is necessary to apply a transparent protective layer which may for instance consist of a lacquer or of thorium fluoride or a similar layer. Thorium fluoride is preferably applied in vapor form. A protective layer of such kind may also be applied to the free face of semitransparent reflecting metal layers made of precious metal which are not subject to dulling. In this case the protective layer will increase the resistance of the mirror layer against wear and abrasion.

Similarly, it is also within the scope of the present invention to include in all embodiments thereof underneath the semi-transparent reflecting layer which serves for the purpose of covering the unsightly yellow or yellowishgreen color of the luminescent mass in daylight, transparent intermediate layers which possess an index of refraction which improves the reflecting ability of the mirror layer.

Special pleasing etfects can be achieved by steaming above or below the semi-transparent metal mirror layer an interference layer. Depending on the direction of incidence of the light and the angle under which the viewer looks at the indicia, such multi-layered bodies show a metallic luster or sparkle in varying colors.

In many cases metallic semi-transparent mirror layers may be replaced by interference mirror layers, provided that the composition and thickness of the interference mirror layer is so chosen that the same wil cover the luminescent body in daylight and will permit passing of light rays emanated by the luminescent body so that the same will become visible in the dark.

The location and width of the portion of the spectrum which will be able to pass through the interference mirror layer depends on the thickness of this layer and on the material of which it is formed, whereby for instance cryolites, sullides, thorium fluorides or quartz may be used, which usually are applied, similar to metal mirror layers, under vacuum in vapor form.

If an irridescent eiect is not desired, layers as above described may be applied in greater thickness and will then serve only as transparent protective layers for an underlying semi-transparent metal mirror layer in order to protect the same against mechanical wear and chemical corrosion.

Such layers of cryolite, quartz or the like may also be used according to the present invention as protective layers between a semi-transparent metal mirror layer and the luminescent body. Especially when the semi-transparent metal mirror layer covers the inner face of an indentation, for instance on the back side of disks of transparent artificial glass-like material or polystyrene, and when the cavity formed by the indentation is then to be filled with luminescent material which contains as a binder nitrolacquers with acetone thinners, it has been found that such lacquers may penetrate through the metal mirror layer and cause a partial aking off of the same from their support by attacking the material of the support, for instance polystyrene.

Such damage to the metal mirror layer can, according to the present invention, be avoided by steaming onto the metal mirror layer prior to application of the luminescent material a protective layer of, for instance thorium fluoride. The usual solvents of the binder for the luminescent material that is, solvents for nitrocellulose lacquers cannot penetrate through the protective layer of thorium fluoride. In place of thorium fluoride, also transparent lacquers may be used for forming the transparent protective layer, provided thatsuch lacquers will not be attacked bythe solvent in the luminescent body. Especially lacq'uers which form viscous-elastic, although relatively soft layers will be doubly suitable when they are applied between the semi-transparent metal mirror layer and the synthetic resin support on which the metal mirror layer is steamed on, since these lacquers will cause a better adherence and a greater brilliancy of the mirror metal layer and at the same time will protect the synthetic resin support from harmful effects of the luminescent body.

,Y Another possibility for avoiding the above-mentioned dvar'nageA to the synthetic plastic material consists in the use Vof luminescent materials which are prepared with binders other than nitrocellulose lacquers, such as water soluble methyl cellulose or gum arabic, water containing emulsions of alkide resins and vinyl resins. It seems, however, especially important to subject the Vmetal mirror layer steamed on the support to a heat treatment in order to improve their adherence.

As will now be shown, it is frequentlyk important according to the present invention to use luminescent materials in combination with beta-rays emitting atoms as source of exciter radiation. Frequently it is necessary to excite the luminescent material with beta-rays in order to achieve the emission of visible light rays of sufficient intensity so as to penetrate the mirror cover layer in Sullicient quantity for being visible in the dark. The semitransparent reflecting mirror layer has to be of sufficient thickness to completely cover the daylight color of the luminescent material. A certain percentage of the visible light rays emitted by the luminescent material will not be able to pass through the mirror layer and consequently the total output of visible light rays by the luminescent material must be suiciently great so that the fraction of the visible light rays which will pass through the mirror cover layer will be suicient to make the indicia formed by the luminescent body visible in the dark.

lt has been found, for instance, that semi-transparent gold layers have to be of such thickness in order to fully cover the color of the underlying luminescent material in daylight that only between 20 and 40% of the visible light rays emanated by the luminescent body are capable of passing through the gold mirror layer.

It is generally required that indicia and indicating arms of pocket and wrist watches and indicia of indicating dev iceszwhich are to be read in the dark must have a brightness of at least 0.16 lux. Consequently, the brightness of4 the light emitted by the luminescent body must be at least about 0.5 lux in order to achieve after covering of the luminescent body with a gold mirror layer a passing' through of at least 0.16 lux.

