Metamerism and Illuminants
What is Metamerism?
Kinds of Metamerism
Importance of Light in Metamerism
What are Illuminants?
What is the difference between a light source and an illuminant?
Different Industrial Illuminants
Impact of Brightness and Position of the Light Source
Use of Matching Cabinets
Reducing the Effects of Metamerism
Apparel production is a complex process, where the finished product is a mixture of various materials. Colour matching all these materials is important to meet the needs of the final consumer.
The supply chain consists of many different stages and whilst a garment may pass colour matching at one stage of the distribution, there might be a difference in shade observed at another stage. This is because of the condition of metamerism.
This technical bulletin will explain how you can reduce the effects of metamerism by adopting standard illuminants and colour matching cabinets to assess colour.
Metamerism is a scientific description of a common colour phenomenon where two colour samples which appear to match under one light source, no longer match when viewed under a different light source. Metamerism can only occur when comparing two objects; the colour of two objects match under one set of conditions but not under another set of conditions.
An example of Metamerism
A factor often confused with metamerism is colour constancy / inconstancy. The colour of an object will appear to vary depending on the light source which is used to illuminate it. For example, you park your bright red car at the roadside in daylight. Darkness falls and although you would still say you have a red car it may well appear to be brown under sodium lighting. This phenomenon is known as colour inconstancy.
An example of Colour Inconstancy
Metamerism is the differing effects upon the colours of two objects when compared under various viewing conditions. Several kinds of metamerism exist:
- Illuminant Metamerism occurs when two objects match under a certain light source to a certain observer, but do not match under a different light source to the same observer
- Observer Metamerism is caused by a difference in colour vision between observers; it is highly subjective and cannot always be avoided. However by proper selection and training of colourists the risk can be reduced
- Geometric Metamerism can be controlled by viewing the samples at the same distance from the observer and at the same (45 degree) angle to the light source
The next few sections will describe illuminants.
The energy of the light source used by the observer to view products is the most important factor affecting their appearance and in assessing the effect of metamerism. The two major factors that influence what we see are:
- The amount and quality of light illuminating an object
- Colour of the background against which an object is viewed
Accurate visual perception depends on the light source being a 'controlled' source, where the colour properties and quality is defined such as in a standard matching cabinet.
An illuminant is a theoretical source of visible light with a profile which is published. In general, the illuminant values are a measurement of the spectral energy distribution of an artificial temperature radiator that radiates heat with a specific colour at the defined temperatures of the light source, unit [K].
- Illuminants have standard values which never vary, while the energy of a light source can vary
- A light source is a physical emitter of radiation, such as a candle, a tungsten bulb and natural daylight, while an illuminant is the theoretical representation of the light source
- All light sources can be specified as an illuminant, but not all illuminants can be physically realised as a light source
|Illuminant Designation||Lamp Type||Operating Temperature||CRI (Colour Rendering Index)||Usage / Kind of Illumination|
|CIE Standard Illuminant A (INCA-A)||Tungsten halogen||2856ºK||100||Incandescent or tungsten light, generally seen in home environments|
|CIE Standard Illuminant D65||Phosphor daylight fluorescent||6500ºK||93||Most commonly used illuminant resembling average daylight of northern sky|
|Fluorescent Illuminant TL84||European commercial fluorescent||4100ºK||85||Narrow band fluorescent which simulates office or store lighting; generally used by many merchandisers and retailers|
|Illuminant CWF (Cool White Fluorescent)||USA commercial fluorescent||4150ºK||62||Cool white fluorescent light, generally seen in office environments|
|Illuminant UV||Ultra-violet||N/A||N/A||Reveals the presence of fluorescent dyes and optical brighteners|
|Illuminant D50||Phosphor daylight fluorescent||5000ºK||92||Resembles 'horizon' daylight; it is the reference for the printing and graphic arts industry|
|Illuminant D75||Phosphor daylight fluorescent||7500ºK||94||Used to evaluate opaque materials, resembles northern sky daylight at noon|
|Illuminant U30||USA commercial fluorescent||3000ºK||85||Commercial narrow band fluorescent light|
|Mercury Vapour||High intensity||4100ºK||70||Generally used in stores and factories|
|High Pressure Sodium||High intensity||2100ºK||50||Generally used in streets and factories|
Note: CRI (Colour Rendering Index) is defined as the ability of the illuminant to render colours as they would appear under true daylight. An index of 100 represents a perfect simulation of daylight.
Fluorescent Lamp Variants:
- CW – Standard Cool White
- WW – Standard Warm White
- CWX – Deluxe Cool White
- WWX – Deluxe Warm White
- ES – Energy Saving
- HO – High Output
Colour temperature is a method for describing certain colour characteristics of light sources. It is a reference number that quantifies the appearance of light. The terms ‘warm’ and ‘cool’ in lighting refer to subjective experiences, such as a warm flame or a cool winter sky.
Cool and Warm Light Sources
Light sources that have higher colour temperatures are 'cool' light sources, while those that have lower colour temperatures are 'warm' light sources.
Applications of Colour Temperature
|Colour Temperature||Kelvin Range||Associated||Effects & Moods||Appropriate Location||Applications|
|Restaurants||Hotel Lobbies, Boutiques, Office Areas, Libraries|
|Public Reception||Office Areas, Showrooms, Book Stores|
|Office Areas||Conference Rooms, Classrooms, Mass Merchandisers, Hospitals|
|Daylight||5,000K to 6,500K||Bright||Alert
|Galleries||Museums, Colour Matching Booths, Jewellery Shops, Medical Examination Areas, Textile Industry, Printing Companies|
The brightness of illumination affects the apparent purity of colours and the amount of detail visible in shadows. In general, the brighter the illumination, the more saturated colours appear and the greater the amount of detail that may be seen in shadow. The geometry of the illumination also affects the parent colour and contrasts of a product. Whether the illumination is coming from a small source or a large diffused one, the angle at which the light strikes the objects and angle of view all affect appearance.
A matching cabinet is the standard equipment for the visual assessment of colour and they are best used for colour appraisal with the below parameters:
|Overall Area||1,560 mm||780 mm||620 mm|
|Viewing Cavity||1,520 mm||560 mm||590 mm|
- The cabinet interior should be a neutral, grey colour e.g. Munsell Grey N5
- For best results, glare, extraneous lights and anything which will mal-adapt the visual response must be avoided
- Any windows near the cabinet should be fitted with grey blinds
- No direct lights should be in the field of view and the general lighting in the vicinity should be 'Artificial Daylight' to a level of 200 to 300 lux
- The wall area behind the cabinet should be finished in light grey emulsion e.g. BS00A01
- The shade matching cabinet utilises neutral grey surroundings to accommodate the widest range of colours
- The distance of the Colour Matcher to the samples being viewed should also be constant regardless of the colour of the samples. The samples should ideally be viewed at a 45º angle of incidence to the light source.
We have seen that metamerism and illuminants are important considerations in controlling and assessing colour in the manufacturing process of any coloured object. In textiles it is therefore important to use consistent quality dyestuff and to consider and use the appropriate illuminants and conditions in matching colour.
Please contact your local Sales Office to find out more about metamerism and illuminants.