A pointing device is an input interface that allows a user to input data to a computer using physical gestures - point, click, and drag - for example, by moving a hand-held mouse across the surface of the physical desktop and activating switches on the mouse. Movements of the pointing device are echoed on the screen by movements of the pointer (or cursor) and other visual changes.
While the most common pointing device by far is the mouse, many more devices have been developed.
This is the most common form of pointing device for computers. Typically it has up to three keys and may have a scrolling wheel. The mouse's motion typically translates into the motion of a pointer on a display, which allows for fine control of a user interface.
The shape of some mice make them unsuitable for a left-handed person, but there are some mice specifically designed for left-handed users. Resting the fingers on the keys on a mouse should not activate the key. Tactual and auditory feedback should be provided on activation of a key.
The user should have the ability to modify the speed of movement of the cursor, the distance moved by the cursor and the double-click interval.
Motion and targeting
Most computer operating systems allow the user to adjust the speed that the pointer moves on the screen when the mouse is moved. People with restricted movement can use very small motions to move the pointer a long distance on the screen. Alternatively for those who struggle with targeting accuracy the pointer movement can be slowed so that targets are easier to hit.
Targets are items or areas on the screen that can be clicked, such as buttons and links on web pages. Targets can also be made easier to click if they are made larger. This can be accomplished by reducing the screen resolution or fine-tuning the display options.
There are also some third-party accessibility utilities available to help people who have difficulty with motion and targeting, particularly for those with a hand tremor.
Many users have difficulty with double-clicking. The maximum delay required for two consecutive clicks to be interpreted as a double-click is not standardized. According to Microsoft's MSDN website, the default timing in Windows is 0.5 seconds.
Most computer operating systems let the user adjust the speed required to perform a double click. This can be increased to help avoid inadvertent double clicks. Lots of people also find it difficult to know when to use a double-click and when to use a single-click. Fortunately, some operating systems have an option that disables all double-clicking across the entire computer so that only single-clicking is ever required. Some specialist joysticks have a button dedicated to double-clicking.
Click and dragging
Clicking and dragging is an important feature of mouse access as it allows the user to highlight text and drag and drop elements around the screen. It is also one of the most cognitively and physically challenging tasks that can be undertaken with the mouse as the button has to be held down during the entire process.
Windows ClickLock is an accessibility feature built into Windows XP that makes it easier for some users to access clicking and dragging. The feature works by locking the mouse button down, allowing the user to concentrate on moving the pointer. A fairly lengthy firm click initiates the ClickLock and another releases it. Many specialist joysticks and rollerballs have a special button to assist with clicking and dragging.
The default pointer on most operating systems can be rather small and many people struggle to find it or track its movement across the screen. Operating systems normally come with some alternative pointers.
Microsoft Windows also has an accessibility feature that helps to find the pointer on the screen by pressing the CTRL key. The feature needs to be enabled before it can be used. Tracking the movement of the pointer can also be made easier by enabling pointer trails.
Alternative pointing devices
Roller / Track Ball
Roller or trackballs consist of a small ball rotating freely in a fixed bed containing sensors to detect rotation of the ball about two axes. These devices are sometimes incorporated in public access terminals since, unlike a mouse, they do not need a surface to work on and they are more resistant to vandals.
People with a mobility impairments use trackballs as an alternative input device. The control surface of a trackball is easier to manipulate and the buttons can be activated without affecting the pointer position. Some physically impaired users find trackballs easier since they only have to move their thumb relative to their hand, instead of moving the whole hand, while others incur unacceptable fatigue of the thumb. Elderly people sometimes have difficulty holding a mouse still while double-clicking; the trackball allows them to let go of the cursor while using the button. However trackballs can be problematic for users with poor manual dexterity, and they have a tendancy to be slightly slower than a mouse.
Many people find that the most difficult part of using a mouse is holding it steady while pressing the button. One solution is to consider adding an external switch to act as the left or right mouse button, or both.
Specialist joysticks, classed as an assistive technology pointing device, are used to replace the computer mouse for people with fairly severe physical disabilities by pluging into the USB port and controlling the mouse pointer.
They can be of either displacement or force type. Displacement joysticks are by far the most common type and are what most people envisage when they consider joysticks. Force applied to the handle causes it to move over several degrees (usually between 20 and 30). This movement is detected and converted to an electrical signal by a potentiometer or a non-contacting technology device. Displacement joysticks are better for positioning accuracy than positioning speed, but it is essential that they return to the default position (usually the centre) when released.
Force sensitive or "stiff stick" joysticks use strain gauges to convert applied force into a proportional electrical output. Typically a movement of less than 1mm is needed to achieve full scale output. Force joysticks pose particular problems for visually impaired and dexterity impaired people.
Joysticks are often useful to people with athetoid conditions, such as cerebral palsy, who find them easier to grasp than a standard mouse. Miniature joysticks are also available for people with conditions involving muscular weakness such as muscular dystrophy or motor neurone disease.
