Patent Application
20090044417
1. Field of the Invention
The present invention discloses to a new type of optical device, the omni-directional lens, which can focus the sun beam from all directions onto a sharp spot with a long depth of field, which then applied to sundials and solar compasses. The sundials and solar compasses made with the omni-directional lens are easy to use and accurate.
2. Description of the Prior Art
Sundial is one of the earliest scientific instruments created by the mankind. Thousands of years ago, almost every culture of the world independently discovered the principle of determine the day in a year and the time in a day by observing the position of the sun, and invented some type of the sundial. Although the mechanical clock was invented in the 16th century, until late 19th century, the sundial was still used as a reliable time piece in Europe. Even in recent years, novel types of sundials are continuously being invented, as shown in the Reference List. Most of the sundials use a gnomon, an opaque piece of solid material which can project a shadow on a panel. The gnomon can be a long and thin opaque stylet, see U.S. Pat. No. 6,604,290 and U.S. Pat. No. 7,114,262; or an opaque sphere, see U.S. Pat. No. 4,835,875 and U.S. Pat. No. 4,945,644. Because the sun has a finite radius, the shadow is always fussy. Some sundials use a hole, such as U.S. Pat. No. 4,384,408; and or equivalently, a mirror, see U.S. Pat. No. 5,197,199; to project a bright spot on the panel instead of a dark shadow, as also used in some European churches. Because the angle of rotation of the sun often exceeds 180 degree, the hole can only be effective for a small range of angles, thus its usefulness is limited. To improve the sharpness of the image, some sundials use a concave mirror, see U.S. Pat. No. 4,520,572, or a cylindrical mirror, see U.S. Pat. No. 6,301,793, to focus the sun beam. However, the focus surface is a special curved surface in the three-dimensional space, which must be strictly arranged and followed. And, similar to the case of curved mirrors, if the sun beam is seriously off the axis, the sharpness of the image is low.
Another problem with the traditional sundials is that the angular position of the sun depends on the day of the year. The difference of the solar time and the average time is represented by the well-known equation of time. The error could be a large fraction of an hour. Therefore, the accuracy of the sundial is limited, especially the stylet and cylindrical-mirror type. Usually, a conversion table or conversion chart is attached to a sundial for the equation-of-time correction.
Magnetic compass is a widely used for determining directions. However, the position of the magnetic North Pole is off about 10° from the true North Pole, and the magnetic South Pole is off about 25° from the true South Pole. In the United States, the error (magnetic inclination) could be as large as 20°. The magnetic inclination also varies year by year. Furthermore, the magnetic compass is greatly affected by the ferromagnetic materials in the neighborhood of a compass, for example, iron ore in the ground or any steel or iron pieces. In 1834, W. A. Burt invented the solar compass which uses the position of the sun to determine the true north. Because of its reliability and accuracy, since the middle of the 19th century, the US government defined the solar compass as the standard for land surveying. The solar compass is also used in the military for reliably determining the directions in the battle field. Recent improvements of the solar compass were disclosed, for example, in U.S. Pat. No. 4,899,451 by Dandurand, and U.S. Pat. No. 5,424,178 by Steele et al., and U.S. Pat. No. 5,459,931 by Waltho. However, the operation of all those solar compasses is complicated, which requires the calculation of the local solar time versus the local standard time at the time of measurement, and requires elaborate manual adjustments. When a gnomon is used, the same inaccuracy problem with the sundials, the fussiness of the image and the equation of time, is present.
It is well known that a convex lens can focus sunlight into a sharp spot, as used in U.S. Pat. No. 9,428. However, it works only when the position of the sun is aligned with the axis of the lens. When the sun is slightly off the axis, the image is distorted. If the sun is seriously off the axis, the image is grossly distorted and eventually disappears. Furthermore, the depth of field is usually quite shallow. The use of convex lens in solar compass requires manual adjustment to align the axis with the sun.
The core of the current invention is a novel optical device for projecting the center of the sun from any direction to form a sharp spot of light onto a panel, see
The center of the light spot has a much higher intensity than the direct sunlight. It makes the center spot very easy to be identified. To avoid burning the panel, at least one of the spheres is made of a heat absorbing material, for example, doped with copper sulfate. With copper sulfate, only the blue light can go through the lens. Furthermore, by using a panel with dark blue background, the brightness of the area exposed to direct sunlight is substantially reduced, and the bright blue spot projected through the omni-directional lens becomes even more eye-catching.
Because of the large depth of field, the center of the sun can be projected on a cylindrical panel without loosing its sharpness over the entire area. A precise printout of the path of the sun can be easily made, which can provide a highly accurate reading. Because both the day of the year and the time of the day can be identified, the correction due to the equation of time is done automatically and accurately. By using two panels per year (from one solstice to another solstice), the daylight saving time can be marked directly.
If the time and the date are known, the instrument can be configured as a compass. The principle of the solar compass is not new. However, to use existing solar compasses, the instantaneous position of the sun must be calculated from astronomical data one by one, and the operator must wait the predetermined time to come. This consumes a lot of time and requires a profound knowledge on astronomy. For the solar compass based on the omni-directional lens, the astronomical information is explicitly marked on the panel. Therefore, it operation is independent of time, very intuitive, and easy to use.
The principle of the omni-directional lens is shown in
all rays with the same h will converge at a point Q (109) on the axis with a finite distance f (110) from the center of the sphere P. In general, for rays with different h, the focal length f is different. Since the rays with the same offset h converge at the same spot at the axis, it creates a light spot with very high intensity. However, all the rays with different h will be divergent and have much lower intensity comparing with that of the central spot. Because the lens is spherically symmetric, parallel light rays coming from any direction will be focused the same way. Therefore, the effect of light focusing is omni-directional.
The theoretically predicted focusing effect is experimentally verified. For example, for n1=1.50 (Lucite) and n2=1.33 (water), a lens with R1=25 mm and R2=9.3 mm generates a focal distance from 60 mm to 75 mm. In other words, the average focal length if 67.5 mm and the depth of field is 15 mm.
For applications in sundial and solar compass, it is not necessary to have the omni-directional focusing effect over the entire sphere. For the longitude, 360 degree is required. However, since the tropic circle is 23.5 degrees from the equator, a 60 degree latitude range is sufficient. This fact is important for the fabrication and mounting of the spherical omni-directional lens, see
As shown in
An example of a stationary sundial using a spherical omni-directional lens is shown in
The panel 302, preferably having a blue background and dark-blue of black markings, is designed according to the local longitude to correct for the difference between the local solar time and the local standard time. To ensure accurate readings, it is preferable to have a Spring panel (from the Winter solstice of the last year to the summer solstice of the current year), and a Fall panel (from the Summer solstice to the Winter solstice). If the size of the panel is a relatively large, the accuracy of the sundial could easily reach a single day except in the neighborhood of the solstices. Therefore, the daylight saving time can be marked on the panel.
The above sundial can be used only at a specific location. A sundial can be used for any location, a portable sundial, is shown in
The design of the panels is shown in
By letting the device to rotate horizontally, the sundial with an omni-directional lens can be configured as an accurate and easy-to-use solar compass. An example of the design of a solar compass is shown in
To use it, first adjust the base plate using screws 720 with the help of level 703. Then, adjust the latitude and longitude to match the location of measurement using screws 704 and 710. To find the true north, just rotate the base place such that the sun beam is focused on the current local time and the current date. The tips should point to the true North and true South. It is worth noting that when rotating the base place, both the date reading and the time reading would change. This will provide a consistency check.
