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Name: Michael 
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Question:
We make acrylic baths and like most of the UK, they are in white. Our end users sometimes say the bath is not the white they ordered but this is because the acrylic seems to absorb the reflected light from surrounding materials and does appear to change colour. We know this is not the case but the consumers insist the bath is taken out.......no surprise in daylight the bath is OK. Is there a name for this phenomena?



Replies:
Hi Michael -

I think our language has not quite adapted well yet to our bold new abilities in color. To refer to this effect, people first start talking about colors and illumination, then they use the phrase "color bleed". The context differentiates between this color bleed and the traditional meaning: the partial transfer of dyes, facilitated by liquid contact. Because this color bleed is due to cross-illumination, I would like to name it "illuminant color bleed". No clue whether my new name will stick.

When I searched Google for "illumination color bleed", I got lots of references to photo-realistic 3-D picture-rendering software, which often has this color bleed problem (worse, even) as it tries to draw pictures of objects described using only word-models. So programmers and artists making or using this software think about this problem often enough. Programmers want to their more advanced software to show color bleed "accurately", as it would appear in the real world, precisely to be used for interior decoration planning. Artists making computer pics with the software always have to deal with color bleed, pro or con, much as your business does. So it is largely the same problem as yours even if the software is abstract.

It might be worthwhile to find modestly priced software to do comparative studies on lighting shapes and color arrangements over a white tub. Some consultant might be worth hiring briefly too.

"White" is defined as near-100% reflectivity, but with random return directions. The reflected light is scattered, diffuse, not specular. Most real objects accomplish whiteness by being transparent except for having lots of little break-points inside. Like broken glass, bleached sand, or refined salt, the breakpoints are transparent but partially reflecting, because the index of refraction changes at them. They are partial mirrors, but tiny and chaotically arranged. So every white substance has a scattering depth, the depth light penetrates, on the average, before being turned outwards. If this depth is larger than the object's dimension, the object is "translucent". If comparable, it's "milky". If much shorter, the object is "white".

Your acrylic is more translucent than, say, porcelain-on-steel. Light entering it has a longer penetration depth before being bounced back out in random directions. However I am not sure this matters much to the color bleed.

Your acrylic is whiter than most ceramics. This might matter for a couple of reasons. One, adding a little darkening, making the material's color 80% white instead of 90%, noticeably shortens the effective scattering depth. Many of the rays that try to bounce around deepest inside are absorbed before they escape. Two, the substance's pigment might be off-white, tinted towards a color that is aesthetically compatible with the cross-illuminating color.

More relevant, I think acrylic has more glossy surface reflection than porcelain. Maybe its index of refraction is a little higher, and that would be why. Or maybe because ceramic is more like rough rock with the protruding tips chopped off flat, whereas acrylic is all gloss, like smooth glass with white paint behind it. Anyway, the portion of light that reflects off the gloss is highly like to come from the tub surround. That might be a noticeable distinction.

It is possible for a white substance (plastic or ceramic) to be not quite "Lambertian". For example, if your white plastic were filled with tiny glass spheres, some of the reflection will be retro-reflection instead of randomly scattered over wide angles. The shape of the filler powder, compared with the shapes of the mashed-together crystals and voids in ceramic, might make one of them reflect more side-incident illumination towards the observer. Of course we probably can not afford to influence this; the materials may be too exotic or expensive, and the amount of experimentation great.

It is a pretty tough problem. I am not sure how much you will be able to really do about it. Customer education before the sale might be the biggest thing, so they choose an arrangement that can work right.

One thing comes to mind. If the white rim of the tube has a white vertical skirt or tile-border adjacent to it, that white skirt covers a large solid angle as seen from a spot on the tub rim. The rim, at least, will then look quite white, with the white border above it. Perhaps the inside walls and bottom will then pick up the purple ceiling and look much more tinted than the rim. After all, the inside is a white cavity; light definitely bounces around in there, and at least half of it faces the colored surround. So it is only a partial fix, or perhaps even an interesting deliberate effect, white with color-in-the-hole.

The shape of the ambient illumination probably matters a lot. Direct light from one spot will be different than diffuse illumination reflecting off a wide white wall directly opposite the tub. I am not sure which is better or worse. I suspect that direct illumination minimizes color bleed and diffuse makes it worse. Or is it the elevation of the light source? That sounds more sensible to me. White light shining down on the tub will allow it first chance to reflect white light back up to the observer. Side lighting is more likely to bounce off the purple wall around the tub before illuminating downwards, exacerbating color bleed. It implies that keeping a white ceiling is important, so that diffuse daylight illumination comes largely from above, and so not all the surroundings are colored. I think it also implies that illumination coming through a window within the tub surround will usually have problems. Its light will usually be horizontal and bounce off all walls multiple times, picking up strong color tint. If it is a south-facing (northern hemisphere) window it will not be so bad, because direct sun rays will often be slanting down through it, heading first towards your white tub. Light straggling in from a window at the far end of a narrow bathroom will always be miserable. If one could install a solar-tube, or reflect the window with down-tilted mirrors near the ceiling, that would fix it. Having at least the top half of the wall opposite the tub painted white would help some too. It constrains your fashion a little, but good illumination is worth learning in its own right.

Due to their diverse situations some customers will not have this problem, others will and might be glad if you could show them why and what kind of things fix it.

Offer interesting white skirts, show photos pro and con, educate about illumination. Maybe offer tints and look into increased filler opacity (if it even matters). Learn for yourself by reading and experiments (in studio or in software). That is all I could hope to suggest.

Sorry to be so longwinded. Actually an interesting question.

Jim Swenson


This is a color spectrometric phenomenon called "metamerism". This is the technical term for the fact that the apparent "color" of light reflected from a surface depends upon the spectral distribution of the incident light. Stage and hair colorists, as well as paint and artist colorists, are very familiar with this effect. The effect can be quite subtle or dramatic. Where this problem is addressed, at least in the US, in paint stores is the presentation of paint swatches in a mix of incandescent and sun light boxes. This way the customer can "see" whether the color selected matches the color they will "see" when the paint is applied indoors or outdoors -- not perfect, but as good as it gets. Chemically, acrylics should not be the culprit. Acrylics are insensitive to all wavelengths of visible light. However, the same cannot be said of the "white" pigments that go into the formulation. The "color" of a bath in sunlight is going to be quite different than the same material in candle light. That is the problem you have to address.

Vince Calder



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