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An Examination Of NASA's Color Methods

Some consistent errors are responsible for the incorrect colors in NASA images

Note:  the filter errors referred to have since been corrected by NASA

    Some rather interesting and uncorrected errors are responsible for the poor quality of the color images that we have recently seen released by NASA.  There are three errors in particular that contribute to the overall problem.  The first is that in some images, the filter values have been incorrectly listed.  The second is that the left and right camera filter values have been swapped in some images.  The third is simply that the wrong filters have been selected to assemble some of the images.

    Taken together, these explain exactly why some images are odd or misbalanced in color, and others are outright wrong.  I will show how the calibration targets ended up with wrong colors and how to correct them, and also how to explain in greater detail why the Martian sky ends up being the wrong color.

    The first error is in assigning filter frequencies.  I have selected three frames of data for assembly into a full color image.  To get right to the root of the problem, I will use data from Opportunity, Sol 001.  This is where some of the errors are most easily seen and verified by anyone.

   Here are three raw monochrome image frames from Sol 001.  The filters used to produce them are (from left to right) L2, L5, and L6.  "L" indicates that the left panoramic camera was used to take these images.

   The original images can be seen by clicking each.
   This image is from the left panoramic camera using filter L2.  The filter is being used as the red data for the color picture.  The caption on the image is as follows:

Left Panoramic Camera Non-linearized Full frame EDR acquired on Sol 1 of Opportunity's mission to Meridiani Planum at approximately 16:02:12 Mars local solar time, camera commanded to use Filter 2 (19 nm). NASA/JPL/Cornell

   In other words, this image used a filter at a wavelength of 19 nm.  But this is clearly impossible.  19 nm would be gamma rays!

   In fact, the true wavelength of this filter is stated as 753 nm by NASA.  What is posted is the "width" of the filter- the pass bandwidth.

   This sort of error would not be noticed by anyone other than somebody familiar with optics or physics.  But the image assembly team surely is aware of this error.

   As a side note, the wavelength of 753 nm is not red light- instead it is infrared.  Human eyes are barely sensitive to this wavelength and it contributes no effective light to a normal color picture.

   To see the original caption on the NASA site, click the image or click here.
   This image is from the L5 filter.  This filter is stated as being at a wavelength of 535 nm.  However, the caption on the NASA site is thus:

Left Panoramic Camera Non-linearized Downsampled EDR acquired on Sol 1 of Opportunity's mission to Meridiani Planum at approximately 16:02:45 Mars local solar time, camera commanded to use Filter 5 (25 nm). NASA/JPL/Cornell

   In other words, this is supposed to be at a wavelength of 25 nm, but once again, this would be very energetic gamma radiation, not visible. light.

   This sort of "light" would spontaneously render matter radioactive.

   This frame represents green light present in the scene that the rover camera was imaging.

   To see the original caption on the NASA site, click the image or click here.
   Finally we come to the blue frame.  This is using filter L6 and is at a wavelength of 483 nm.  This is a good blue for most purposes and is fine for the color imaging process.  The NASA caption reads:

Left Panoramic Camera Non-linearized Downsampled EDR acquired on Sol 1 of Opportunity's mission to Meridiani Planum at approximately 16:03:16 Mars local solar time, camera commanded to use Filter 6 (24 nm). NASA/JPL/Cornell

   And indeed, we see the same mistake has been made.  The passband of 24 nm has been listed, but that tells us nothing of the color.  It only tells us how wide the spectrum is that the filter will admit.

   What we really need to know is that filter L6 passes lightwaves with a length of 483 nm, and allows waves of that frequency, plus or minus 12 nm.  So light from 495 nm to 471 nm is let into the camera when you use this filter.

   To see the original caption on the NASA site, click the image or click here.

   If we use these raw "monochrome" images, we can recreate the color of each and then make a full color image from that data.  Here is how.

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