colour inconstancy

8/8/2019 Colour Inconstancy

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Colour Inconstancy



What is color inconstancy ?

Colour Rendering Property: (For light source) - Thecharacteristics of source to reproduce the colour observedunder reference light source. E.g. The fluorescent tube andmercury lamp have poor colour rendering properties.

Color inconstancy is the change in color of a single sampleunder different lights.



The magnitude of color inconstancy can be defined by DE

CMC of the sample between two lights.

The ISO standard for shade (in)constancy is called

CMCCON and it refers to the DE CMC for a sample

between D65 and a second light source.

Given the CIE L*a*b* values for a sample in D65 and in a

second light source it is possible to calculate the

CMCCON for a shade.



As shown in Figure 1, product color can changesignificantly in going from store light to daylight.

This change can be unexpected and could leave theconsumer less than satisfied with their purchasing

decision.

Therefore, color inconstancy is a problem that needs amore complete analysis.

Figure 1 : A color named POPPY viewed in 3

different lights (Ultralume, D65, CWF)



How is it different from metamerism ?

Shade inconstancy is the change in color of a single

sample.

Metamerism, on the other hand, is the change in color difference between a pair of samples.

Normally, luminant metamerism is of greatest interest

although factors such as observer and geometry can alsoplay a role.



The magnitude of metamerism can be measured by the

change in DE CMC for a sample pair between any twoluminants. So there are two DE CMC values involved. Ametameric pair could have a DE CMC=0 in one light anda high DE CMC in the second light.

Creating shades with a low inconstancy in itself does notimprove the chances of getting matches with a lowmetamerism.

It is entirely possible to get high metamerism matches for color constant shades and it is also possible to get

nonmetameric matches for color inconstant shades.



Metamerism can only be controlled by using the similar

colorants in the color standard and the sample.

Attempting to reduce the color inconstancy of a color standard does not in any way reduce the possibility of

metamerism.



By analyzing 2300+ colors in a textile palette covering a

wide gamut it was found that 10 to 20% of the colors

have a very high color inconstancy in going from D65 to

incandescent light with color shifts of DE CMC greater

than 6 units. Most colors showed a color shift of morethan 2 color units.

Calculations with other color palettes and light sources

showed similar results.

What is the extent of color inconstancy ?



Which colors show the highest inconstancy ?

Upon taking a closer look at the shades with high color inconstancy between D65 and incandescent light it wasseen that red-oranges and bright blues particularly those

of medium depth show the most pronounced color inconstancy.

Figure 3 shows the top 100 colors with the highest values

for CMCCON.



The exact set of colors that show the highest color inconstancy depends on the pair of light sources beingused.

Figure 4 shows the results for D65 and TL84. Here also

we see that colors covering a certain region of the color space show the highest inconstancy although this regionis somewhat different from what we saw for D65 andincandescent lights.



Further insight is gained by considering the reflectance curves for

the colors with a high inconstancy. If we normalize their reflectance

curves (divide by maximum reflectance for each curve), these

curves fall into two broad families as shown in Figure 5.



These are colors that

show a large

proportional change in

reflectance between the

low and high

wavelength limits of the

visible range. Thespectral energy

distribution for

incandescent light

shows a monotonic

increase over the

visible range.

Therefore, colors with the highest change in reflectance are the most

inconstant between D65 and incandescent light.



The results obtained here are not a characteristic of theparticular color palette used in this study.

Calculations with other color palettes showedqualitatively similar results.

Color inconstancy therefore depends primarily on thetype of color and the luminant pair.

Metamerism, on the other hand, depends on the type of colorants used and the luminant pair.

Color constancy depends on the broad shape of thereflectance curve for a color whereas metamerismdepends on smaller differences in reflectance curves.



Can shade inconstancy be controlled ?

Yes and no. Modest changes in shade inconstancy can be made by

altering the recipe used to make a color. Inconstancy isprimarily determined by the color itself and onlysecondarily by the colorants used. Some colors withcertain characteristic curve shapes will ALWAYS becolor inconstant regardless of the recipe used.

In attempts at reducing shade inconstancy for the top100 inconstant colors it was found that only modestgains were possible by altering the recipe.



There is some room to maneuver by recipe changes butfor the most part it is true to say that shade constancy ismostly determined by the shade at hand not so much bythe recipe itself.

Further, a less shade inconstant recipe does not in anyway reduce the chances of metameric matches.



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colour inconstancy

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