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Spirit/Opportunity Color Images

Color Calibrating the Images

 Obtaining properly balanced color results from the Pancam is a fascinating operation which can be done on most common imaging softwares. Color reproduction from multispectral filter greyscale images such as provided by Pancam basically consists of obtaining the proper visible band images and combining them to RGB, or Red, Green, and Blue as per their wavelength values.

 Given the band images provided are adjusted within the proper brightness response curve for each of the three respective colors, your result from simple RGB combination will be a "true" color image. Of course this would be quite a rudimentary image, and normally in need of further balancing, cosmetic, and aesthetic adjustments, but the components are all present and significant further color enhancement is usually unnecessary, in some extreme cases detrimental to image truity .

 Now we get to the Pancam, which is a remarkable piece of imaging technology, and simply explained for being so complex. It has two "eyes", left and right. They both "see" differently.. The left side sees almost entirely in visible light, the right is almost exclusively infrared. Each is equipped with eight different filters, which allow only certain wavelengths of the spectrum
to be seen in each of the images it captures. Here are the filter positions by image/wavelength.

LEFT CAMERA        RIGHT CAMERA
    
   L1. EMPTY               R1. 430 (SP) *
L2. 750 (20)              R2. 750 (20)
L3. 670 (20)              R3. 800 (20)
L4. 600 (20)              R4. 860 (25)
L5. 530 (20)              R5. 900 (25)
L6. 480 (25)              R6. 930 (30)
  L7. 430 (SP)*            R7. 980 (LP)*
        L8. 440 Solar ND      R8. 880 Solar ND

*SP means short pass filter, *LP means long pass filter.
One lens on each side is coated so it can observe the sun (solar ND)
L1 is clear, no filter

 We all know or should know that "true" RGB wavelengths are something that cannot be nailed down to a definite value, the response is more of a curve.  There is no absolute "true" because color responses vary depending on observer and other factors such as lighting and atmospheric conditions.. The average human response for RGB is a given however, and these values are..  Red 650nm +/-, Green 510nm+/-, and Blue 445nm-475nm +/-

If you need references for that... 

What Wavelength Goes With a Color?

http://webmineral.com/help/Reflectivity.shtml

 http://hyperphysics.phyastr.gsu.edu/hbase/vision/cie.html#c2

 We can quickly see by referencing the filter wavelengths above in the left column that none of the filters provided by Pancam are exactly equivalent to the actual and normally applied RGB values, nor monitor colors.  This being, it is not purely a "true color" camera, even though the filters do fall absolutely near the accepted values. The closest approximations to "true" RGB it natively supplies are..
Red L4 600nm ( or L3 670nm)  Green L5 530nm  Blue L6 480nm ( or L7 430)

 Although these bands are not precisely perfect color, beautiful close color images without the overdriven reds can be produced from this data without much further processing.

This can be as easy as combining normalized bands L4, 5, and 6, reducing the blue and green saturation levels accordingly (red will rarely need reduction, instead it will need slight boosting if L4 is used for red), then aesthetically cleaning and contrast balancing the image. In most of the resulting products it is necessary to adjust the uneven field illumination.

L4,5,and 6 color balanced calibration images, Spirit and Opportunity.

  

      Opportunity's calibration image      

 Using this process will show us as close to true images as possible from both rovers. To the right is an Opportunity calibration image. Notice the sky reflectors for both have a blue/grey/pink ruddy tint.

We must note that as of yet we haven't a clue as to the actual brightness levels in these images, but naturally red is more intense than green which is more intense than blue. Red covers a short range, green a middle intensity, but wider spread, and blue the most diffuse, but wider still. The web "raws" were stretched according to their wavelengths, or normalized, so HSL reduction on the green and blue will be necessary to bring the colors into more proper adjustment. This is required due to the nature of using multispectral imaging equipment to get true colors.

 Each filter only sees in the intensity of it's assigned wavelength, therefore on RGB combination each color channel is maximized, needing reduction according to visual response curves. A round estimate for this reduction falls near the following RGB ratio; 1,-3,-6. A strong amount of color imaging "knack" is also quite helpful. This is color imaging, not rocket science, and the best balance is an aesthetics tuned process that represents the calibration dial accurately.

