Re: Iron Meteorite on Mars (Color Photo)

From: Christopher M. Jones (christopher.m.jones_at_gmail.com)
Date: 01/20/05 

Date: Wed, 19 Jan 2005 22:03:57 -0600

Spockstuto wrote:
> I have noticed that pictures from the rovers with bright areas tend to
> make bright streaks and cause problems. If you notice, sometimes the
> pictures of the rovers aluminum parts outside have very bright streaks
> or spots when they observe a shiny area. The metallic meteorite is shiny
> and causes white streaks. I wonder if the camera on the rover is
> sensitive to bright objects??? Look at the heat shield pictures they
> have the same anomaly.

This is called "blooming" and is a phenomenon related to CCD
imaging systems. In a CCD (Charge Coupled Device) the imaging
pixels are arranged along a series of columns (rows if you
rotate 180 deg.), each pixel represents a "bucket" where charge
(electrons) can build up during exposure. These buckets are
kept separated from each other by careful application of
voltages to the different parts of the bucket and its neighbors.
After exposure the charge in each pixel is transfered from
bucket to bucket (this is the "charge coupling") along the
columns to a top row, that top row is transferred in similar
manner and each lump of charge is sent into an analog to
digital converter which reads the charge that was in each pixel
(which corresponds directly to the intensity of light that had
been shining on it), then the columns are shifted up again,
etc, until the whole frame is read out. However, each bucket
has a maximum number of electrons it can hold (the "full-well
capacity") due to the nature of the way the charge is held in
place and the specifics of the device's design. If this
full-well capacity is exceeded, then electrons will bleed or
"bloom" to adjacent pixels on the same column. So, when a CCD
imager takes a picture of a scene with very bright elements
and allows those parts to be overexposed, the result is that
those bright spots cause blooming in the image, effectively
overexposing other parts of the image along the same column as
the bright parts. There are designs with can prevent bloom
but they come at a cost in CCD sensitivity, such protections
are fairly common on consumer equipment but not on CCD based
scientific instruments (the Hubble Space Telescope, for
example).

Here's what looks to be an incredibly informative (though
somewhat long winded) page on CCD technology:

http://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.html

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