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Name: Cherlyn
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A digital sundial displays the current solar time in digits, words, or pictures.

Wetter belgien [belgium]

Two closely-spaced parallel masks project different images depending on the angular position of the sun in the following way: The first mask, a regular array of thin vertical slits, casts a striped light pattern onto the second mask. This light pattern is independent of the height of the sun.

The second mask is composed of narrow stripes of the digits, words, or pictures to be displayed. The striped pattern of sunlight cast by the first mask illuminates exactly those stripes of the second mask corresponding to the image representing the current time.

The light shining through both masks is projected onto a translucent viewing screen mounted closely behind the second mask, which in a digital display of the time. A plate of light-refracting material can be inserted between the two masks, effectively linearizing the motion of the light pattern cast onto the second mask.

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Using this linearized version, it is possible to construct not only a sundial displaying the hours, but also a minute display which, for example, repeatedly displays the minutes of the current time in five-minute intervals. Sundials have been in use since ancient times and usually involve some means of casting a shadow onto an analog scale containing time markers. Similar to the hour hand of a regular clock, the position of the shadow on this scale indicates the current time.

While many different variations of this basic principle have been proposed, few deal with the disadvantages of an analog display, which requires a certain skill of the observer and is limited in accuracy.

Using a digital display overcomes these disadvantages, as evidenced by the success of digital clocks and watches. The object of the present invention is to transfer this advance to the domain of sundials. A fictitious digital sundial was described in Ian Stewart's column "Mathematical Recreations" Ian Stewart, "What in heaven is a digital sundial? In his article, Stewart builds on an idea first presented in Kenneth Falconer's book "Fractal Geometry" on 89 Kenneth Falconer, Fractal Geometry - Mathematical Foundations, John Wiley and Sons, : In order to illustrate a theorem on the projection of fractals, Falconer describes a hypothetical digital sundial based on a three-dimensional fractal that casts different shadows in the shape of s.

This device, although mathematically plausible, can not be realized in practice for several reasons.

Wetter belgien [belgium]

Fractals have infinitesimally small structure, which would impede manufacturing of the device; furthermore, the theorem does not yield a method of constructing such a fractal. Most importantly, the theorem relies on a point-shaped light source and on geometrical optics including straight-line projectionneither of which is true in the physical world, since a the disk of the sun subtends an angle of about one-half degree, and b diffraction of light imposes a lower limit on the size of any optical structure, so that a fractal with its infinitesimally small detail can not be used.

Thus, even if it were be possible to manufacture such a fractal device, the laws of physics would prevent it from working. A holographic sundial has been proposed which overcomes some of these problems by exploiting the wave nature of light A.

Stuart, "Holographic sundial", Applied Optics, Vol. The main disadvantage of this approach is the long and costly manufacturing process of the device, in which each displayed image has to be recorded separately, and the photographic material needs to be reoriented for each exposure.

Furthermore, shrinkage of the photographic emulsion limits the angular precision. Although the title of the patent claims a digital sundial, the device is just a variation of the traditional analog sundial, reversing the roles of shadow casting gnomon and time scale. Similar such variations include U.

This invention actually comprises a physically realizable digital sundial, but it has the drawback that the device is quite complex, and thus expensive to manufacture. Furthermore, since the light gathering tube and the display are two separate units only connected by a cable of optical fibers, it is not immediately obvious to the observer that the sun is responsible for creating the image of the time on the display as opposed to, say, an electronic circuit.

Having many components makes the device difficult to install and prone to damage. Finally, the principle underlying the invention only allows displays with a small of discrete elements such as seven-segment displaysand increasing the of elements also increases the complexity of the device. In light of the above, objects and advantages of the present invention are to provide a digital sundial which is physically realizable, which can be manufactured easily and inexpensively, which consists only of a few components and thus is robust, which is unlimited in the contents to be displayed, and whose function as a digital sundial is immediately apparent to an observer.

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A more general object of the present invention is to provide an optical apparatus for digitally displaying the angular direction of a remote light source. The present invention embodies a digital sundial, a device that displays the current time in digits, words, or pictures that change with the direction of the sunlight.

Two closely-spaced parallel masks create the different images in the following way: The first mask, a so-called stripe mask, is an array of vertical slits that cast a striped pattern of light onto the second mask, a so-called digit mask, which is composed of narrow stripes of the digits, words, or pictures to be displayed. The sunlight shining through the first mask generates a light pattern that illuminates exactly those stripes of the second mask corresponding to the digits of the current time.

This in a digital display of the time. A diffusion screen, mounted closely behind the second mask, allows reading of the displayed time from oblique viewing directions. A plate of light-refracting material can be inserted between the two masks, effectively linearizing the motion of the light pattern on the digit mask.

Using this linearized version, it is possible to construct not only a sundial displaying the hours, but also a minute display showing, for example, the 12 five-minute intervals. As opposed to the hour display, where each digit is displayed only once during the day, in the minute display the same set of digits is repeatedly displayed every hour, as the same parts of the digit mask are illuminated by different slits of the stripe mask in an hourly cycle. Variations on possible mask des include displaying the time in seven-segment style, roman numerals, words or pictures.

By using a pivotal mount allowing the sundial to be rotated about an axis parallel to the earth's axis of rotation, the displayed time can be adjusted easily to correct for the changing difference between solar and actual clock time, and to switch between standard and daylight savings time. By using two displays arranged at an angle the range of displayed hours can be extended.