A touchscreen is a computer display screen that is sensitive to human touch, allowing a user to interact with the computer by touching an active area, target or control such as pictures or words on the screen. Touchscreens are activated by the insertion or removal of the fingertip or by pressing the controls, active areas or targets with a mouthstick, headstick, or other similar device (stylus).
For more detailed guidance on making touchscreens accessible, see the Touchscreens Guideline.
A touchpad (also trackpad) is a pointing device consisting of specialized surface that can translate the motion and position of a user's fingers to a relative position on screen. They are a common feature of laptop computers and also used as a substitute for a computer mouse where desk space is scarce. Touchpads vary in size but are rarely made larger than 10 square centimetres.
The most common technology used to operate touchpads entails sensing the capacitance of a finger, or the capacitance between sensors. Because of the property being sensed, capacitance-based touchpads will not sense a stylus or other similar implement. Gloved fingers or prosthetic limbs will generally also be problematic.
A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's monitor. It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touchscreen but with greater positional accuracy. A light pen can work with any CRT-based display, but not with LCD screens, projectors and other display devices.
The light pen became moderately popular during the early 1980s. Due to the fact that the user was required to hold his or her arm in front of the screen for long periods of time, the light pen fell out of use as a general purpose input device.
For some people whose only, or most reliable, movement is with their head, a head operated mouse may be the best solution. It translates the movements of a user's head into directly proportional movements of the computer mouse pointer. Head operated mice are now quite small and compact units which attach to the front of a desktop or laptop computer screen, and may also be used with many communication devices.
Head operated mice function by a wireless optical sensor which uses infrared light to track a small disposable target that is placed on the user's forehead or glasses.
Fatigue can be an issue when a lot of keystrokes are required in order to accomplish a task however head operated mice can offer accurate mouse operation.
Eye tracking devices can be a powerful alternative for individuals with no control, or only limited control, over their hand movements.
A camera mounted on a computer monitor is focused on one eye. The available software processes the camera image to determine where the user is looking (gaze point), and the cursor is then placed at the gaze point. "Mouse clicks" are done with either a slow eye blink, eye gaze or a supplementary switch. Special software allows the person to type, and may include word-completion technology to speed up the process. These systems can be very costly.
Positioning of pointing devices
It is vital that any device you choose should be correctly positioned so that it is physically accessible and comfortable for the user. Even the perfect device, placed into an inappropriate position, can be extremely difficult to use.
Sometimes pointing devices, and especially switches, need to be positioned very carefully using universal mounting arms, trays or other mounting equipment.
Problems encountered by disabled people and the ageing population using pointing devices
People with visual impairments may not be able to see the cursor or pointer on a screen so would find it advantageous to have the facility to execute pointing functions from a keyboard or to interact with a computer by receiving tactile feed back.
Some people with physical impairments may have difficulty gripping or moving a standard mouse. Others find that when they click a button the pointer moves slightly and they have clicked the wrong thing.
People with a more severe cognitive difficulty may find it difficult to understand the link between the movement of the mouse, or pointing device and the resulting movement of the pointer on screen.
- The user should be allowed to execute pointing functions from the keyboard
- Provide the user with the facility for modifying cursor speed, distance and double-click interval
- Provide the user with the facility to change the size, color and symbol for the on-screen pointer and cursor
- The pointing device has the facility to automatically move the on-screen pointer onto the active window, button, or menu
- The user should be able to change the function associated with the buttons on the pointing device, for example, the functions click, double click, and the use of the right or left button
- The operation of a pointing device should not require two simultaneous hand movements
- The power needed to operate a pointing device should be between 0.3 and 0.6 Newton (Nordic Cooperation on Disability, 1998)
- The user is provided with feedback of an action using a visual, auditory or tactile form
- The text and background colour combination should have high contrast
- Avoid shades of blue, green and violet for conveying information since
they are problematic for older users
- There should be no noticeable flicker on the screen
- White or yellow type on black or a dark colour is more legible, provided that the typeface, weight and size are suitable
- Controls are labelled in a large high contrast font
- Audible output or tactile output is provided for identification of controls and for results of activating controls
- Controls should be operable by a mouthstick, headstick or other similar device (stylus)
- Commands can be entered by voice
- CAN/CSA Z412 (2000) Guideline on Office Ergonomics
- DS/ISO/DIS 9241-20 (Draft) Ergonomics of human-system interaction - Part 20: Accessibility guidelines for information/communication technology (ICT) equipment and services
- DS/ISO/TS 16071 (2003) Ergonomics of human-system interaction - Guidance on accessibility for human computer interfaces
- ETSI TR 102 068 (2002) Human Factors (HF): Requirements for assistive technology devices in ICT
- ISO 13406-1 (1999) Ergonomic requirements for work with visual displays based on flat panels. Part 1 - Introduction
- ISO 13406-2 (2001) Ergonomic requirements for work with visual displays based on flat panels. Part 2 - Ergonomic requirements for flat panel displays
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