Light reflectance with relation to colour

15 Mar Light reflectance with relation to colour

The difference in lightness between two colours is probably the most important factor contributing to the visual experience of a pattern or a form.

If the lightness difference is very small, you tend to read
a form as a whole. While with a large lightness contrast between two colours, you tend to read a form as two separate entities.

Additionally, from a distance, the contrast of lightness between colours on buildings is almost the only apprehended contrast. The difference in hue barely stands out.

At close range, the contrast of lightness is a very important element when, for example, reading or distinguishing parts of a façade.

Lightness is not a quality of a unique colour but can be determined by comparing it to a grey scale or through instrumental measurement – it’s also important to remember that lightness is not the same as whiteness.

Luminous reflectance factor Yl, by some called LRV, is
a physical measure of how much of the incident light is reflected from a colour material. It is given as a percentage of how much a perfectly reflecting diffusor (white) would reflect under similar conditions. White has the value Yl = 100 and black the value Yl = 0. If the scale between white and black is divided into equal luminous reflectance steps, this scale does not represent equally large visual lightness differences, as does however the NCS Grey Scale. (A grey colour which is visually perceived to lie midway between white and black has a value of Yl = 26%.)

NCS Lightness, on the other hand, is indicated with the visual lightness value v on a scale between 0-1. Black has v = 0.0 and white has v = 1.0 on a scale with uniform visual steps. (A grey colour visually in the middle has v = 0.5.)

A yellow would have the highest lightness value – as can be seen from the NCS Triangle below showing the light grey lines with the luminous reflectance factor (Yl) and the visual lightness value v close to the whiteness value

 A blue by contrast would have the lowest lightness value – as can be seen from the NCS Triangle below showing the light grey lines with the luminous reflectance factor (Yl) and the visual lightness value v close to the blackness value

 Around the world we are seeing increasing regulations governing the use of colour externally. These regulations
tend to focus on the light reflection for reasons of heat absorption of surfaces and the affects this has on construction and bonding materials. In most cases the luminous reflectance factor (Yl), must not fall below 20 or 25, v = 0,4 or 0,5. In many countries there are also institutions that determine how public places should be planned in order to obtain a good environment for partially sighted people. In conclusion, it is lightness and the difference in lightness between two colours that plays a primary role in how we consider our surrounding environment.

 Lightness and reflectance information are given for the colours in NCS Exterior and can be found in the Exterior Block. In addition, values for all 1950 NCS Original colours are also offered in the NCS Atlas.

 For comprehensive information the NCS system and it’s application as the national colour standard for South Africa – SANS 1091 – 2012 see http://www.store.sabs.co.za/sans-1091-2012-ed-2-01

 The NCS Colour course is recognised in South Africa by the South African Institute of Architects and the South African Institute of Interior Design Professions. Its forms part if the continued practice development points – CPD – for the built professional. Each day course has a value of 2 CPD points. Please see www.ncscolour.co.za for available dates.

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