Abstract:
The invention relates to a display device for a wristwatch comprising a luminescent surface for illuminating the background of a display element consisting of one or more alphanumeric characters. The display device comprises a window in an opaque watch face in which these characters can be displayed. The luminescent surface covers at least one partial area of a supporting surface situated behind said window. The alphanumeric characters of the display element have an opaque material and are located between the watch face and the luminescent surface.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a display device for a wristwatch having a luminescent surface for the background illumination of a display element. 
   2. Description of the Related Art 
   It is known from the prior art for digital displays for dates, days of the week and/or chronometer functions to be provided with numbers, in which case an analog movement mechanism simultaneously displays hours, minutes and seconds using hands. 
   Such timepieces have the advantage that they can be configured purely mechanically and thus do not require any energy source such as a battery and/or a solar cell in order to function. The problem with these analog timepieces, then, is to render the display visible in the dark. For this purpose, it is possible to provide a battery or rechargeable storage battery, by means of which a luminous unit is supplied with energy. U.S. Pat. No. 6,208,591 utilizes a luminescent layer which is arranged in and beneath the then transparent face. This luminescent layer illuminates the hands of the timepiece from behind. However, it is thus not possible to render additional digital information of the timepiece readable in the dark. 
   It was also known to apply luminescent materials to the hands in order for the latter to be luminous in the dark. The application of these materials to numbers which indicate the digital time units, however, is associated with two significant problems. On the one hand, the application of these materials results in these digital time displays becoming considerably thicker. Furthermore, such displays only appear in a window during the corresponding time unit and are otherwise covered by the face, with the result that they are not recharged to a sufficient extent. 
   There is no question of using radioactive products for sales of timepieces in the civilian sector. 
   Wristwatches, finally, are often provided exclusively with a digital display of time units. These watches are usually with an LCD display. Such watches require a power source. 
   SUMMARY OF THE INVENTION 
   Taking this prior art as the departure point, the object of the invention is to configure a display device of the type mentioned in the introduction such that the digital time display is also visible in the dark. 
   Since it is exclusively the region behind the window in the face which is coated with the luminescent material, it can be ensured that this region is constantly recharged when the watch is worn, provided the watch is not covered by clothing. The numbers which appear in the window either are introduced in a transparent disk or consist essentially of a punched-out material, the parts of which are connected to one another merely via thin connecting crosspieces. This has the advantage that it is only in the region of this window that account has to be taken of the considerable thickness of the luminescent material, but the number disks, which bear the digital time-display units, may be of very thin configuration, in the customary manner. 
   Further advantageous embodiments are characterized in the subclaims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example using exemplary embodiments and with reference to the attached drawings, in which: 
       FIG. 1  shows a schematic plan view of a wristwatch with a digital date function according to a first exemplary embodiment of the invention, 
       FIG. 2  shows a schematic plan view of a wristwatch with a digital chronometer function according to a second exemplary embodiment of the invention, 
       FIG. 3  shows an exploded view with a schematic illustration of the region around the window of the first exemplary embodiment according to  FIG. 1 , and 
       FIG. 4  shows an exploded view with a schematic illustration of the region around the window of the second exemplary embodiment according to  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a plan view of a wristwatch with a face  1 , an hour hand  2 , a minute hand  3  and a second hand  4 . Also provided are two windows  5  and  15 , in which it is possible to see the current date, in this case the number  15  for the fifteenth day of the month, and, in this case, the letters “Sat” for the current day of the week (Sat for Saturday). The display elements are designated in general terms by  42 . It is, of course, also possible to provide just one window  5 . The configuration according to the invention of the region of the window or windows  5 ,  15  is illustrated in the detail-form drawing of  FIG. 3 . 
     FIG. 2  shows a different wristwatch, this time configured as a chronometer. The same features are indicated by the same designations in all the figures. This watch according to  FIG. 2  likewise has the hour hand  2 , the minute hand  3  and the second hand  4 . A further coaxial second hand  14  is provided for the chronometer function. The stopped minutes can be seen in the window  25  (in this case eight minutes) and the stopped and elapsed time in hours (in this case one hour) can be seen in the adjacent window  35 . 
   The hands  2 ,  3  and  4  and also  14  in the exemplary embodiments according to  FIGS. 1 and 2  are each provided with conventional luminescent coatings  8 , with the result that the hands can also be read in the dark. The windows  5 ,  15  and  25 ,  35  are configured in accordance with the invention in order that the alphanumeric characters there can also be read in the dark, this being described in more detail by way of two exemplary embodiments in conjunction with  FIGS. 3 and 4 . 
     FIG. 3  shows a schematic exploded view of a first exemplary embodiment for displaying the number “8” in the window  5 . The configuration for the windows  15 ,  25  and  35  may be identical in each case. The window  5  is an opening, in this case of square form, in the face  1 . At least one disk  21 , preferably rotating about the hand axis, is arranged beneath the face  1 . An essentially square section of the disk is illustrated here, this section having light acting on it through the surface area of the window  5 . It is possible to provide one disk  21  per display unit, i.e. two disks for the day of the week and date, and three disks for hours, 10s of minutes and minutes for a chronometer function. 
   The disk  21  which is illustrated here by way of example comprises a region  23  of transparent material which has arranged in its interior  25  or on its surface at the top  24  or bottom  26 , preferably at the top  24 , an opaque display element, in this case the number “8”, which is designated  22 . Arranged beneath this rotating disk  21 , opposite the window  5 , is a disk  19  which is fixed in relation to the housing of the watch and, in its central region, is provided with luminescent material, designated  29  here. The surface coated by the luminescent material  29  is advantageously somewhat larger than the opening of the window  5 , with the result that even slanting incident light through the window  5 , this light passing through the disk  21 , illuminates border regions of the luminescent material and thus also charges the same. 
   The disk  21  is advantageously highly transparent and has a slightly diffusing action on its underside  26 , with the result that, irrespective of the region covered by the display element  22 , those sub-regions of the surface  29  which are located directly beneath the display element  22  also receive light. It is thus the case with each type of display, i.e. with each date and/or each day of the week and/or the chronometer numbers, that most of the surface  29  in each case is illuminated, with the result that, when it is light, the (dark) display element  22  can be read against the light background and, in the dark, it is visible as a negative in relation to the light-emitting element  29 . 
     FIG. 4  shows another embodiment of the invention, the window  5  in the face  9  and the luminescent surface  29  in the region  19  being the same. The difference in relation to  FIG. 3  relates to the type of display elements, in this case designated  32 . The display element  32 , in this case-the number “0”, is connected via small crosspieces  33  to a carrier structure  34  which, as it were, replaces the disk  21 , which for each display of an alphanumeric character, in this case of a number, is located outside the window  5 . The carrier structure may be connected to the inner region of the carrier via a radial connecting crosspiece  35 . All the crosspieces  33 ,  34  and the character  32  are of very thin configuration and, for reasons of simplicity, are illustrated in two-dimensional form without any thickness. 
   It is thus possible for the light passing in through the window  5  to act on the luminescent surface  29  without obstruction, with the exception of the region covered by the display element  32 . All the crosspieces  33  and/or the carrier structure  34  may be of transparent configuration and only the display element  32  is opaque, or is made of the same transparent material as the crosspieces  33  but coated with an opaque material. 
   Both the cases mentioned in  FIGS. 3 and 4  have the advantage that the wristwatch provided with this system remains readable in the dark for a long period of time even after the luminescent materials on the surface  29  have been recharged for only a short period, this being the case both for the traditional hour, minute and second display elements and for the additional chronometer function and/or date/day of the week function.