Reflectivities of common materials.

[donjoe]

Does anyone know of any database/list of the reflectivities of various common materials one would be likely to encounter while shining their lasers around? You know, concrete, wood, plastic, clear glass etc. This kind of information could be very useful in deciding what stuff to stay away from while using lasers of various power levels + eyewear of various densities.

AFAIK, clear glass will reflect up to 8% of incoming light if it's hit at a normal angle (or is this wrong?) and progressively more as the angle increases, reaching almost 100% reflectivity at almost 90 degrees (i.e. light beam barely grazing the surface of the glass).

 

[Benm]

Albedo - Wikipedia, the free encyclopedia - scroll down a bit for some example materials.

This only goes for matte surfaces. Afaik the reflection of a single pane of window glass is ca 4% at a normal angle, so a double pane window could refect 8% in total (and at a normal angle these would overlap).

 

[donjoe]

Cool, thanks, I wouldn't have thought of the term "albedo" in a non-climate context.

So from Wiki and from this we have so far:

• Material, Albedo
  
• New asphalt, 0.04 - 0.05
• Black acrylic paint, 0.05
• Aged asphalt, 0.1 - 0.12
• Conifer Forest, 0.08 - 0.15
• Bare soil, 0.17
• Deciduous trees, 0.15 - 0.18
• "White" asphalt shingle, 0.2
• Green grass, 0.25
• Aged concrete, 0.2 - 0.3
• Desert sand, 0.4
• New concrete (traditional), 0.4 - 0.55
• Ocean ice, 0.5 - 0.7
• New concrete with white portland cement, 0.7 - 0.8
• White acrylic paint, 0.8
• Fresh snow, 0.8 - 0.9
• Water, 0.03 - 1.0

 

[Bluefan]

Albedo - Wikipedia, the free encyclopedia - scroll down a bit for some example materials.

This only goes for matte surfaces. Afaik the reflection of a single pane of window glass is ca 4% at a normal angle, so a double pane window could refect 8% in total (and at a normal angle these would overlap).

The reflection of a single glass to air or air to glass transistion is ~4% at normal incidence. A single sheet of glass would have two of those reflections and at an angle the reflection can go up a lot.

 

[joeyss]

Hey guys just so you know. COLOR is very imporatant for things that aren't black or white. THe opposite of the color will be like black. Ex hitting a green object with a red laser will not reflect much while a green will. Keep this in mind.

 

[Benm]

Thats true, albedo is specified for the average of the visible spectrum unless otherwise indicated.

The reflection of a single glass to air or air to glass transistion is ~4% at normal incidence. A single sheet of glass would have two of those reflections and at an angle the reflection can go up a lot.

Not really, the reflection from exiting the glass is very small. Just try boucing a laser off a doubly glazed window: this results in two major reflections at any angle other than 90 degrees, not in four.

There IS a reflection of the exit sufrace, but its usually very dim, and at shallower angles it hardly there at all since you get total inner reflection.

 

[Bluefan]

I checked, the reflection at normal incidence is given by:
R = ((n_t - n_i)/(nt + ni))^2
And thus it doesn't matter if the interface is glass to air or air to glass, going from an idex of 1 to 1.5 or the other way around gives a 4% reflection per interface. A single glass plane has 2 interfaces. A doubly glazed window will have 4 reflections but because of the thin glass planes they will be overlapping.

 

[Benm]

Hmm.. did you calculate that the reflection for the far transition (air into glass) also has to be transmitter by the near plane?

Just try for yourself and shine a laser at a ca 45 degree angle onto a single pane of glass. You will see 2 refections, but the one from the near plane is much brighter than the other one.

 

[Bluefan]

I didn't include that, in case of normal incidence the 4% loss when entering the glass in calculating the second reflection can be neglected. If we don't neglect it: the second reflection will have two extra losses, it will enter the glass and lose 4%, reflect and lose 96% (4% left) and again lose 4% when exiting the glass. That leaves 3.6864% from the second surface, roughly 4% as approximated.

At angles these two can be futher apart because the second reflection only starts with what the first has left passing, which can be more or less depending on angles and polarization. I neglect futher bouncing in the glass or interference effects.

My point was that the second interface reflects the same fraction of the incoming light, it does not matter if it's glass air or air glass. But it's true that the reflections aren't equal in intensity. In double glazed glass the two reflection from the second glass plane will both be lower in intensity than the reflections of the first glass plane (if we don't neglect losses from earlier reflections that cause this). The further away the lower the intensity of the reflection.

 

[Benm]

I'm quite sure the difference is much bigger. When i look at the reflection of a laser from a double glazed pane of glass, i can see 2 reflections that are about equally bright, and 2 that are much dimmer (but about the same intensity between them).

The only plausible explanation for this is that the reflection from the far edge of a glass pane is less intense. Obviously the reflection of the second pane is a few percent lower that that of the first, but this isnt visible by eye.

 

[Bluefan]

At what angle did you try this? Unless you have like mirror coated glass (or something besides normal glass that I didn't account for), the physics isn't different.

 

[Benm]

It seems you're right - at decent angles all 4 dots are very close in brightness. I've noticed the bright-dim-bright-dim pattern at very shallow angles. Perhaps some curvature of the glass also has to do with it, as these are household doubly glazed windows which arent exactly flat to within a wavelength.