Membrane keys consist of thin layers of material covering the underlying circuitry. Membrane keypads are most commonly found on microwave ovens. They are also typically found on security alarm controls and on medical measuring instruments.
Individual membrane keys can be difficult to distinguish for visually impaired users. From a visual perspective, the keys do not have obvious edges and high contrast colour combinations are not always used. For example, many microwave ovens use low contrast (e.g. white/grey or silver) colour combinations. This can make it very difficult to distinguish the key itself from the surrounding area, and can make the labels difficult to read.
Similarly, individual membrane keys can be harder to distinguish by touch, since the height of the key is minimal, when compared to mechanical keys (e.g. the keys on a computer keyboard or an ATM). If the entire key cannot be raised, a raised area surrounding each individual key will make the key easier to tactually distinguish.
Numeric keys should have a different shape to function keys. Numeric and function keys should be placed in obvious groupings.
When command keys are vertically arranged, 'cancel' should be the uppermost key and 'enter' the lowest. When the command keys are horizontally arranged, 'cancel' should be located the furthest left, 'enter' the furthest right. It is better to position the command keys to the right of the numeric keys. They are then less likely to be inadvertently touched when entering numerals. Where command keys are positioned beneath the numerical keys they may pose a problem to visually impaired persons because they are likely to be pressed accidentally when entering numbers. Command keys should be as large as possible so that the words on them can be larger and thus easier to read.
In numeric keypads, there should be a raised tactile marking on the number 5.
The results of an investigation into the tactile features of membrane keypads suggest that, with regard to tactile navigation of a keypad, button shape and button texture are both acceptable means of distinguishing between buttons on a keypad.
Regarding button texture, in the abovementioned study it was revealed that a rough texture provided the best tactile feedback for button distinction. In cases where a rough texture is not feasible, an acceptable alternative is to provide two different textures on the keypad - a smooth texture on the buttons and a rough texture on the surrounding area.
Some membrane keys make an obvious 'click' when pressed. This can be both felt and heard. This provides a tactual and audible indication that the key has been successfully pressed.
The membrane keys should be as raised as possible, to make the key and surrounding key area as tactually distinctive as possible. Another alternative is to have a raised edge surrounding the key. If possible, the raised area should also be rougher than the surrounding area, to further improve tactual distinction.
With respect to colour there are three areas on the keypad:
- The key
- The key label (a number, letter, word or symbol)
- The area surrounding the key
Ideally these should have three different shades or colours.
High contrast colours should be used for the key itself and the surrounding area. A dark key colour on a lighter surrounding area will make the key stand out.
High contrast colours should also be used for the key background and the key label. A light label on a dark background is effective.
If possible avoid using the same colour for the key label and the area surrounding the key, as this can reduce the effectiveness of the high 'key:key label' contrast.
The conventional colour coding for the following function keys exists:
Red: Cancel Yellow: Clear or Correct Green: Enter or Proceed Blue: Help or Information
Matt and anti-glare materials should be used where possible.
The force required to press the key should not be excessively high. However the force should not be so low that the keys can be accidentally pressed when a user is investigating the location and function of the buttons by touch. Some guidelines suggest that it should not be greater than 22.2 Newtons (irrespective of the orientation of the control). However, other guidelines specify a pushing force of not more than 2 Newtons.
Keys should not require fast sequential pressing (i.e. pressing the button two or more times in quick succession).
A larger number of keys is preferable to having a small number of keys, each with multiple functions.
Keys should not have to be held down for an extended period of time.
- ES 201 381 (December 1998) Telecommunication keypads and keyboards: Tactile identifiers.
- ETSI ES 201 381 (December 1998) Telecommunication keypads and keyboards: Tactile identifiers.
- IEC 73 (1990) Colours of pushbuttons and their meanings.