I thought it could be amusing to try this:

Click to view attachment

It's a composite of rear hazcams from sols 1256, 1264 and 1268 with enhanced, unaltered colors. Oppy didn't move during those days, and took images of the storm at roughly the same time each sol. Since tau was changing, the color of the ambient light must have been different in each one --apparently enough to distinguish the blueberries and some bright rover pieces, and perhaps the sky and the dust too.

I don't follow you. The hazcams are panchromatic imagers and any color you see here can only be due to:
1) contrast stretch differences
2) moving shadows

You simply cannot judge anything about the color of ambient light based on monochrome images from 3 sols.

I would echo UG's comment. Any inference of 'colour' from Hazcam (or indeed Navcam) is pure fiction.

Hang on a minute - I think this does make sense. If you illuminate something with three different colours of ambient light surely you can make some kind of pseudo-colour image. Isn't that how the Phoenix scoop camera works?

Very clever Fran.

It will be interesting to use the Radiometrically Calibrated images to do this exercise when they are available.

I agree with Nigel. Depending on sun angle, the illuminating light would be filtered. High up in the Earth sky, it's white (well, yellow-white + the blue scattered illumination from the sky). Lower down, the light would be yellowish as more blue light gets scattered away. And then right at sunset, the light is that golden yellow.

So I suppose you could try to recombine the images knowing the illumination wavelength.

Heck, you might even be able to dig out spectral information. (Not much, since you are only varying illumination source wavelength, not detector wavelength.) At the best you might be able to say: "That object must be reddish since it stays brighter when the sun is lower in the sky, that object must be bluish since it gets much darker when the sun is lower in the sky."

Didn't they use this kind of analysis when looking at Huygens descent images?

-Mike

Interesting Idea: this way the three different tau ambient light conditions could possibly act like three different "filters", thus producing albeit not a true color image (of course) but still could potentially reveal interesting information as a false color image.

However, i do suspect that the different "tau-filters" would just reflect differences in overall brightness, thus unfortunately not revealing much additional "spectral structure".

To test this hypothesis I did a quick principal component analysis (PCA)



on the composite image's color space which shows that the additional "filtes" do indeed not reveal any significant additioanl "intrinsic" dimensions to the data other than the 1-dimensional brighness information.

Wow! Nice work!

-Mike

Yup - you do. Unless someone has installed street lighting......

The effect might be subtle but I can see how differing tau levels would change the distribution of flux intensity at various frequencies. Certainly as Tau increases the overall color cast would tend towards "reddish" due to the increase in flux from light reflected from atmospheric dust vs the low tau scenario where the overwhelming majority of incident flux is unimpeded direct solar insolation.

The change from low tau (0.5) to high tau (2.0) changes the ambient light from a 90:10 ratio of direct unimpeded solar flux( "clear" or to be more accurate flux that has a frequency distribution that comes from our own dear Sun): reflected diffuse flux ("reddish" or whatever color reflected solar flux from martian atmospheric dust tends to be) to something closer to a 50:50 ratio.

So if you take a panchromatic image under the 90:10 illumination regime and compare it to a panchromatic image taken under the 50:50 regime then you will see different details.

Edited to add: I see Nirgal's point but I can't comment on the limits it places in our ability to see real variation between images taken under varying flux regimes as I haven't done the numbers on this one. That said I'm happy to accept that the effect is may be too subtle to detect given the 8bit contrast scale we're working with on these images but I think it should be plausible even if it is undetectable in the images in question.

Real or not, it can be gorgeous. Spirit landing raw composite:

Click to view attachment

I've got a couple of pictures more, but this is already off-topic.

Yes, it is very off-topic - Moderator

Ture, but this is an interesting idea, and that one of the lander is lovely whatever it really shows, great work Fran.

Split to a new thread.

Interesting indeed. It really seems to work to some degree, somewhat like an image made with just an orange and a cyan filter. Perhaps with some calibration and comparison with RGB images the color effect can be calibrated and/or confirmed.

The lander one is mostly or completely a lighting artifact.

The following composites are 600 days apart and both in the solar panel shadows, so they receive scattered light only.
In both, one cable has apparently a distinct hue. Saturation is stretched, though, and the jpeg artifacts are awful.

Click to view attachment

I googled around but I only found this picture with cables being manipulated in the back side (large version):

http://mediaarchive.ksc.nasa.gov/detail.cfm?mediaid=18868

Isn't that something of an oxymoron?

Very interesting !

I did the PCA (Principal Component Analysis) again on that image (after converting from RGB to LAB in order to pre-decorrelate the channels)
and the result is stunning:




with all three Eigenvalues significantly above zero:

Eigenvalues:
1.96515
0.68486
0.34999

and at least five very distinct clusters probably corresponding to different spectral properties of different parts of the image (sky, soil, lander parts)


interesting stuff

@fran: how exactly did you compose that lander image ?

Hum, I loaded three pictures from the same Sol, sorted them and composited them as RGB, nothing else.

I'm not convinced now that any of the pictures shows much if any spectral data. Maybe only color changes in a flat, featureless surface can be slightly meaningful.

I.e. in this one you can see that the features that face the rover are redder, and everything else bluish, due to changing shadows/reflections.

Click to view attachment

----> EDIT: I was too premature in my conclusion: Although there is multi-dimensional structure in the data, it could be due to different light source angles rather than spectral properties of the objects ....

Also note the distinctly colored details at the rock (a crop I did from your image above where I directly used the principal components
for the false color composite)

The distinct color of the rocks comes from their rugged texture and changing illumination. It's not a coincidence that the ground appears more bland because it's pretty flat and the sand grains look similar at different illumination angles. Some of the rocks appear bland in color due to their face being illuminated more or less the same by sunlight at different times of sol, but other rocks are angled such that significant shadowing occurs at certain times and hence these "filter" differences.

I'm sorry, but I just cannot believe a slight solar elevation angle alone could cause such a different illumination color so as to act as a filter for the three exposures. The last composite frames in particular had to be taken with sun high up within a couple of hours. How much does daylight color appear to change around noon on Earth? I expect the same on Mars.

Moreover since the intrinsic color variations in rocks and soil are very subtle in "natural" color, they're only distinct in false color composites. This method, at best, would generate images at *slightly* different colors and even then, it would be only useful if you could freeze the illumination angle.