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Metamerism and Illuminants

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Contents Jeans in store lighting

Introduction
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
Colour Temperature
Impact of Brightness and Position of the Light Source
Use of Matching Cabinets
Reducing the Effects of Metamerism

Contact us about Metamerism and Illuminants


Introduction

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.

What is Metamerism?

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

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

Ferrari lightFerrari dark

Kinds of Metamerism

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.

Importance of Light in Metamerism

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:

  1. The amount and quality of light illuminating an object
  2. 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.

What are Illuminants?

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].

What is the difference between a light source and an illuminant?

  • 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

Different Industrial Illuminants

Illuminant DesignationLamp TypeOperating TemperatureCRI (Colour Rendering Index)Usage / Kind of Illumination
CIE Standard Illuminant A (INCA-A)Tungsten halogen2856ºK100Incandescent or tungsten light, generally seen in home environments
CIE Standard Illuminant D65Phosphor daylight fluorescent6500ºK93Most commonly used illuminant resembling average daylight of northern sky
Fluorescent Illuminant TL84European commercial fluorescent4100ºK85Narrow band fluorescent which simulates office or store lighting; generally used by many merchandisers and retailers
Illuminant CWF (Cool White Fluorescent)USA commercial fluorescent4150ºK62Cool white fluorescent light, generally seen in office environments
Illuminant UVUltra-violetN/AN/AReveals the presence of fluorescent dyes and optical brighteners
Illuminant D50Phosphor daylight fluorescent5000ºK92Resembles 'horizon' daylight; it is the reference for the printing and graphic arts industry
Illuminant D75Phosphor daylight fluorescent7500ºK94Used to evaluate opaque materials, resembles northern sky daylight at noon
Illuminant U30USA commercial fluorescent3000ºK85Commercial narrow band fluorescent light
Mercury VapourHigh intensity4100ºK70Generally used in stores and factories
High Pressure SodiumHigh intensity2100ºK50Generally 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

Lighting conditions

Colour Temperature

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 TemperatureKelvin RangeAssociatedEffects & MoodsAppropriate LocationApplications
Warm3,000KFriendlyIntimate
Personal exclusive
RestaurantsHotel Lobbies, Boutiques, Office Areas, Libraries
Neutral3,500KFriendlyInviting
Non-threatening
Public ReceptionOffice Areas, Showrooms, Book Stores
Cool4,100KNeatClean
Efficient
Office AreasConference Rooms, Classrooms, Mass Merchandisers, Hospitals
Daylight5,000K to 6,500KBrightAlert
Exacting colouration
GalleriesMuseums, Colour Matching Booths, Jewellery Shops, Medical Examination Areas, Textile Industry, Printing Companies

Impact of Brightness and Position of the Light Source

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.

Restaurant lighting

Matching Cabinet 2Use of Matching Cabinets

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:


Dimensions

 WidthHeightDepth
Overall Area1,560 mm780 mm620 mm
Viewing Cavity1,520 mm560 mm590 mm

Cabinet Interior

  • 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

Viewing Backgrounds

  • The shade matching cabinet utilises neutral grey surroundings to accommodate the widest range of colours

Viewing Distance

  • 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.

Reducing the Effects of Metamerism

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.