A method and apparatus for mapping between the positions of fibers at opposite ends of incoherent fiber-optic bundles have been invented to enable the use of such bundles to transmit images in visible or infrared light. The method is robust in the sense that it provides useful mapping even for a bundle that contains thousands of narrow, irregularly packed fibers, some of which may be defective.
In a coherent fiber-optic bundle, the input and output ends of each fiber lie at identical positions in the input and output planes; therefore, the bundle can be used to transmit images without further modification. Unfortunately, the fabrication of coherent fiber-optic bundles is too labor-intensive and expensive for many applications. An incoherent fiber-optic bundle can be fabricated more easily and at lower cost, but it produces a scrambled image because the position of the end of each fiber in the input plane is generally different from the end of the same fiber in the output plane. However, the image transmitted by an incoherent fiber-optic bundle can be unscrambled (or, from a different perspective, decoded) by digital processing of the output image if the mapping between the input and output fiber-end positions is known. Thus, the present invention enables the use of relatively inexpensive fiber-optic bundles to transmit images.
The invention calls for a two-part calibration or mapping process. Part 1 of the process (ordinarily performed by the fiber-bundle supplier) takes place on the apparatus depicted in Figure 1. A computer that controls the apparatus and processes its measurement causes a video monitor to generate a test pattern described below. The input end of the fiber-optic bundle is equipped with an objective lens and is positioned so that the test pattern on the video monitor is focused onto the input plane. Another lens focuses the image from the output plane onto a charge-coupled-device (CCD) video camera. The output of the camera is digitized and fed to a frame grabber in the computer.
At first, the test pattern is a solid bright screen, so that the output ends of all the fibers (except the defective ones) appear bright. The digitized image of the output plane is subjected to a sequence of digital processing steps in which the centroids of the output-fiber-end subimages are computed, as illustrated in the upper part of Figure 2. Thereafter, these centroids are deemed to be the positions of the output fiber ends for the purpose of mapping.
As shown in the lower part of Figure 2, a test pattern in the form of a bright horizontal line is then swept vertically across the input end, in increments corresponding to one pixel of the CCD; at each increment of position, the brightness at each pixel location on the CCD is recorded. Next, the same thing is done with a test pattern in the form of a bright vertical line swept horizontally. On the basis of the brightness-vs.-pixel-position data from the horizontal and vertical sweeps, the horizontal and vertical line positions that result in maximum brightnesses at the previously determined centroids are computed. These horizontal and vertical line positions are converted to coordinates of fiber ends on the input plane. Thus, the relationship between the coordinates of the input and output ends of each and every fiber (not including defective fibers) is established. This relationship, which is the desired mapping, is recorded in a lookup table (LUT). Each fiber-optic bundle is characterized by a unique LUT.
The mapping as determined thus far is unique to the pixel coordinate system and the setup of the mapping apparatus; as such, the mapping is subject to change along with changes in magnification, translation, and rotation of images when the fiber bundle is removed from the mapping apparatus and installed in another apparatus in which it is to be used to transmit images. The mapping as determined thus far is also subject to change associated with focus adjustments, change of the CCD camera, or insertion or removal of an infrared or visible-light filter in the image-transmission system.
Part 2 of the calibration process involves a partial remapping to compensate for such changes. Part 2 (ordinarily performed by the end user) takes place once the fiber-optic bundle has been installed in the imaging system. The input end of the bundle is illuminated with a solid bright test pattern and the user visually compares the video image of the output end with the corresponding image recorded previously in the first step of Part 1. The user identifies the ends of four fibers in the two images for use as fiducial points. Then software computes a preliminary new LUT based on the previous LUT and the coordinates of the fiducial points in the previous and present coordinate systems. The new LUT can be tested visually by computing the output fiber centroids and overlaying them on the video image. If necessary, the transformation coefficients can be modified in an iterative subprocess until the fiber centroids computed by use of the new LUT appear to lie at the centers of the fibers in the video image.
This work was done by Harry E. Roberts, Brent E. Deason, Charles P. DePlachett, Robert A. Pilgrim, and Harold S. Sanford of SRS Technologies for Marshall Space Flight Center. MFS-31520