Re: [magnitude6] Asphalt roads are not black

[Jan Hollan <jhollan@amper....muni.cz>]

Dear Pierantonio,

you are quite true, albedo can concern solar (or optical) radiation (UV,
visible, shortwave IR). However, if I am not mistaken, it can concern, as
in planet astronomy traditionally, light (photopic sensitivity, as planets
are bright enough). The paper on heat islands should concern solar
radiation, but, as I remember, it is not emphasised there. This is the
reason why I wrote that visual ones may be a bit lower (fresh asphalt is
less black in the IR, I have no info about old ones) and not that they ARE
lower.

I've measured a sort of photopic albedo by a canon EOS D60 camera, and the
results are the same, 12 to 15 per cent see (a letter sent just to DSLF,
but somehow never arrived there)
 Re: Asphalt and Concrete surfaces
        URL: http://amper.ped.muni.cz/jenik/letters/public/msg00120.html
 (later, from the images of sunlit white papers, I've changed the R and B
multipliers, so that R is enhanced and B suppressed,
and as R/G and G/B ratios for asphalt are some 1.2,
the albedos of asphalt should be even 1.07* higher than I computed the
first time).

The possibility that the higher asphalt reflectivity in the IR
could influence the results seems improbable -- if IR would be perceived
by the camera, it could be expected in the R-filtered pixels, but it's not
the case. Asphalt has no large R-excess.

I made some rather bad images of solar spectrum by the camera to check its
infrared sensitivity. Just one spectral feature is clearly visible (apart
from the D line in one image).  I assume it to be the B band, so the
recorded spectrum ends surely before the A band, as it is not visible.
Visually, it is visible, just there is hardly any light behind it. So, the
camera does not go behind 760 nm, there is no worry that it records IR
radiances. One of the spectra I took is in the
http://amper.ped.muni.cz/light/luminance/spectrum .

Another image there shows the detail of the grating there
(a segment of a CD) and the shadow of the telescope (I took a series of
such images to check if there is no dependence on the battery being either
full or almost empty, there is no over ten per cent, but I should make
some more images to prove it). The third image shows the apparatus -- for
seeing the spectrum, there is a cardboard tube with a slit made from two
gilettes (or one broken to halves, I don't remember any more) and a tilted
piece of a CD on the other end, in a perforated Al tube from an
effervescent C-vitamine.

Perhaps a better spectrum could be photographed by simply mimicking the
way of looking at the solar spectrum with a whole CD from a window (to
avoid illumination of the disk from above). A small lens (my Canon had
just a large one) is preferable, with a large focal ratio. The real proof
of the camera sensitivity needs to record at least three Fraunhofer lines,
so that the wavelengths could be properly calibrated.

(I had to finish the grant for the ministry of environment, so my answer
is rather delayed.)

wishing you some rain instead of sun & drought all the time,

 jenik

PS.

The next step, after getting some albedos (better could be obtained in an
open space with an overcast sky of uniform luminance) of roads etc., would
be to get their BRDF. It is definitely possible with a clear sky. The
contributon of dispersed light can be subtracted when positioning some
shadowing objects on the surface at some points. During one day (or even
one afternoon) an almost complete BRDF could be obtained (of course Sun is
never in zenith in our latitude...). I hope to start taking such images in
August or September.



> <The typical albedo is perhaps 12 per cent, 15 being quite common as
> well, or even more.
>
> Dear Jan,
>
> Do you have the spectral distribution of asphalts too? The reflection in CIE
> Photopic band (eye) or in V-band might not be similar to the albedo, which
> is defined in a larger wavelength range. What is important for us is the
> reflectance in scotopic and photopic bands. As you know there are
> differences for surfaces with non-flat spectra (typical example are plants
> which clorophill absorbs strongly light in photopic band so their reflection
> factor is smaller than the albedo).