Spatial Calibration for Accurate Long Distance Measurement Using Infrared Cameras

A new spatial calibration procedure has been introduced for infrared optical systems developed for cases where camera systems are required to be focused at distances beyond 100 meters.

Army Combat Capabilities Development Command Armaments Center, Picatinny Arsenal, NJ

All commercially available camera systems have lenses (and internal geometries) that cannot perfectly refract light waves and refocus them onto a two-dimensional (2D) image sensor. This means that all digital images contain elements of distortion and thus are not a true representation of the real world. Expensive high-fidelity lenses may have little measurable distortion, but if sufficient distortion is present, it will adversely affect photogrammetric measurements made from the images produced by these systems. This is true regardless of the type of camera system, whether it be a daylight camera, infrared (IR) camera, or camera sensitive to another part of the electromagnetic spectrum.

The most common examples of large-scale lens distortions are known as barrel and pincushion effects, which are illustrated in figure 1. If these images were a truly planar reproduction of the real world, the curved lines in the images would appear as straight lines. Essentially, this can be thought of as the focal length (conversion from pixel distance to real-world distance) not being uniform throughout the image. Spatial calibration aims to build a transform to correct for large-scale distortion effects and effectively flatten an image so that the focal lengths (x and y) are uniform throughout the field of view.