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Apparatus for photoelectrically reading a translucent answer document having a bias bar printed thereon
| Details |
Inventors: Thibodeau, James;
Assignee: National Computer Systems, Inc. (Minneapolis, MN)
Primary Examiner: Moore; David K.
Assistant Examiner:
Attorney, Agent or Firm: Peterson, Palmatier, Sturm & Sjoquist, Ltd.
An answer document or form, on which the user darkens certain discrete response areas with a graphite or lead pencil to denote his selected choices or data entries, has a bias bar printed across one end with the same color ink used in printing the remainder of the document. When the document is read by the optical scanner, the light passing through the translucent paper also passes through the printed bias bar. Analog voltage signals are obtained from each of the photocells in the optical scanner and these signals are capacitively stored, then converted to digital values. The digital values are averaged and the individual digital values for each photocell are compared with the computed average to check for bad cells. If a poor photocell response persists for a predetermined number of answer documents, the entire scanning operation is stopped. Concomitantly, a predetermined value is added to the average response value derived from the bias bar so as to provide a light mark threshold with which the various response values obtained from the discrete response areas in each row are compared. Only the photocell values for those response areas having an opacity darker than the light mark threshold, that is those passing the comparison test, are further processed and scored. The need for individually calibrating the various photocells for various types of document paper and ink marks is obviated when utilizing my system. |
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DETAILED DESCRIPTION I claim: 1.
In combination, an answer document having a bias bar and various response areas printed thereon, both said bias bar and response areas being printed with ink possessing the same optical characteristics, and optical scanning apparatus for scoring said answer document including first means for deriving a first signal in analog form having a value representative of the optical characteristics of said bias bar, second means for storing a second signal in digital form having a value corresponding to the value of said first signal for subsequent comparison with additional signals derived from said response areas, and means for converting said analog signal to said digital signal, said digital signal having a value which is the substantial digital equivalent of the amplitude of said analog signal, said amplitude being said value representative of the optical characteristics of said bias bar.
2.
The combination set forth in claim 1 including a digital computer, said second means constituting a first memory section contained in said computer.
3.
The combination set forth in claim 2 in which said first means includes a plurality of photosensors and means for advancing said sheet in a direction so that said bias bar precedes said response areas to provide said first signal and thereafter provide said additional signals, said additional signals also being in analog form, said third means successively converting each of said additional signals from its analog form to corresponding additional digital signals each of which additional digital signals has a digital value which is the substantial digital equivalent of the amplitude of the additional analog signal from which it was derived, said computer having a second memory section for storing said additional digital signals to enable comparison of said additional digital signals with said first-mentioned digital signals.
4.
The combination set forth in claim 3 in which said bias bar provides a plurality of first signals in analog form, said third means converting each of said first signals to said second signals each having a digital value which is the substantial digital equivalent of the amplitude of the additional analog signal from which it was derived, and said second means storing all of said second signals
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Cell Phones 
