Contents

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

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

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

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

Coats Admiral Product Range

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	75	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

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