Computers Printers and Scanners

How a Fax Machine Scans a Document

Published at 03/20/2012 20:22:26


Fax machines once were the cornerstone of business, spitting out thousands of pages of documents every day from different parts of the world. With the advent of internet technology, the importance of fax machines has faded somewhat, but they still enjoy a large, loyal fan base. This is mainly due to the fact that documents sent over telephone lines using a fax are considered to be more secure than an unencrypted email sent over the internet. Now, modern businesses use fax servers which retrieve faxes and save them in their internal memory while also sending them to various departments via email or as print outs.


Fax machines have evolved a lot over the last fifty years. But the idea behind how a fax machine scans a document has not changed.

The older version of fax machine scans using a rotating drum. The document to be scanned will be placed with the printed side facing outwards. Inside the machine, there is a photo-sensor that picks up light signals. It can recognize a light or dark area. This photo sensor is attached to a long arm that can move over the document as the drum rotates. The photo sensor focuses on a small area, as small as 0.1 centimeters squared. This little area of the paper will either be black or white in color. If the color in this area is black, the fax machine will send a particular tone at a particular frequency, and if it’s white it will send a tone at a different frequency. As the paper moves over the drum, the fax machine scans each line at a time, converting the page to electronic tones of different frequency.


A modern fax machine scans the paper without using any rotating drums and instead uses a moving sensor attachment. This sensor scans the document each line at a time. Some fax machines have a paper feed mechanism so that the fax machine can scan a large number of pages automatically. Like the older generation machine,the modern fax machine converts the black and white spots in each line of the paper to a digital signal and it is passed over the line. The modern fax machines can scan and send pages at a peak data rate of 14,400bps, and if there is too much noise in the line the data transfer rate drops back to lower speeds.

The latest generation of fax machines has a photo-diode or CCD sensing array. These machines have 1,728 sensors, arranged horizontally, and scan a whole line simultaneously. These types of fax machines scan the document at a very high speed. Once a fax machine scans a page, it compresses the data, so that the page can be transmitted at a faster rate. A typical modern day fax machine in an office is known as a CCITT (ITU-T) Group 3 Facsimile machine. These machines scan a document and can communicate with another group 3 machine. They scan a document at a horizontal resolution of 8 pixels per mm, and vertical resolution of 98, 196 or 391 lines per inch. The required resolution can be selected, after which the machine scans the document at the specified resolution. The higher the resolution, the more time it will take to scan and transmit the data over a telephone line.

Tips and comments

Even with the advent of internet technology, as long as faxing is considered to be an essential part of doing business, fax machines are here to stay.