Patent ID: 8721093

Claim:
A method of automatic focusing of a projecting device, comprising the steps of: a. providing an image light source to project an image onto a plane through a first image lens; b. providing an image sensor to sense the image on the plane through a second image lens; c. moving the second image lens in a predetermined direction of an optical axis for a predetermined distance, and activating the image sensor to sense the image on the plane to get a definition of the image after every movement of the second image lens, and locating the second image lens at a position where the definition is greatest; d. moving the first image lens in a predetermined direction of an optical axis for a predetermined distance, and activating the image sensor to sense the image on the plane to get a definition of the image after every movement of the first image lens, and locating the first image lens at a position where the definition is greatest; wherein the definition of the image defined in the step c and step d is contrast of the image; and wherein the step c further includes the steps of: c-1. activating the image sensor to sense the image on the plane through the second image lens, and getting a contrast of the image; c-2. storing the contrast gotten in the step c-1 in a memory; c-3. moving the second image lens in a first direction of the optical axis for a predetermined distance; c-4. activating the image sensor again to sense the image on the plane through the second image lens, and getting a contrast of the image; c-5. comparing the contrast gotten in the step c-4 with the contrast stored in the memory in the step c-2, and moving the second image lens in the first direction of the optical axis for a predetermined distance when the contrast gotten in the step c-4 is greater than the contrast stored in the memory in the step c-2, or moving the second image lens in a second direction, which is opposite to the first direction, for a predetermined distance when the contrast gotten in the step c-4 is smaller than the contrast stored in the memory in the step c-2; c-6. storing the contrast gotten in the step c-4 in the memory; c-7. activating the image sensor again to sense the image on the plane through the second image lens, and getting a contrast of the image; c-8. comparing the contrast gotten in the step c-7 with the contrast stored in the memory, and moving the second image lens in the direction, which is that same as the last movement, for a predetermined distance when the contrast gotten in the step c-7 is greater than the contrast stored in the memory; c-9. storing the contrast gotten in the step c-7 in the memory; and c-10. repeating the steps from the step c-7 to the step c-9 until contrast gotten in the step c-7 is less than the contrast stored in the memory, and then holding the second image lens at the present position.