A screen reader is a software application that attempts to identify and interpret what is being displayed on the screen (or, more accurately, sent to standard output, whether a video monitor is present or not). This interpretation is then re-presented to the user with text-to-speech, sound icons, or a Braille output device. Screen readers are a form of assistive technology (AT) potentially useful to people who are blind, visually impaired, illiterate or learning disabled, often in combination with other AT, such as screen magnifiers.
A person's choice of screen reader is dictated by many factors, including platform, cost (even to upgrade a screen reader can cost hundreds of U.S. dollars), and the role of organizations like charities, schools, and employers. Screen reader choice is contentious: differing priorities and strong preferences are common.
Microsoft Windows operating systems have included the Microsoft Narrator light-duty screen reader since Windows 2000. Apple Inc. Mac OS X includes VoiceOver, a feature-rich screen reader. The console-based Oralux Linux distribution ships with three screen-reading environments: Emacspeak, Yasr and Speakup. The open source GNOME desktop environment long included Gnopernicus and now includes Orca.
The most widely used screen readers are separate commercial products: JAWS from Freedom Scientific, Window-Eyes from GW Micro, Dolphin Supernova by Dolphin, System Access from Serotek, and ZoomText Magnifier/Reader from Ai Squared are prominent examples in the English-speaking market. The opensource screen reader NVDA is gaining popularity.
Types of screen reader 
CLI (text) screen readers 
In early operating systems, such as MS-DOS, which employed command-line interfaces (CLIs), the screen display consisted of characters mapping directly to a screen buffer in memory and a cursor position.[clarification needed] Input was by keyboard. All this information could therefore be obtained from the system either by hooking the flow of information around the system and reading the screen buffer or by using a standard hardware output socket and communicating the results to the user. This was relatively easy to engineer.
In the 1980s, the Research Centre for the Education of the Visually Handicapped (RCEVH) at the University of Birmingham developed Screen Reader for the BBC Micro and NEC Portable.
GUI screen readers 
Off-screen models 
With the arrival of graphical user interfaces (GUIs), the situation became more complicated. A GUI has characters and graphics drawn on the screen at particular positions, and therefore there is no purely textual representation of the graphical contents of the display. Screen readers were therefore forced to employ new low-level techniques, gathering messages from the operating system and using these to build up an "off-screen model", a representation of the display in which the required text content is stored.
For example, the operating system might send messages to draw a command button and its caption. These messages are intercepted and used to construct the off-screen model. The user can switch between controls (such as buttons) available on the screen and the captions and control contents will be read aloud and/or shown on refreshable Braille display.
Screen readers can also communicate information on menus, controls, and other visual constructs to permit blind users to interact with these constructs. However, maintaining an off-screen model is a significant technical challenge: hooking the low-level messages and maintaining an accurate model are both difficult tasks.
Accessibility APIs 
Operating system and application designers have attempted to address these problems by providing ways for screen readers to access the display contents without having to maintain 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. Existing APIs include:
- Apple Accessibility API
- Microsoft Active Accessibility (MSAA)
- Microsoft UI Automation
- 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. For example, a screen reader can be told that the current focus is on a button and the button caption to be communicated to the user. This approach is considerably easier for screen readers, but fails when applications do not comply with the accessibility API: for example, Microsoft Word does not comply with the MSAA API, so screen readers must still maintain an off-screen model for Word or find another way to access its contents. One approach is to use available operating system messages and application object models to supplement accessibility APIs: the Thunder screenreader operates without an off-screen model in this way. (Note: the latest version of Thunder also includes an off-screen model but has one that does not involve installing a device driver. Consequently it can be used on a memory stick without any files needing to be installed.)
Screen readers can be assumed to be able to access all display content that is not intrinsically inaccessible. Web browsers, word processors, icons and windows and email programs are just some of the applications used successfully by screen reader users. However, using a screen reader is, according to some users, considerably more difficult than using a GUI and many applications have specific problems resulting from the nature of the application (e.g. animations in Macromedia Flash) or failure to comply with accessibility standards for the platform (e.g. Microsoft Word and Active Accessibility).
