Editing Optical mark recognition (section)

===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.<ref name=HowEverythingWorks.org>{{cite web|last=Bloomfield|first=Louis A|title=Question 1529: Why do scantron-type tests only read #2 pencils? Can other pencils work?|date=29 May 2006 |url=http://howeverythingworks.org/wordpress/2006/05/29/question-1529|publisher=HowEverythingWorks.org}}</ref> As some phototubes are mostly sensitive to the blue end of the [[visible spectrum]],<ref>Mullard Technical Handbook Volume 4 Section 4:Photoemissive Cells (1960 Edition)</ref> blue pens could not be used, as blue inks reflect and [[Transmittance|transmit]] blue light. Because of this, number two pencils had to be used to fill in the bubbles—[[graphite]] is a very [[Opacity (optics)|opaque]] substance which absorbs or reflects most of the light which hits it.<ref name=HowEverythingWorks.org />

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 (this is 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 is printed in.<ref name=HowEverythingWorks.org /> 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 filled, the scanner will detect it as filled in.