What are Screen Readers?

A screen reader is a software application that identifies and interprets what is being displayed on the computer screen. This information is then presented to a blind user as speech or by through a Braille display. People with little or no functional vision use screen readers.

There are two types of screen readers the CLI screenreader and GUI screen reader.

CLI screen readers

Command Line Interface (CLI) screen readers were used in early operating systems such as the Linux Console or the old MS-DOS. The screen display consisted of characters mapping directly to a screen buffer in memory and a cursor position. The information was obtained from the system and reading the screen buffer and then communicating the results to the user.

GUI screen readers

A Graphical User Interface (GUI) has characters and graphics drawn on the screen at particular positions and does not require textual representation of the graphical contents of the display. GUI screen readers gather messages from the operating system and use these to build up an “off-screen model,” which is essentially a representation of the display in which the required text content is stored.

Information on menus, controls and other visual constructs are also communicated by the screen readers. However, translating the low-level messages and maintaining an accurate model are both complicated tasks.

Accessibility APIs

To address these problems, operating system and application designers have attempted to provide ways for screen readers to access the display contents without maintaining an off-screen model. These involve the provision of alternative and accessible representations of what is being displayed on the screen accessed through an API. Examples include AT-SPI, Microsoft Active Accessibility and the Java Access Bridge. Screen readers can query the operating system or application for what is currently being displayed and receive updates when the display changes. Screen readers find this approach significantly easier but fail when applications do not comply with the accessibility API.

Screen readers may access all display content that is not intrinsically inaccessible. Some of the applications used successfully by screen reader users include Web browsers, word processors, icons, windows and email programs. However, using a screen reader is considerably more difficult than using a GUI and many applications have specific problems resulting from the nature of the application or failure to comply with accessibility standards for the platform.

Main Screen Readers

There are some screen readers that are available in only one language and one country but the world leading screen reader manufactures that have multilingual support are GW Micro (Window Eyes), Freedom Scientific (Jaws) and Dolphin (Hal). The most used in the US is Jaws, with Window Eyes second and Hal third.