1. Field of the Invention
This invention relates in general to methods and apparati for calculating the image focus on cathode ray tubes.
When monitors and television sets are manufactured, not only the electrical values must be tested and adjusted, but also the image geometry and other image parameters must be set.
The automation of image adjustment not only makes it cheaper to manufacture such equipment, but also increases the yield and improves the quality of the sets.
Identification of image height, image width, image position and image brightness can, for example, be accomplished with known sensor systems. By contrast, the automatic identification of the image focus is very difficult to realize and requires very expensive equipment.
An additional problem is acquiring images from monitors or television sets having different standards. There are television sets that operate with 50 Hz or with 60 Hz field frequencies; in a monitor assembly line, for example, monitors that have three different field frequencies must be adjusted.
The invention enables the adjustment of the image focus of monitors, including those having different field frequencies, at a reduced cost and reduced complexity of apparatus.
2. Description of the Prior Art
Images, scripts, and graphics are produced on cathode ray tubes for television monitors. An optimum image focus is desired and since the image focus can differ at different locations on the monitor screen, the entire image content must be utilized for evaluating the image focus.
Until now, the picture screen of a monitor has been observed by the human eye. With adequate training, an employee looking at the picture screen can recognize the difference between the image focus at the edge and the focus in the middle. The adjustment of image focus occurs by adjusting a potentiometer in the deflection assembly.
Until the present time, such a procedure could only be automated using an extremely expensive apparatus.
For the automatic adjustment of the image focus, measuring marks on a part of the monitor image are recorded, for example by a solid state camera, with known methods. The camera signal is interpreted and the image focus is obtained with special and complicated hardware logic.
When monitors with multiple field frequencies must be adjusted, the known solution for 50 Hz monitors can only be applied in principle. An extension of the method would require further development of the hardware circuitry and of the method of measuring the average image focus over the entire image, such that a rapid adaption to the various field frequencies is possible.
It is known also known how to measure the image focus by way of the gradient distribution in the gray-scale image using an involved image processing system. In addition to the high cost for hardware and software, there is the disadvantage as in the previously discussed solution that an average image focus must be calculated using many measurements at different locations of the CRT screen. The time to acquire the image focus thus is usually too long. Times of a few hundred milliseconds are desirable in automated manufacture.
The following art discusses various cathode ray tube adjustment and monitoring systems. U.S. Pat. No. 4,163,308, U.S. Pat. No. 4,411,120, U.S. Pat. No. 3,962,722, U.S. Pat. No. 4,551,653, U.S. Pat. No. 4,193,086, European Patent No. 105,792, Patent Abstracts of Japan, Vol. 10, No. 297 of Oct. 9, 1986, No. 61-114437, the article entitled "A Technique for the Automated Measurement of the Resolution of Shadow Mask CRT Displays" by Veron and P. O'Callaghan SID 87 Digest Pages 211-213, German No. 2,030,417, German No. 2,805,691, German DE No. 3,107,042, German No. 2,746,969, German DE No. 3,047,340, German DE No. 2,839,187, French No. 2,497,399, European patent No. 0,119,282 and publication in Funkschau of May 1986, Pages 40-42.