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Name: Bah I.
Status: Student
Age: 18
Location: N/A
Country: N/A
Date: June 2004


Question:
What the difference between image resolution and spatial resolution?



Replies:
"Spatial resolution" is a specific term that describes how highly detailed a picture is. For example, the spatial resolution of a flatbed scanner may be 1200 pixels per inch , or the spatial resolution of a spy satellite may be one meter. As far as I can tell, "image resolution" is a general term that pertains to the overall quality of the image. Sometimes image and spatial resolution are used interchangeably. The quality of the image depends also on detail of the color and the brightness in the picture. It is possible to have good spatial resolution but poor color resolution; for example, if a computer is set to display only a limited range of colors.

The best quality pictures should resolve many colors and shades of gray, as well as have high spatial resolution.

Bob Erck


Suppose you are taking a photograph of an object painted with many light/dark stripes.

1- The stripes have a physical width, say in centimeters.
2- The camera looking at these stripes from a distance, from which point of view each stripe has an angular width, say in degrees or radians. The camera lens is imperfect, and will blur these stripes to muddy gray if they are narrower than some width, in radians.
3- The stripe depicted in the finished photograph has some width, say in pixels of a digital image, or in millimeters on a photographic paper. The stripes will be missed or at least blurred if they aren't more than 1 pixel wide, in a digital photograph.

This scenario lays out:
- 3 frames-of-reference (object-space, view-angle, and picture-coordinates)
- 2 performance limitations (lens blurring, data storage).

This leaves lots of room for confusion when trying to compare cameras and photographs. People do try to resolve the ambiguities, by augmenting "resolution" with terms like "spatial" and "image". But it's still a bit of a zoo, which you must figure out each time someone is talking.

My definitions:

Spatial resolution is the maximum resolvable number of line/space pairs per meter, on the object being photographed. It might devolve to line-pairs per radian, when the distance from camera to the object is not known, or is preferred to remain indefinite.

Image resolution is the maximum resolvable number of line/space pairs per unit in picture coordinates, whatever they are for your recording medium.

But, then, I am sure that any one party could manage to swap those definitions. They might, for example, refer to the spatial resolution in the plane of the photographic paper.

A distinction can be drawn between physical units such as meters and radians, and image-relative units, such as pixels; or taking 1 unit to be 100% of the image width. In this dichotomy, "spatial resolution" would use physical units, "image resolution" would use one's image-relative units.

In digitally-recorded photography it takes at least two "pixels" to show one line/space pair. That can throw in another factor-of two confusion which you may need to clarify. 640x480 pixels/image is roughly equivalent to 320x240 line-pairs/image.

And, of course, things can be specified as a size or a frequency. Such as: micro-radians, or, line-pairs per radian. These are both intended to communicate the same thing.

Good luck and good sense. You may need them.
Jim Swenson



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