Luminescent materials with three times the brightness -have'to be incited at least three times as strongly, practically four to tive times. as strongly. Consequently, four to ve times as many radioactive inciting atoms must be distributed in or act upon the luminescent material.

Lu'minescent paints of such high brightness and containing natural radioactive inciting atoms such as radium or thorium atoms have, however, the undesirable quality that the light intensity reduces very quickly. Such strongly activated luminescent materials as are commonly4 knownhave with respect to the intensity of their luminescence a half-decay period of three to four months, so that already after about a year and a half a luminescent indicia of this kind would practically cease to glow in. the dark. i v

Furthermore, the alpha rays of such natural radioactive masses attack the lacquer layers to. a considerable extent so that also the desired daylight effect, namely the brilliant gold color, is lost within a short period of time. Consequently, by using customary luminescent materials' it would take only one or two years after the luminescent and gold shining indicia and arms of the Watch have been produced until the brilliant golden appearance in day time and the night glow will disappear.

--l'iurthermore, according to thepresentinvention, larger' quantities of luminescent paint materials have to be ap-V plied. If those larger quantities of luminescent paint materials would be used in combination with natural radioactive elements, the radioactivity would have to be in creased sixteen to twenty fold whereby a considerable part of the radioactive radiation energy would emanate as hard gamma-rays which are practically unscreenable.

The carrier of a watch or the like which would have been provided with luminescent indicia in the above described manner would then be exposed to a dosage of gamma rays which would by far exceed the allowable maximum in accordance with the London recommendations of 1951 and which probably would cause damage to the wearers health.

Completely different, however, are the conditions if luminous paints are used which are activated with pure beta-rays emitting atoms, for instance promethium 147. The beta-rays emitted by promethium have an energy of about 0.22 million electron volts and are practically completely absorbed by 0.2 millimeter of sheet brass or 0.5 millimeter of synthetic glasslike material. Consequently, even greatest activation of the luminous paint will not cause any health hazard to persons wearing or exposed to the indicating device. Furthermore, luminescent materials which emit light rays of great brightness when incited with pure beta-rays practically do not show any decrease in their illuminating power. Furthermore, the working with luminous paints which are activated withl promethium 147 is much less dangerous, since this element, in contrast to radium or thorium is very quickly eliminated by the human body.

Since one also has not to fear any damage to the semitransparent gold mirror layer by the beta-rays, the use of luminescent indicia which are activated with pure betaray emitting atoms such as promethium 147, nickel 63 or carbon 14, and by which the radiation energy is considerably below 0.5 million electron volts has extraordinary important advantages in combination With semi-transparent metal mirror layers which serve for covering the daylight color of the luminescent material.

It is also within the scope of the present invention to provide beta-rays emitting materials only for the required additional activation of the luminous paint mass, while the basic activation is accomplished by the use of natural radioactive elements in the then only required small quantity.

lt will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of indicating devices differing from the types described above.

While the invention has been illustrated and described as embodied in an indicating device provided with luminescent indicia, it is not intended to be limited to the details shown, since various modifications and structural changes may be made Without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended tc be comprehended within the meaning and range or" equivalence of the following claims.

What is claimed asnew and desired to be secured by Letters Patent is:

l. In a dial, in combination, a luminescent body having a predetermined configuration for indicating an indicia; and a partly translucent reflecting metallic mirror layer superposed upon said luminescent body, whereby said partly translucent reflecting metallic mirror layer is visible when the intensity of outside light reflected by said translucent reflecting layer exceeds the intensity of light i1 rays emitted by said luminescent body and passing through said partly translucent reflecting layer.

2. In a dial, in combination, a luminescent body having a predetermined conguration for indicating an indicia; a transparent support having a smooth face arranged in front of said luminescent body; and a partly translucent reecting metallic mirror layer on said smooth face o f said transparent support, so that said indicia when illuminated from an outside light source will be ot glossy metallic appearance due to reflection of such outsid e light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reecting metallic mirror layer.

3. In a dial, in combination, luminescent body .having a predetermined coniguration for indicating an indicia; a transparent support having a smooth face arranged 1n front of said luminescent body; and a composite layer consisting of a partly translucent reflecting metallic mirror layer and an interference mirror layer being translucent for light rays having a wave length corresponding to the wave length of the major portion of the light rays emitted by said luminescent body on said smooth face of said transparent support, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reflection of such outside light by said composite layer, and when viewed in the dark, said indicia will be visible due to the light rays b eing emitted by said luminescent body through said partially translucent relecting composite layer.

4. In a watch, in combination, a dial formed with at least one indentation having a predetermined configuration for indicating an indicia; a luminescent body substantially illing said indentation; and a partly translucent reilecting metallic mirror layer covering the free face of said luminescent body, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reection of such outside light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reilecting metallic mirror layer.

