1. Field
Embodiments of the invention relate to an infrared (“IR”) camera and a method for focusing an IR camera.
2. Related Art
Over the last few years, an increasing number of IR cameras also comprise a visual light imaging subsystem for generating digital images in the visible light range. Such cameras often also comprise functions for fusing or blending the visible light images and the IR images in various ways. Typically, the visible light imaging subsystem has a fixed focus, or an autofocus function according to prior art methods.
The visible light imaging subsystem may have a substantially larger field of view than the IR imaging subsystem. Also, the imaging focal plane array of the visible imaging system may have a higher resolution than that of the IR imaging system. Therefore, the IR and visual images must be aligned before they can be blended, or fused. Aligning the images may include applying a scale factor and X/Y offset to one of the images to make objects in the plane of focus appear in the same scale and position in both images when they are overlaid each other using the same pixel grid. Typically the scale factor is fixed and the offset is a linear function of focus distance. It is known in the art how to adjust the two images to the same scale.
The displacement or offset between the IR and visual images means that an object will be found at different positions in the two images. Two main factors contribute to this displacement. First, optical subsystems for IR imaging and visible light imaging are placed at a distance from each other on the camera, which causes a parallax error. Second, the optical axes of the IR and visual optical subsystems are typically not parallel and slightly skewed. This must be handled in each camera individually, which is typically done by registering the displacement between the IR and visual images at different distances to an object during calibration of the camera. How to do this is also known in the art.
It is common in the art to use a focus motor for focusing the IR optics. The focus motor must be controlled according to some focusing algorithm to focus the IR optics properly. Typically this is done by moving the lens to one of its extreme positions and then sliding it through its whole range to determine the position that gives the best focus. The lens is then moved to this position. This is time consuming and interrupts the imaging procedure. Thus, focusing can only be performed at times when the imaging procedure is not active. Accordingly, there is a risk that the camera will not keep its focus throughout the imaging procedure.