Depth of Field in Modern Thermal Imaging
This text is an abstract. The full text is available at the SPIE Digital Library.
Modern thermal imaging lenses for uncooled detectors are high aperture systems. Very often, their aperture based f-number is faster than 1.2. The impact of this on the depth of field is dramatic, especially for narrow field lenses. The users would like to know how the image quality changes with and without refocusing for objects at different distances from the camera core.
The Depth of Field approach presented here is based on the lens specific Through Focus MTF. It will be averaged for the detector area. The lens specific Through Focus MTF will be determined at the detector Nyquist frequency, which is defined by the pixel pitch. In this way, the specific lens and the specific FPA-geometry (pixel pitch, detector area) are considered.
The condition, that the Through Focus MTF at full Nyquist must be higher than 0.25, defines a certain symmetrical depth of focus. This criterion provides a good discrimination for reasonable lens/detector combinations. The examples chosen reflect the actual development of uncooled camera cores. The symmetrical depth of focus is transferred to object space using paraxial relations. This defines a typical depth of field diagram containing three functions: Hyperfocal distance, nearest and furthest distance versus sharp distance (best focus).
Pictures taken with an IR Camera illustrate the effect in the depth of field and its dependence on focal length. These pictures confirm the methodology. A separate problem is the acceptable drop of resolution in combination with a specific camera core and specific object scenes. We propose to evaluate the MTF-graph at half Nyquist frequency. This quantifies the resolution loss without refocus in accordance with the IR-picture degradation at the limits of the Depth of Field. The approach is applied to different commercially available lenses. Pictures illustrate the Depth of Field for different pixel pitches and pixel counts.
Keywords: Thermal Imaging, depth of focus, Depth of Field, Nyquist frequency, Diffraction limit, Through Focus MTF