What do we mean by colour contrast?Typically we think of contrasting colours as ones that come from the opposite sides of the colour circle.
We might say yellow contrasts with blue or red contrasts with green. These are contrasting hues. But colour contrast has two aspects: tone and hue.
Tone and hue
Tone is best understood by imagining what a building or room would look like on a black and white photograph. The Disability Rights Commission suggests that a very good way of assessing the contrast in a particular environment, is to take a photograph and then reduce the image to black and white. It's not too difficult to understand tone and the relevance it has.
Another way of getting an idea of contrast is by almost completely closing your eyes and just peeping out.
Bright red and blue contrast greatly in terms of hue, but very little in tone, while light grey and dark grey have no hue contrast but very good tonal contrast.
A difference in tone is usually more important than a difference in hue for people with impaired vision. And yet it's interesting to think how many taps indicate hot and cold by red and blue detailing.
Where hue is concerned Yellow seems to be particularly easy to see and it stands out in many situations, especially against black. While Red stands out in good lighting conditions it tends to disappear as light fades. Lighting is an important factor in achieving adequate contrast and avoiding harsh glare.
People who are what's called 'colour blind' have a deficiency on one pair of receptors in their eyes - and red/green is the most common. This combination should not be used alone to denote on/off switches and controls. Text or pictograms should be used.
LightnessThe lightness of a surface can be judged in two ways: by visual comparison with samples having known lightness values, for example a grey scale, or by instrumental measurement. The first method gives us visual lightness.
Grey page from NCS AtlasIf we look at a grey scale with equal intervals between white and black, like the one on the right hand side of this NCS Atlas page, black has the least visual lightness and white has the most. But it's very confusing to find that the middle grey S 5000-N has a visual lightness of 0.5 on a scale of 0-1 but a light reflectance value of only 26 on a scale of 0-100.
There is a constant relationship between visual lightness and light reflectance values and on the NCS Atlas pages the lines that plot visual lightness and LRVs are the same, however it's an indication that lightness is not an intuitive thing.
On each of the pages in the NCS Atlas there is a scale of nine points of visual lightness and LRV. This allows the LRV to be known when selecting colours. The pages illustrated have the visual lightness 0.5/LRV 26 marked by arrows. You will see the position and angle of the lines change on each page. This is due to the different wavelengths of the colours.
Now look at the four samples from the atlas pages illustrated. Each has the same nuance of 2060: that is the same perceived blackness of 20 and the same perceived chromaticity of 60. But the light reflectance values change from 20 to 40.
30 points colour contrastThe guidance in the Equality Act depends on instrumental measurement of lightness and requires a difference of at least 30 points LRV between relevant adjacent surfaces, for example between the door, the wall and the floor. People with impaired vision tend to scan the floor area up to 2 metres in front of them and up to shoulder height in order to navigate their way around. So the features in this field of vision are the most important, while the ceiling is often the largest area of uncluttered colour and helps them to gauge the size of the room.
The following clauses in Part M relate to accessibility for people with impaired vision:
0.29d definition of visual contrast
1.26f,m Ramps and kerbs
2.17d Door furniture
2.21g Manual door controls
2.24a Manifestations on glass
3.18 Signs for lifting devices
3.28d,e Lift control buttons and face plates, lift floors
3.37,3.34i,k, 3.43k, m Lifts
3.8, 3.10e,f,g,h Door openings, furniture, frames, leading edges, vision panels
4.24 Fire alarms
4.7 Seating in audience facilities
4.28 Switches, outlets and controls
4.30 Lighting pull cords
5.4k Sanitary fittings
9.5a Step nosings
In BS8300:2009 new guidance suggests that in some cases e.g. between door furniture and doors, 15 points difference in LRV might be sufficient. A graph is provided for guidance on situations where a difference of 20 points or more might be acceptable for surfaces with illuminance of 200 lux or more.
While 30 points difference in LRV seems very straightforward, Part M uses the term visual contrast in its subject index and contrast visually in its Definition 0.29. So definitions can be confusing.
Both RNIB and DRC refer to visual lightness in their practical guides for DDA.
When we look at black and white versions of a scene we are judging visual lightness not LRV. It is important not to confuse visual lightness and visual contrast.
LRV - Light reflectance valueLRV is an instrumental measurement made using a spectrophotometer. It is equivalent to CIE Y and is the proportion of visible light reflected by a surface, weighted for the sensitivity to light of the human eye.
LRV is expressed on a scale of 0-100 where absolute white has a value of 100 and absolute black has a value of 0. In practice white may be about 85 and black about 6.
For people with adequate vision, difference in hue or chroma (colour intensity) provide sufficient visual contrast. But for people who are visually impaired the main feature of a surface which determines the ability to identify differences in colour is the amount of light the surface reflects, or its LRV.
Colour specification which complies with Part M requires the right information, the colour specifying tools - and more knowledge about colour.
Next > >
NCS – Natural Colour System®© property of NCS Colour AB, Stockholm 2019. References to NCS®© are used with permission from NCS Colour AB.