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aostling
01-03-2011, 06:09 PM
In San Francisco last week I came across this street sign. The lettering on the Peralta sign appeared to be white lettering on a black background. In fact it was black lettering on a white background, just as for the Rutledge sign. The specular reflection at the slight grazing angle between the sign surface and the sun had effected some sort of magic.


http://i168.photobucket.com/albums/u183/aostling/tonereversal.jpg


As you can see below, the effect disappears at larger viewing angles. Has anybody heard of this phenomenon before?


http://i168.photobucket.com/albums/u183/aostling/unreversedtones.jpg

dp
01-03-2011, 06:17 PM
I thought those things only happened at Haight and Ashbury. That is an intersting effect, though.

Evan
01-03-2011, 06:22 PM
Judging by the appearance of the white background it is retroreflective material. The black isn't for obvious reasons. The reteroreflective material isn't allowing a significant amount of forward scatter as it is designed to produce back scatter. The black enamel is highly reflective at grazing incidence and therefor appears lighter.

Incidentally, the full Moon is about twice as bright as the nearly full Moon. This was an unsolved problem until samples of the soil were returned. It happens that the soil is a fairly effective retroreflector and returns a significant amount of the sunlight back to it's source regardless of angle of incidence. This makes the Moon appear brighter than it should when the Sun is almost directly in line with the Earth and darker than it should at other times.

aostling
01-03-2011, 08:40 PM
I thought those things only happened at Haight and Ashbury. That is an interesting effect, though.

It's a logical certainty that at some angle the effect will be neutralized, the background and the lettering appearing as the same uniform shade of gray. I would have photographed that if I'd thought about it at the time.

I can't think of any application for this phenomenon. Something to file away in memory, though.

Evan
01-03-2011, 10:42 PM
I have seen the effect other places involving non specular reflectors. I can't think just where right now. There are effects very similar to that which are exploited in the construction of gas lasers. It is called the Brewster effect and the manifestation is the Brewster Angle of various non specular materials, notably glass. At a particular angle light of one polarization passes through the material without loss while the other polarization is reflected. Changing the angle changes the ratio of reflected light of each polarization.

Hmm. I wonder if that is how the Real 3D projectors work. A spinning Brewster Window....There would be no flicker as the illumination would change sinusoidally.

Optics Curmudgeon
01-04-2011, 01:06 AM
Evan's dead on with the retroreflection thing. Get a piece of Scothchlite or equivalent at the home center, the white or red stick-on stuff. If the background (behind you) is dark, then the surface looks dark. The playa in dry lakes in the Nevada desert exhibits a similar effect, the prevailing wind orients the grains and you can see the effect on reflection and polarization from the air, with the right apparatus.

Joe

john hobdeclipe
01-04-2011, 08:26 AM
You can get a similar effect with a black & white photo negative.

If you hold it so that the background behind it is very dark, and there is some light coming from behind you, you can wiggle it around until the light reflects off of the emulsion, but you look right through the parts of the negative that more or less clear. The result is reasonable representation of a positive image.

Evan
01-04-2011, 08:54 AM
You just jogged my memory. The original Tintype process depends on a similar effect. The Tintype is actually a negative image formed on japan black laquer. By using a black background the dark areas of the tintype appear lighter than the backgound and the light areas allow the background to show.

Mark Hockett
01-12-2011, 12:07 PM
The instrument cluster in my Toyota truck has a similar effect. During the day it has a white background and at night when the dash lights are on the background turns black.