 NOTE: There are now plentiful brightness calibration images being supplied for even better accuracy.

To sum this up

 Proper techniques and adjustments are the difference between images that look like this which is nice, but very ruddy, and this. The NASA/JPL color images we've been seeing and the first one linked above have been made with the "wrong" red filters, the near infrared,  reportedly done so in order to better identify surface compositions. I would prefer to see the sights as they really appear.. so would many others. Use of any other than actual red green and blue filters will not produce an accurate color image. Use of any other filters and then calling the result "true color" is inaccurate and misleading.  IR is not visible red. Use of an IR filter for red is not accurate in close or "true color" imaging, period. It's called pseudo-color when the proper components are not present and one or more is manufactured from other sources.
What it does is make for a paradigm fitting "pretty picture" with salmon skies and too dark red surfaces. Brightness reduction for human response to ccd imaging is not a secret, nor
is any of the color imaging done with the pancam

Here is a "pancam" picture taken on earth. (No IR for red on this one)

  Now before anyone goes running off shouting conspiracy, trying to blame anyone or agency of dishonesty (Especially the hard working MER personnel of whom I am colleague), or even dismissing this issue as irrelevant, perhaps we should step back and take a look at the bigger picture.

 It is most definitely relevant we have proper true color imaging of the Martian surface, however, it takes a little time and work to apply the personal beautifying touch to these images. Because the bands are not quite RGB proper, it takes a little more than cursory attention to attain the proper color aspects. While I have poured a good bit of attention into meager sets of full band data in order to make these Spirit and Opportunity color images, they are at best a decent guess until we get full resolution images and full ancillary data as it is to be delivered to the PDS. Take them as they are.. Stunningly pretty pictures of Mars in color.

 The MER team has a daily influx of not only photos but mission data. It is a bonus that we get frequent new images, but it might be a bit unrealistic to expect fully processed and calibrated totally accurate true color beauties out of every downlink. This will happen gradually as the full record is archived and made available. The world is clamoring for big fresh images on top of this daily influx.. All that work, being done, yet to be done, and still coming in to be done... Thus it is that the huge, undefined, hastily constructed pans with the ruddy colors from using a "wrong" red,  bad stitch lines, and beige sky come to pass. I don't think they do justice to the actual beauty of the Martian surface.

While there is no doubt in anyone's mind that Mars is in fact a very ruddy color, at times it is very much yellowish brown, earth-toned, and bluish skied. The differing ruddiness of the images constructed from the proper color bands is directly dependent upon ambient lighting conditions such as time of day, dust and water cloudiness, distance and angle of surface being photographed to the sun and rovers, materials being photographed, or any combination of such.

 What is undoubtedly the most important thing to remember is that no matter what the representations of "true color" show, real red green and blue is...

Red 650nm+/-   Green 510nm+/-  Blue 440-475nm+/-

If it's not that, or very near, then it's not "true" color

 I offer here an alternative view, one done with a fair degree of accuracy using common and measurable techniques.
One which also shows that Mars may be more than we ever imagined.

                           Two Worlds, One Sun.

 I look forward to both Rovers continuing massive success.. I also look forward to expanding this study as new and more total data releases of this magnificent imagery are acquired. Thanks for your interest, and keep checking back at my collection for new images and accuracy improvements on the ones already displayed.

 Planetary color imaging, a high form of both art and science..

  -- Keith Laney --

 

For more insight into making sense of the images and the way Pancam captures them, I recommend reading this article from "Above Top Secret", which is fairly accurate as a whole despite the author's errant assertion that it is acceptable to substitute an IR channel for red. http://www.atsnn.com/story/30048.html

 If you'd rather have an unbiased look at the actual scientific process behind Mars color imaging,  I suggest this article from Dr. Levin over the above article

Solving the color calibration problem of Martian Lander images

 

Spirit Color Images

 

Keith Laney Productions™ © 2002-2012

All custom imagery use restricted without permission. All rights reserved

 

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