The marks on the paper are detected by the reflection of light from the light source that fall on to the sensor. Each sheet is printed in a colour invisible to the OMR, known as the drop-out colour. The light used is either red, or infra-red, so anything on the sheet printed in red will not be detected by the sensor. The drop-out colour may also be green. Typical OMR sheets will have the title and sub-headings printed in black and the areas to be filled are in red.
OMR sheets must be printed and guillotined to very precise tolerances, otherwise the marks made on them could be slightly out of alignment with the OMRs read heads, causing the sheets to be misread. Also, the precise shade of the drop-out colour must be carefully controlled during the printing process. For these reasons, OMR sheets are normally pre-printed by the OMR manufacturer to SIMS specifications.
The sheet to be read is considered to be divided into a matrix of read areas, each with rows and tracks. Typically, an OMR has exactly 42 tracks and typically 66 rows.
Each track corresponds to an individual light sensor, and is parallel to the reference edge of the sheet and a fixed distance from it. The normal distance of this track is 0.2 inch.
The reference edge of the sheet is printed with a series of black blocks arranged vertically which may or may not be the same width, depending on the particular sheet. These correspond to the number of rows in each sheet. The black blocks are called the clock track or timing track. For example, the read areas available on a sheet used with the Attendance module consist of 38 tracks and 66 rows. Each track is 0.2 inches in width and there are 6 rows to an inch.
In actual use, it is unlikely that more than a few hundred of these read areas would be used on most sheets. A typical A4 sheet contains about 2400 read areas.
There are smaller blocks (see diagram) which are used to make sure that the paper is not skewed. In the track 2 position, there are a series of thin black rectangles, which identify the sheet.
This is a line printed in the drop-out colour (usually red). It is used to determine black and ‘white’, or mark and no mark. It is essential that the line is not marked in any way.
It is possible that due to a mark on the line or a fault in the read head, the line is read, and the test fails.
Black Line Test
At the bottom of the sheet, there is a long black line. This line checks that every read head will register marks. If any part of the line is not read, the test fails.
The OMR performs its own auto-calibration. If the OMR passes a sheet backwards and forwards a number of times, it is calibrating its sensitivity to black and ‘white’ (in this context, ‘white’ is effectively the drop-out colour).
Consult your OMR user manual for more information on setting up these tests.
The white line, black line and skew tests are used to check that the OMR is working properly and that the sheet is being fed through the reader correctly.
Further details concerning OMR use with SIMS modules can be obtained from:
Priory Business Park
Telephone: (01234) 838080
Fax: (01234) 838091
Pre-printed data entry sheets for use with the OMR can be obtained from: