The Imager for Mars Pathfinder (IMP) and the Surface Stereo Imager (SSI) are nearly identical.  The IMP flew on the Mars Pathfinder mission, and took more than 16,000 images of the Martian surface in Ares Vallis between July 4 and September 27, 1997.  The SSI will be launched in January 1999 as part of the Mars Volatiles and Climate Surveyor package, on the Mars Polar Lander '98 mission.  In late 1999 it will land near the south pole of Mars.  In this page, for convenience, IMP images are discussed.  However, the same facts apply to SSI images.

How the IMP Records Images  | Anatomy: Dissecting an IMP Image | How the IMP Filters Work | more information is available in How the IMP Works

Perhaps the best way to begin thinking about the way that the IMP records images is to begin by considering the nature of cameras.  We all look at pictures, but do we always think about what they really are?  Cameras are devices which record the light that is reflected off the objects in the scene with which they are presented.

Unlike a conventional camera, the IMP does not record images on film.  Instead, light reflected off objects is focused within the camera head and projected onto a "CCD," a Charge Coupled Device.  To understand how our CCD works, and why we chose to use a CCD camera for the Pathfinder mission, let's begin by thinking about traditional cameras, which record images on film.

When you take an image with a traditional camera, using ordinary photographic film, you expose film to the light being reflected off  of objects in the scene.  That light strikes the chemicals on the film, and causes them to react.  With black and white film, there is one layer of chemicals which is affected by all of the visible reflected light.

With color film, there are multiple layers of chemicals coating the film.  Each layer is affected by a different wavelength of light.  One layer of chemicals is affected by green, one by red and one by blue.  Thus, when we have a color print made from a negative, the colors that we see are composed of red, green and blue, but these colors are inseparable.
 
 

In the IMP, the CCD takes the place of film.  The CCD is a small chip, which, unlike film, is broken up into very small finite blocks (we'll call them cells).  Light travels through our camera's eyes, is reflected by mirrors, passes through filters and lenses, and then is reflected by a prism onto the surface of the CCD detector.  Just like film, the CCD is affected by the photons of light which hit it.  Each time a photon hits the surface of the CCD, an electron in one of the cells is knocked out of place.  Bright areas of the scene reflect many photons of light, dark areas reflect fewer photons.  Each area of the scene is focused onto a specific area of the CCD, thus, some areas are hit with many photons, and some with fewer photons.  Larger image of the CCD, with electronics

Conventional mechanical 35 mm camera:

The camera opens the shutter, focusing light from objects in the scene

The light reacts with the chemical coating.  Bright areas of the scene reflect more light, forming a pattern on the film.

After being chemically developed, an analog image of the scene is left on the film.

The processed film can be used to make slides or prints of the image.

 
The IMP:

The CCD is exposed to light, after it is focused and has passed through a chosen filter.

The photons of reflected light hit the CCD detector. Each individual cell of the CCD builds up a charge proportional to the number of photon which have struck that cell.

The charge on each cell is read out, cell by cell by a piece of electronics.

Another piece of electronics is used to assign each cell a number, according to the amount of light which struck that cell.

The numbers are sent through space via radio waves.  Once on Earth we can recreate the Martian scene by assembling the bright and dark pixels in order.

 
As you can see, the first stages of taking an image consist of either chemical or electronic reactions in the film or CCD.  The last stages of taking an image with the IMP are quite different, and it is those differences which make the CCD more practical for planetary exploration.

One reason is that the CCD images begin their existence in segments, the cells, which makes it easy to convert them to  pixels.  Each cell becomes a pixel after it has been assigned a numerical number.  Images recorded on photographic film are analog -- they are continuously variable instead of being broken into small bits.  This fact makes it harder to represent the images as numbers.

Why would you want to break an image down into numbers?  Scientists need a method of analyzing the image data which can be quantified.  With an analog image, like a photograph, you can say "rock #1 looks darker than rock #2," and that does teach you something.  However, with our digital images, we can assign numbers to the differences between the two.  This is only one example of many -- we can also take images of the sun and determine the qualities of the atmosphere by seeing quantitatively how much light is passing through it. 

The image below shows the path of light (indicated by yellow lines) as it travels through the camera and is projected onto the CCD.

(for simplicity, many parts of the camera have been left out, including the filter wheel. This image simply shows the light path to the CCD Detector within the camera head.)

This image of the SSI without its top cover shows the eyes , the filter wheel, and the top of the CCD.  Click on the image for a larger version.