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View Full Version : What's the difference between a 'white' material, and a mirror?


alex101
10-23-2014, 06:18 AM
They both reflect all visible light, so why do they appear so different? :confused:

Patrick1
10-23-2014, 06:23 AM
A mirror produces specular reflection; an incoming light ray bounces off in a single direction. A matte white surface produces diffuse reflection; incoming light rays reflect off in many directions (called scatter). A glossy white surface will produce some degree of both these effects.

alex101
10-23-2014, 06:26 AM
But what about a glossy white surface vs a matt reflective surface? White still looks white, and the reflective still looks... different (not white) ...

Patrick1
10-23-2014, 06:27 AM
I'll add: although we naturally tend to think of white surfaces as being brighter than a mirror, reflected light in a mirror (or especially the reflection of the sun on a curved mirror-like surface) can be a zillion times brighter than a white surface.

davidbriggs
10-23-2014, 09:23 PM
Alex, I'm not sure if this is what you mean by a "matte reflective surface" but a metal with a microscopically rough surface reflects light entirely at the surface, but over a wide range of angles because that surface is uneven. A glossy white object on the other hand reflects 5-10 percent of the light in a specular fashion at the surface, and most of the remaining light in a diffuse fashion from within.
http://www.huevaluechroma.com/021.php

WFMartin
10-26-2014, 12:33 AM
Consider the sun as the white light source....

A white material only reflects a fraction of the white light of the sun as the sunlight strikes it. If is it a white cloth, there is a tremendous amount of random scattering of the light rays as it is reflected. Even a white egg, or a shiny Corvette only reflects a fraction of the total sunlight that strikes them, but more than a cloth napkin.

Logically, this makes sense, since you can gaze directly at a white napkin in bright sunlight, or even at a white Corvette in direct sunlight, whereas if you were to gaze at the sun, you would damage your retinas.

On the other hand, a mirror reflects the sunlight with a profoundly greater efficiency than a white "thing" or "object".

However, simple, understandable, laws of physics dictates that even a mirror is not perfectly, 100% efficient in terms of reflecting light. Even when painting reflections in a mirror, they should be created just a tiny bit darker in value than the objects being reflected in it.

It is for this reason that when painting reflections of a scene in water, I always try to paint the reflections just a tad bit darker than the objects that created them.

That's one reason that reflections of skies in water always appear darker than the sky, itself, if only a small amount. This is assuming, of course, that the goal of the painter is that of reality.;)

davidbriggs
10-26-2014, 04:33 AM
White objects and mirrors attain a similar range of percentage reflectance - e.g. around 90% for a titanium white paint (Munsell value 9.5) and up to 98-99% for a magnesium oxide coating, compared to 90-95% for ordinary metallic mirrors up to more than 99% for HR mirrors. As Patrick said, the difference is that a mirror produces a concentrated specular reflection while a matte white object produces a diffuse reflection.

Bastet469
10-27-2014, 01:05 AM
Funny you ask this question. I was about to ask if anyone has tried Tri-Art's Liquid Mirror acrylic paint. They hype up it's reflectivity quite a bit.

"It is an opaque pigment that can be described as metallic or platinum in colour.... it has superior reflective qualities than all other pigments, including Iridescents."

The company already makes iridescent paint in both silver and pearl but claim that Liquid Mirror is markedly different. The exclusive shine properties of Liquid Mirror are attributed to the use of something called bismuth oxychloride. The result, according to art guru Marion Boddy-Evans of About.com, is:

"a pale, very shiny, silver color. Like the shiny highlights on silver jewelry, the closest to what I visualize when I think 'silver' that I've come across in paint."

But she admits that it can't be truly appreciated in online photos or videos.However at a whopping $20 average price for 60ml, I'm not prepared to pick up Liquid Mirror on faith alone.

What do you guys think?

Patrick1
10-27-2014, 05:48 AM
Bastet,
Yes...there is no substitute for seeing a real color sample...especially 'special effect' colors. I have a Tri-Art hand-painted color chart. Dontcha love companies that actually make these things? :angel:

Their Liquid Mirror is much brighter than all other acrylic 'silver' colors I've seen. It looks like platinum. I suppose if you applied it smoothly with a knife it would look somewhat mirror-like, although in my sample you can't see your reflection in it...it's not chrome-like...it's more like whitish-silver.

Incidentally, Tri-Art's Iridesent Pearl is very similar in color; from most angles it's similarly bright, but is less opaque. It also costs a fraction of what Liquid Mirror does. I've found it useful in abstract work for bright splashes and accents.