AUTOMATIC BRIGHTNESS CONTROL FOR PHOTOGRAPHIC PROJECTORS
First Claim
1. A photographic projector comprising a. projecting means for projecting an image of a transparency along an optical projection path, said projecting means comprising i. light source means for illuminating a transparency optically aligned with said light source means in said optical projection path, and ii. projection lens means for projecting an image of the transparency, said projection lens means being aligned with said light source means along said optical projection path with the transparency interposed in said optical projection path between said light source means and said projection lens means;
- b. light intensity sensing means mounted adjacent to and offset from said optical projection path for sensing the intensity of a light image of the transparency projected upon and reflected from at least a portion of a surface of said projection lens means facing the transparency prior to passage of said light image through said projection lens means; and
c. potential control means operably connected to said light source means and said intensity sensing means for controlling the potential applied to said light source means in accordance with the intensity of the light image projected upon at least said portion of said surface of said projection lens means.
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Abstract
A photographic projector having means for automatically controlling the amount of illumination generated by a projection lamp in accordance with the intensity of the light image of a transparency projected onto a projection lens. The projector in addition to the projection lamp and the projection lens, includes light intensity sensing means and potential control means connected to the light intensity sensing means and the projection lamp for controlling the potential applied to the projection lamp. The light intensity sensing means preferably is a photoconductive cell, such as a Cadmium-Selenium cell. A solid state circuit is provided for incorporation into the projector as the potential control means.
75 Citations
7 Claims
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1. A photographic projector comprising a. projecting means for projecting an image of a transparency along an optical projection path, said projecting means comprising i. light source means for illuminating a transparency optically aligned with said light source means in said optical projection path, and ii. projection lens means for projecting an image of the transparency, said projection lens means being aligned with said light source means along said optical projection path with the transparency interposed in said optical projection path between said light source means and said projection lens means;
- b. light intensity sensing means mounted adjacent to and offset from said optical projection path for sensing the intensity of a light image of the transparency projected upon and reflected from at least a portion of a surface of said projection lens means facing the transparency prior to passage of said light image through said projection lens means; and
c. potential control means operably connected to said light source means and said intensity sensing means for controlling the potential applied to said light source means in accordance with the intensity of the light image projected upon at least said portion of said surface of said projection lens means.
- b. light intensity sensing means mounted adjacent to and offset from said optical projection path for sensing the intensity of a light image of the transparency projected upon and reflected from at least a portion of a surface of said projection lens means facing the transparency prior to passage of said light image through said projection lens means; and
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2. The photographic projector of claim 1, wherein said light source means is of the incandescent filament type.
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3. The photographic projector of claim 1, wherein said projection lens means is mounted in a first wall of a housing and said light intensity sensing means is mounted on another wall of said housing and is adapted to sense the intensity of a light image projected upon said projection lens means and said first wall.
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4. The photographic projector of claim 1, wherein said light source means is connected to a source of alternating potential by a common terminal and said potential control means, and said potential control means comprises a terminal connection to another terminal of the source of alternating potential, a first capacitance and a first resistance connected in series between said terminal connection and said light source means, a second capacitance and a second resistance connected in series with each other and together in parallel with said first capacitance, a triac connected between said terminal connection and said light source means, and a diac connected between said triac and the junction between said second capacitance and said second resistance, said intensity sensing means being connected in parallel with said first capacitance whereby the alternating potential supplied to said light source meAns is controlled in accordance with the intensity of the light image sensed by said intensity sensing means.
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5. The photographic projector of claim 4, including means for varying said first resistance to selectively adjust the potential supplied to said light source means.
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6. The photographic projector of claim 4 wherein said intensity sensing means comprises a photoelectric cell.
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7. A photographic projector comprising a. projecting means for projecting an image of a transparency along an optical projection path, said projecting means comprising i. a housing, ii. light source means disposed at least partially within said housing for illuminating a transparency removably disposed within said housing and optically aligned with said light source means in said optical projection path, and iii. projection lens means disposed at least partially within said housing for projecting an image of the transparency outwardly of said housing, said projection lens means being aligned with said light source means along said optical projection path with the transparency interposed in said optical projection path between said light source means and said projection lense means;
- b. light intensity sensing means mounted adjacent to and offset from said optical projection path for sensing the intensity of a light image of the transparency projected upon and reflected from at least a portion of a surface of said projection lens means facing the transparency and a portion of said housing adjacent said projection lens means prior to passage of said light image through said projection lens means; and
c. potential control means operably connected to said light source means and said intensity sensing means for controlling the potential applied to said light source means in accordance with the intensity of the light image projected upon at least said portion of said surface of said projection lens means and said portion of said housing adjacent said projection lens means.
- b. light intensity sensing means mounted adjacent to and offset from said optical projection path for sensing the intensity of a light image of the transparency projected upon and reflected from at least a portion of a surface of said projection lens means facing the transparency and a portion of said housing adjacent said projection lens means prior to passage of said light image through said projection lens means; and
Specification