Overview of Radiometric Calibration

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[edit] Why perform radiometric calibration?

Both vidicon cameras (such as those carried on-board the Viking and Voyager missions), and charge coupled device (CCD) cameras (such as on the Clementine, Mars Reconnaissance Orbiter, and other contemporary missions) produce digital images with the inherent artifact known as camera shading. Camera shading results from the non-uniform sensitivity across the field-of-view of the imaging instrument.

uploaded image: Flat field This image (acquired during pre-flight calibration by a Mars Exploration Rover Microscopic Imager) illustrates camera shading. Ideally, every pixel in the image should have the same DN. Radiometric calibration corrects this type of non-uniform brightness.
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Flat field This image (acquired during pre-flight calibration by a Mars Exploration Rover Microscopic Imager) illustrates camera shading. Ideally, every pixel in the image should have the same DN. Radiometric calibration corrects this type of non-uniform brightness.

Perhaps the best way to illustrate camera shading is to imagine acquiring a digital image of a target of uniform brightness, say a screen that has been painted a uniform shade of gray. If the camera sensitivity across the fields-of-view were ideal (and the flat-field target exactly the same brightness everywhere), then the acquired digital image would have the same DN value for all the pixels in the image. However, because of the non-uniform sensitivity of the camera, the DN values of the resulting image will vary throughout the image array (see the example to the right). A typical camera may have as much as 20% variation across the field-of-view. Camera shading corrections are applied to an image that correct for the non-uniform sensitivity so that, in our flat-field observation example, the radiometrically corrected image would contain pixels of identical value.

[edit] What is radiometric calibration?

Radiometric calibration recalculates the DNs in an image based on numerous factors, such as the exposure time, known values for the camera shading based on flat-field observations, dark current (output current of a detector when no energy is incident on the detector, such as when the shutter is closed), and other factors describing the unique electronics design and characteristics of an imaging system. Camera sensitivity may be time dependent because of the drift of the camera sensitivity throughout the course of the mission. The camera sensitivity is also dependent on the filter, operating modes of the instrument, and temperature of the cameras. Additionally, the camera response may be non-linear at various brightness levels.

[edit] How is the image changed by radiometric calibration?

A radiometrically calibrated image has DNs in radiometric units that are proportional to the brightness of a scene. Radiometric calibration applications in Isis produce output values that represent either:

  • Radiance - The amount of electromagnetic energy emitted or reflected from an area of a planet, in units of µw/(cm2*sr)
  • Reflectance - The ratio of reflected energy to incoming energy (i.e. irradiance/solar flux, often simply called I/F). A reflectance would be 1.0 for an ideal 100% reflector where the sun and camera orientations are perpendicular to the reflecting surface.

Generally, Isis radiometric calibration applications that offer both reflectance and radiance output will output reflectance (I/F) by default. Due to the fact that radiometric calibration is mission dependent, each mission supported by Isis has its own radiometric calibration application. A few examples of radiometric calibration applications in Isis are:

  • vikcal: Viking Orbiter Visual Imaging System
  • moccal: Mars Global Surveyor Mars Orbiter Camera

See Also: Mission Specific Programs

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