Why does black reflect better than white?

[Gweedz]

Go to a new car dealership and you'll see all the back (and dark) cars with an almost mirror-like reflection on the body, yet the white cars just look white.

 

"Common sense" would indicate white would reflect better. Why the disconnect?

[StringJunky]

Assuming the same surface characteristics - glossy - the reflections are the same but the contrast range of the reflections is greater in black. Black starts dark and lightens through shades of grey to white. The reverse doesn't happen so much with white.

[Janus]

While the white does reflect more total light, quite a bit of it is scattered diffuse reflection. This tends to wash out the directly reflected image. With dark or black paint, most of the light you are seeing is from the reflected image and thus it is easy to see it. Assuming equal reflectivity of the surface, the light from the reflected image is the same. It is just easier to see it against the dark paint background than it it to see it against the white paint background.

[Strange]

This reminds of when I used to write 3D graphics code. When modelling reflections (simply) diffuse reflections are based on the colour of the surface while specular reflections take the colour of the light source.

[studiot]

 

contrast range

 

 

This is the correct term when considering these types of effects.

 

You will know you fully understand it when you can explain to someone how a (light) projector can show black on a white screen.

[Sensei]

Such things should be measured by precise instruments, not rely on naked eye.

f.e. place laser in front of detector measuring its power,

then point it on reflective material, and use detector to measure reflected beam power,

repeat with other material..

Compare results.

[Gweedz]

You will know you fully understand it when you can explain to someone how a (light) projector can show black on a white screen.

A white screem shows black by not reflecting light. This is why the darker the ambient lighting, the deeper the blacks. Put the screen in sunlight and everything is washed out.

 

So using the projector screen analogy - it would mean a white car at night should be ideal for showing an image. But the black car at night still provides a clearer (more defined) image.

 

I tested this one night by standing close to my dark blue car and shining an LED flashlight downward to the door at a 45 degree angle. There was a clear light circle on the ground. When doing it to my neighbor's white car I got a huge diffused reflection - almost unnoticeable reflection.

 

I'm tempted to say it is a physical property of the pigments but then again this also happens with glass and a black or white sheet behind it.

Edited June 19, 2015 by Gweedz

[studiot]

Forgive me but I thought a projector projected not reflected, hence its name?

 

[Gweedz]

While the white does reflect more total light, quite a bit of it is scattered diffuse reflection. This tends to wash out the directly reflected image. With dark or black paint, most of the light you are seeing is from the reflected image and thus it is easy to see it. Assuming equal reflectivity of the surface, the light from the reflected image is the same. It is just easier to see it against the dark paint background than it it to see it against the white paint background.

If this were the case then if all ambient light is removed (say in a dark garage) and there was only one light source (say a small bulb on the floor), it should reflect more defined in the white car than black?

I'm trying to devise an experiment I can do to show white is more reflective than black. By reflective I mean "mirror like". Because so far all my observations prove contrary to that, and contrary to what I was taught in school. Assuming the surface quality is the same between the two paint jobs, I would expect the same clarity (but at different contrast).

Forgive me but I thought a projector projected not reflected, hence its name?

 

Sorry about that - when I was saying "projector" I was actually referring to the projector "screen". I edited my post to hopefully make it less confusing. Thanks for catching that.

[StringJunky]

When you measure the light intensity on a dark shiny surface and then measure the black surface with ambient light on it, the difference in luminosity between the two is greater than the same two measurements on a shiny white surface. What this means is that a greater range of tones (contrast,) from dark to light, can be reflected by a shiny black surface. Think about it, in white you've got a range of ambient-lit white to very bright specular white highlights vs jet black to very bright specular highlights which are white.

[swansont]

You need to take care that the material doesn't have a coating, because the reflection from that surface will have nothing to do with the pigment.

[StringJunky]

You need to take care that the material doesn't have a coating, because the reflection from that surface will have nothing to do with the pigment.

Thinking about it's not simple and straightforward it, there's things to take into consideration. Although I haven't read it all through and comprehensively, Feynman's Strange Theory of Light and Matter deals with it ....it's quite an involved subject. IIRC.

[mississippichem]

You need to take care that the material doesn't have a coating, because the reflection from that surface will have nothing to do with the pigment.

Yep. Most coatings contain gloss additives that are formulated to gives certain amounts of specular reflectance at various angles to the surface.

 

The paint formulators for the automotive paint may have chosen, for some aesthetic reason, to make the black version more glossy than others. They would include enough gloss additives to where the reflective properties of the pigment matter very little.

[Asimov]

Gweedz: Yup, the black seems brighter by contrast. However the white reflects more.Black is the absence of all the visible wavelengths. White is all the visible wavelengths combined.