[meteorite-list] color calibration

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 30 May 2008 00:18:01 -0500
Message-ID: <009901c8c214$88c58680$5056e146_at_ATARIENGINE>

Hi, EP, List

    The quarrel is about whether the released photos do
not make Mars look enough like Arizona rather than any
accusation that it is made to look too much like Arizona.

    The cameras on Martian spacecraft are not "cameras;"
they are multi-spectral imaging photometers -- not the same
thing. The bandwidth of the filters used is as narrow as
is possible, ranging from +/- 2.5% bandwidth down to
+/- 0.5%.

    The human eye has peak response frequencies, too,
but the eye's bandwidth is as wide as possible. The eye
is essentially an attempt to have a full spectrum imager,
exactly the opposite purpose from that behind the design
criteria of the spacecraft imagers.

    Some renderers make their Mars images dim and dark
because the sunlight on Mars is less than half the intensity
of sunlight on Earth, and Martian soils are dark, but human
eyes have "auto exposure" built in -- things look "normal"
over a vast range of local illumination. Consider the Moon.

    The Moon reflects 7% of the light that falls on it, more
in the bright areas (11-12%), less in the dark mare (5%).
The albedo of ground anthracite coal dust is 5%, same
as the Moon. When the astronauts jumped out of the lander
onto the Moon's surface for the first time, did they scream,
"OMG! It's black as pitch, black as coal! A black world
under a black sky! It's like being locked in a coal cellar at
midnight! I can't see a thing!"?

    No. They keyed the mike and said, "I'm standing on a
light gray powdery surface..." Hey! Are you blind? It's as
black as COAL! If you think the optical image processor
in the spacecraft imagers or in a Nikon is sophisticated,
it's nothing compared to the human brain. The astronauts
even had to wear gold metallic "sunglasses" (visors) to
be able to see while walking on a coal-black surface!

    And when we take pictures on the Moon, we don't
make them dim and black; we brighten them up to a
"light gray surface," just like our brain tells us to. We
do the same for Mars photography. Notice how dark
the Raw images from Phoenix are? They haven't been
processed yet. Mars has very dark "soils" (more of
that later).
    
    As for Mars' sky "colors," they are very slight tints to
the brightest portion of the photo in most cases. I ( or
anyone) can take the L4-L5-L6 images from the MER
imager and produce a series of processed photos with
EXACTLY the same color calibration target rendering
that have a light pink sky, a light blue sky, a light yellow
sky, a light orange sky, a light green sky, or a light cyan
sky... at the horizon. It's just too close to call.

    The sky tint in the pictures is intensified by the fact
that, in terms of absolute brightness, the Martian surface
is very dark. Not as dark as the Moon of course, but
Mars' albedo is only 15%! (The Earth's is 60% to 70%
depending on how many clouds there are.) To make that
dark surface show up, the images are brightened, which
makes the sky too bright (and slightly tinted). "Red" Mars
is really dark brown Mars, about like the good dark dirt in
your garden (if you're lucky enough to have good dark
dirt in your garden)..

    If you were there, standing on Mars and staring up at
the sky, the most obvious thing about the appearance of the
Mars sky would be that the top of the sky is very, very dark
-- you are looking straight up through a thin atmosphere, as
thin as the Earth's at 90,000 feet altitude -- the top of the sky
is essentially black (or dark midnight blue).

    The "color" of any sky is the color of its scattered light.
The scattered light of Mars is blue and bluest closest to the
Sun. So, how do we measure the color of scattered light in
the Martian atmosphere? How about using the Hubble?

    "Every 2 years, at Mars and Earth conjunction, images
of the planet are taken by the Hubble Space Telescope...
The resolution of the Hubble Space Telescope in sufficient
to allow imaging of the planet's limb... The Hubble Space
Telescope has onboard spectrometers which are used to
study the Doppler red shift of distant stars. These
spectrometers can be used to calibrate the light reaching
the imaging camera on the HST. The spectrometers are
capable of looking at the same stars as the imaging camera
and therefore provide an excellent calibration. A study of
these images was made by Philip James[18], who found
that scattering of the Martian atmosphere as seen from
Earth is predominantly blue... If the results of the Hubble
Space Telescope are correct, the Martian sky cannot be
as red as shown in many published images.."
    18. James, P.B., M.J. Wolff, R.T. Clancy, S.W. Lee,
J.F. Bell, III, and L.J. Martin, "Synoptic Monitoring of
Mars by HST: 1996-1997 Observations," Bull. Amer. Astron.
Soc., 28,1069, 1996.

    Information on the Phoenix Surface Stereo Imager and
and a list of all of its 24 filters and their frequencies and
bandwidths can be found in this LPI meeting paper:
http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2156.pdf

    "True" color images will come from the RightA (604nm),
RightB (532nm), and RightC (485nm) filters; their bandwidths
are 15, 28, and 21 nanometers. The human eye's peak RGB
responses are 559nm, 531nm, 419nm with bandwidths of
about 50-100nm.

    In the paper referenced above, there's a photo of the color
target taken with the ABC filters that looks fairly true but other
published on-Earth photos do not. Even the "good" photo does
not look much like the one taken with a good consumer digital
camera (even though that is not the same as what the eye sees,
either, but it's much closer). Of course, it will be a while before
all the filter and calibration data for the stream of images coming
back is availble. (The calibration data for the MER images is
not available yet, as far as I know, so it takes a while.)

> Realistically, the light that reaches the surface on
> Mars is already pretty ugly by human standards.

    Really, EP, I have no idea what you mean by "ugly"
sunlight. Sunlight is sunlight. I think it looks pretty good,
whether it's shining on Mars or shining on the Earth. Just
because you're on Mars, it still not a good thing to look
directly at the Sun. Everything will look just as bright to
you as it does on Earth. Even though it measures at only
50% of bright Earthly sunshine, your brain processing
will "correct" it.

    It's a bright sunny day on Mars. And it's Spring.



Sterling K. Webb
-----------------------------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <epgrondine at yahoo.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, May 29, 2008 2:01 PM
Subject: Re: [meteorite-list] color calibration


Hi Sterling, all -

NASA has been making Mars look like Arizona for years.
 The Mars "enthusiasts" need that illusion of an Earth
like Mars to continue their folly.

Realistically, the light that reaches the surface on
Mars is already pretty ugly by human standards. I
suppose that is why people set the sky to blue, to
make the place look Earth like. Once more, it's not.

Scientifically, I suppose you'd want to set the light
to white for composition analysis. To put the
landscape in human terms, blue works, as long as it is
identified.

How they'll solve the problem of calibration and
incoming light color at the same time is a good
question. I think its a problem in two variables, with
only one equation, they way they've set it up.

E.P. Grondine
Man and Impact in the Americas








      
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Received on Fri 30 May 2008 01:18:01 AM PDT