Self-voicing applications 
Some programs speak or make other sounds so that they can be used by blind people or people who cannot see the screen. These programs are termed self-voicing and can be a form of assistive technology if they are designed to remove the need to use a screen reader.
Cloud-based screen readers 
Some telephone services allow users to interact with internet remotely. For example,can read web pages over the phone, does not require special programs or devices on the user side.
Web-based screen readers 
A relatively new development in the field is web-based applications like text-to-speech functionality to web content. The primary audience for such applications is those who have difficulty reading because of learning disabilities or language barriers. Although functionality remains limited compared to equivalent desktop applications, the major benefit is to increase the accessibility of said websites when viewed on public machines where users do not have permission to install custom software, giving people greater 'freedom to roam'.that is web portal, managing content like news updates, weather, science and business articles for visually impaired or blind computer users. Other examples are or that add
With the development of smartphones, the ability to listen to written documents (textual web content, PDF documents, e-mails etc.) while driving or during a similar activity in the same way that listening to music, will benefit to a much broader audience than visually impaired people. The best-known examples are Siri for iOS, and Google Now and Iris for Android. With the release of the Galaxy S III, Samsung also introduced a similar intelligent personal assistant called S Voice.
Screen reader customization 
Not only do screen readers differ widely from each other, but most are highly configurable. For example, most screen readers allow the user to select whether most punctuation is announced or silently ignored. Some screen readers can be tailored to a particular application through scripting. One advantage of scripting is that it allows customizations to be shared among users, increasing accessibility for all. JAWS enjoys an , for example.
- JAWS. - An open source Mozilla Firefox extension that simulates how a web page would look in
Verbosity is a feature of screen reading software that supports vision-impaired computer users. Speech verbosity controls enable users to choose how much speech feedback they wish to hear. Specifically, verbosity settings allow users to construct a mental model of web pages displayed on their computer screen. Based on verbosity settings, a screen-reading program informs users of certain formatting changes, such as when a frame or table begins and ends, where graphics have been inserted into the text, or when a list appears in the document.
Some screen readers can read text in more than one language (e.g., Chinese ), providing that the language of the material is encoded in its metadata. Some screen reading programs also include language verbosity, which automatically detects verbosity settings related to speech output language. For example, if a user navigated to a website based in the United Kingdom, the text would be read with an English accent.
See also 
|Look up screen reader in Wiktionary, the free dictionary.|
- List of screen readers
- Screen magnifier
- Speech processing
- Speech recognition
- Speech synthesis
- Theofanos, Mary Frances, and Redish, Janice (Ginny) (November-December 2003). Guidelines for Accessible and Usable Web Sites: Observing Users Who Work With Screen Readers. self-published version. Redish & Associates. Retrieved 2008-07-20.
- "Talking Terminals. BYTE, September 1982". Archived from the original on June 25, 2006. Retrieved September 7, 2006.
- Paul Blenkhorn, "The RCEVH project on micro-computer systems and computer assisted learning", British Journal of Visual Impairment, 4/3, 101-103 (1986). Free HTML version at Visugate. See also "Access to personal computers using speech synthesis. RNIB New Beacon No.76, May 1992". Retrieved August 17, 2005.
- According to "Making the GUI Talk" (by Richard Schwarzenegger, BYTE December 1991, p. 118-128), the first screen reader to build an off-screen model was out Spoken.
- Apple Accessibility API.
- Java Access Bridge
- Luk, R.W.P., D. Yeung, Q. Lu, H.L. Leung, S.Y. Li and Fred Leung (2003). "ASAB: a Chinese screen reader". Software: Practice & Experience (Wiley) 33 (3): 201–219. doi:10.1002/spe.497.