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Image Resolution: Geometric Blur

Neutron beam, showing ideal image and real image While the detector resolution plays a big role in determining the image resolution, one must always remember that geometry also limits the image resolution. The geometric blur is illustrated in the cartoon to the right. Imagine that the ball is perfectly opaque to neutrons, and so its image should be a round, sharp shadow. However, since the ball is not directly in contact with the detector, its edges do not uniformly block all the neutrons coming from the entire aperture. Instead, some squeak by, causing the edge to be blurred. If the ball (or an edge) could be placed directly in contact with the detector surface, the resolution would be just that of the detector’s intrinsic resolution. But, since there is always some displacement, h, between the object and detector surface, there will always be some blur. The geometric resolution, λ is approximately given by:

λ = d h / L.

For instance, with an L/d of 300, and a separation h = 10 cm, the geometric resolution is about 0.33 mm. Another interpretation of the geometric resolution is to imagine placing a pinhole at the object’s location. The geometric resolution is then the size of the minified aperture on the detector, which from similar triangles is simply:

λ = d h / (L >> h).

By assuming that L >> h, one arrives at the approximate formula.

Inquiries or comments: David Jacobson

Online: May 2006