Digital sundial

摘要

A digital sundial displays the current solar time in digits, words, or pictures. 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 results 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. 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.