This invention generally relates to a scanning radiation sensor and, particularly, to such a sensor with a means for varying the focus of the sensor. 
Scanning infrared sensors of the type which collect exitance from a plurality of points along a line on a target and convert the collected radiation, such as infrared light, to a representation of temperature are well known.  The collection of light from the plurality of points is achieved by means of a scanning optical system including a rotating scanning mirror which radially sweeps the target and directs the collected radiation to a focus adjustable lens system interposed between the scanning mirror and an infrared detector.  The infrared detector converts the collected radiation to an electrical signal which is then converted to a representation of temperature. 
Ideally, the scanning optical system focus should be set to minimize the largest size of the optical resolution for all spots along the portions of the scan line where temperatures are to be measured.  There are two types of optical resolution: angular resolution and dimensional resolution.  Angular resolution is defined as the ratio of spot size and distance to the scanner optics and is expressed in milliradians.  Dimensional resolution is defined as the maximum spot area along the scan line and is expressed in square inches. 
A difficulty with known scanning radiation sensors is that most targets are not interior cylindrical surfaces, but are generally flat.  Accordingly, even though the focal length of the optical system remains constant during the scan, because the distance between the lens and the spots on the target vary, the resolution varies along the scan line on the target. 
In known radiation scanners, this problem has been approached by providing a manually adjustable or manually controlled optical system to enable the operator to selectively focus on any point of the scan line.  Some users manually set the focus to minimize the largest angular resolution.  Specifically, it is known to minimize the largest angular resolution for all spots measured by calibrating the optimum focusing distance D.sub.f for a certain perpendicular object distance Do through of the formula D.sub.f =2D.sub.o /(1+cos .alpha.),  where .alpha.= the instantaneous scan angle, D.sub.o equals object distance and D.sub.f, equals focusing distance. 
However, such formulas are difficult for operators to deal with even when reduced to graphs or tables of different focal settings for different parameters.