It is also possible to construct a small table-top version of the sundial in which the display is read or illumintated through a horizontal mirror or a wall-mounted sundial in which the display is read or illuminated through a vertical mirror. The sundial comprises an hour display 12a and a minute display 12b, contained by a frame 16, which is supported by a base 18 and a stand Base 18 serves to position the sundial such that its axis 22 is parallel to the earth's axis of rotation, that is, at an angle to the horizontal which is equal to the latitude of the geographical site of the sundial.

By means of a pivot 24 shown in FIG. As will be explained below, for each angular position of the sun with respect to axis 22, the sundial displays images of digits representing the current solar time in digital form. The particular digital sundial illustrated by FIGS.

Both displays contain a planar stripe mask 30 and a planar digit mask 32, each of which is a thin opaque sheet with a specific pattern of light-transmitting apertures. Masks 30 and 32 are preferably separated by a central transparent plate 34 made from glass, plexiglas, or any other transparent material, but can also be separated by air.

For reasons explained below, the thickness of transparent plate 34a in the hour display is preferably about one-tenth of the thickness of the corresponding plate 34b in the minute display. Masks 30 and 32 are preferably realized by applying an opaque coating to both sides of transparent plate 34 the apertures are formed by leaving areas uncoated.

Alternatively, masks 30 and 32 can comprise a photographically produced film or transparency having opaque and transparent regions, or a thin rectangular sheet made from any opaque material having light-transmitting apertures, positioned adjacent to transparent plate To increase weather resistance and also to ensure planarity of the maskseach display preferably further includes two protecting transparent plates 36 and 38, positioned adjacent to masks 30 and The sun-facing protecting plates 36a and 36b of hour and minute display respectively preferably have different thickness to achieve equal overall thickness of both displays.

Finally, each display preferably also includes a matte light-diffusing viewing surface 40, onto which the different images are projected, thus enabling observation of these images from different directions. Viewing surface 40 is preferably integral with the sun-opposing transparent plate 38, being formed, for example, by etching or sand-blasting the outer surface of plate Alternatively, viewing surface 40 can be embodied by a light-diffusing coating or lamination applied to plate The opaque regions are shown in white, and the apertures are shown in black.

As can be observed, stripe masks 30a and 30b are regular arrays of thin parallel vertical slits, while the apertures in digit masks 32a and 32b are sampled from different digits. We will now explain the operation of the sundial.

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Since the two displays operate in a very similar way, the discussion will focus mainly on the hour display. The minute display will be briefly discussed below. When hour display 12a is illuminated by a remote light source, stripe mask 30a casts a striped pattern of light onto digit mask 32a.

The apertures in digit mask 32a have the form of narrow stripes sampled from all the digits that are to be displayed during the day. The arrangement of these stripes is such that at a given time exactly those stripes that correspond to the digit representing this time are illuminated by the light shining through stripe mask 30a.

For example, at 9am, the striped sunlight coming from stripe mask 30a falls exactly onto those apertures that correspond to the numeral 9. Consequently, the light that passes through both masks forms a pattern of closely-spaced vertical stripes in shape of the numeral 9. This pattern is projected onto viewing surface 40, yielding an image of the 9 in thin, closely-spaced bright stripes, which can be observed from any viewpoint on the sun-opposing side of display 12a.

One hour later, at 10am, the sun has performed an apparent motion of 15 degrees due to the rotation of the earth. The thin stripes cast by stripe mask 30a have moved slightly to the left on digit mask 32a, now falling onto apertures that correspond to the digits 1 and 0, yielding an image of the This process repeats for all the images to be displayed in the course of one day. For an example of the appearance of the displayed images, FIG. The apertures in digit mask 32a in FIG.

During the course of a day, the sunlight falling through a single slit in the stripe mask generates a part of each of those images.

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Thus, each period of the digit mask contains a thin vertical stripe sampled from each of those s. One can observe this periodic pattern in the digit mask in FIG. Note that the periods of stripe and digit masks are the same. The change of the sun's declination during the course of the year does not affect the displayed time, since, similar to the gnomon of a regular sundial, the slits in the stripe mask 30a are parallel to axis 22 which in turn is parallel to the axis of the earth.

Thus, the displayed image only depends on the horizontal angular position of the sun and is independent of the seasonably varying height of the sun. It will be appreciated that the images encoded in digit masks 32a and 32b in FIGS.

Alternate embodiments of the invention not shown can use different digit masks composed from arbitrary pictures which are sampled in thin vertical stripes. Simple variations include using a different typesetting font e.

In fact, any picture whose detail is limited to the resolution of the stripe mask can be displayed, including letters, characters, words, symbols, or pictograms. Furthermore, the time intervals during which different images are displayed can be arbitrarily chosen by varying the width of their corresponding stripes in digit mask The relationship between the geometry of the masks and the time intervals during which the different images are displayed will be discussed in detail below.

For ease of exposition we will first discuss an alternate embodiment of an hour display comprising stripe mask 30a and digit mask 32a, but not including transparent plates 34, 36, and The sun's azimuthal angle i. As is also the case with conventional planar sundials, we have to consider that a shadow cast onto a plane does not move linearly, but with the tangent of this angle. For illustration, FIG.