Optical answer sheet

An optical answer sheet or "bubble sheet" is a special type of form used in multiple choice question examinations. Optical mark recognition is used to detect answers. The most well known company in the United States involved with optical answer sheets is the Scantron Corporation, although certain uses require their own customized system. The terms "Optical answer sheet" and "scantron" have become more or less interchangeable.

Optical answer sheets usually have a set of blank ovals or boxes that correspond to each question, often on separate sheets of paper. Bar codes may mark the sheet for automatic processing, and each series of ovals filled will return a certain value when read. In this way students' answers can be digitally recorded, or identity given.

Reading
The first optical answer sheets were read by shining a light through the sheet and measuring how much of the light was blocked using phototubes on the opposite side. As some phototubes are mostly sensitive to the blue end of the visible spectrum, blue pens could not be used, as blue inks reflect and transmit blue light. Because of this, number two pencils had to be used to fill in the bubbles—graphite is a very opaque substance which absorbs or reflects most of the light which hits it.

Modern optical answer sheets are read based on reflected light, measuring lightness and darkness. They do not need to be filled in with a number two pencil, though these are recommended over other types due to the lighter marks made by higher-number pencils, and the smudges from number 1 pencils. Black ink will be read, though many systems will ignore marks that are the same color the form in printed in. This also allows optical answer sheets to be double-sided, because marks made on the opposite side will not interfere with reflectance readings as much as with opacity readings.

Most systems accommodate for human error in filling in ovals imprecisely, as long as they do not stray into the other ovals and the oval is almost completely filled.

Errors
It is possible for optical answer sheets to be printed incorrectly, such that all ovals will be read as filled. This occurs if the outline of the ovals is too thick, or is irregular. During the 2008 U.S. presidential election, this occurred with over 19,000 absentee ballots in the Georgia county of Gwinnett, and was discovered after around 10,000 had already been returned. The slight difference was not apparent to the naked eye, and was not detected until a test run was made in late October. This required all ballots to be transferred to correctly printed ones, by sequestered workers of the board of elections, under close observation by members of the Democratic and Republican (but not other) political parties, and county sheriff deputies. The transfer, by law, cannot occur until election day (November 4).