5. In a watch, in combination, a dial formed with at least one indentation having a predetermined configuration for indicating an indicia', a luminescent body substatially lling said indentation; a transparent protective layer covering the free face of said luminescent body; and a partly translucent reflecting metallic mirror layer superposed upon said transparent protective layer, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reflection of such outside light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reecting metallic mirror layer.

6. In a watch, in combination, a disc-shaped carrier member having an outwardly directed face; at least one support body arranged on said outwardly directed face of said carrier member, said support body being formed with an indentation of predetermined conguration for indicating an indicia; a luminescent body substantially lling said indentation; and a partly translucent reflecting metallic mirror layer arranged in front of said luminescent body, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reflection of such outside light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reflecting metallic mirror layer.

7. In a watch, in combination, a disc-shaped carrier member having an outwardly directed face; at least one transparent support body arranged on said outwardly directed face of said carrier member, said transparent support body having an outwardly directed face and an inwardly directed face and being formed with an indenta tion in said inwardly directed face, said indentation being of predetermined conguration for indicating an indicia; a luminescent body substantially iilling said indentation; and a partly translucent reflecting metallic mirror layer covering said outwardly directed face of said transparent support body, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reection of such outside light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reecting metallic mirror layer.

8. In a watch, in combination, a disc-shaped carrier member having an outwardly directed face; at least one luminescent body having a predetermined configurationfor indicating an indicia arranged on said outwardly directed face of said disc-shaped carrier member; and a partly translucent reecting metallic mirror layer on the free face of said luminescent body, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reflection of such outside light by said metallic mirror layer. and when viewed in the dark, said indicia Will be visible due to the light rays being emitted by said luminescent body through said partially translucent reflecting metallic mirror layer.

9. in a watch dial, in combination, a transparent support having an outwardly directed face and an inwardly directed face and being formed with an indentation in said inwardly directed face, said indentation having a predetermined configuration for indicating an indicia; a luminescent body substantially filling said indentation; and a partly translucent reflecting metallic mirror layer interposed between said luminescent body and said transparent support, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reection of such outside light by said metallic mirror layer, and when viewed in the dark., said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reflecting metallic mirror layer.

l0, In a watch dial, in combination, a transparent support having an outwardly directed face and an inwardly directed face and being formed with an indentation in said inwardly directed face, said indentation having a predetermined conguration for indicating an indicia; a luminescent body substantially filling said indentation; and a composite layer consisting of a partly translucent reilecting metallic mirror layer and of an interference mirror layer being translucent for light rays having a wave length corresponding to the wave length of the major portion of the light rays emitted by said luminescent body interposed between said luminescent body and said transparent suport, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to rellection of such outside light by said coniposite layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reflecting composite layer.

1l. In a watch dial, in combination, a support consisting of a transparent synthetic resin, said support having.

an outwardly directed face and an inwardly directed face and being formed with an indentation in said inwardly directed face, said indentation having a predetermined configuration for indicating an indicia; a luminescent body consisting of a luminescent material distributed in a solid synthetic resin adapted to be softened by a solvent therefor and including a relatively small quantity of said solvent substantially lling said indentation; and a composite layer consisting of a transparent layer of a material impermeable by said solvent and of a partly translucent reflecting metallic mirror layer interposed between said luminescent body and said transparent support, so that said indicia when illuminated from an outside light source will be of glossy metallic appearance due to reflection of such outside light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays being emitted by said luminescent body through said partially translucent reflecting metallic mirror layer.

l2. In a watch dial, in combination, a transparent support having an outwardly directed face and having an in- Wardly directed face; a luminescent body arranged adjacent to at least a portion of said inner face of said transparent support; a partly translucent reflecting metallic mirror layer arranged on one of said faces of said transparent support in front of said luminescent body; and an opaque cover layer in front of said outwardly directed face of said transparent support, said opaque cover layer being formed with indicia-shaped cutouts in front of said luminescent body and said metallic mirror layer, so that indicia-shaped portions of said metallic mirror layer will be visible when said watch dial is illuminated from an outside light source, and said indicia-shaped cutouts will `be visible when viewed in the dark due to the light rays being emitted 'by said luminescent body through said indicia-shaped portions of said metallic mirror layer.

13. In a watch, in combination, a transparent support having an at least partly smooth front face and having a rear face; a luminescent body having a predetermined configuration for indicating indicia and arranged in the rear of said transparent support in contact with said rear face thereof; a dial member supporting said transparent support together with said luminescent body, with said front face of said transparent support located forward of said dial; and a partly translucent reflecting metallic mirror layer at least on said smooth faced portion of said transparent support, so that said indicia when illuminated from an outside source will be of glossy metallic appearance due to reflection of such outside light by said metallic mirror layer, and when viewed in the dark, said indicia will be visible due to the light rays emitted by said luminescent body through said partially translucent reflecting metallic mirror layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,756,344 Ferrier July 24, 1956

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