Kai Ojala

Random target method for fast MTF inspection

A random target method for fast MTF inspection is proposed. The setup includes a random target, lens under test and a CCD camera with focus adjustment. The target consists of a random black and white pattern of a flat spectrum. The MTF of the lens is acquired by imaging the random target on the CCD using the lens under test, and then analyzing the spatial frequency content of the image. Frequency range up to about 50 cycles/mm is possible using commonly available CCD imagers. Measurement speed and precision depend on the sample matrix size used in calculation. A matrix of 128*128 samples per measured field point provides better than 2% precision and a few seconds total execution time (ordinary PC-computer) per lens including best focus evaluation and the measurement of tangential and sagittal MTF curves of 5 field points. Thus fast MTF inspection of low to medium quality lenses seems possible.

Fast lens testing using random targets

The performance of a random target method for fast MTF measurement of a lens is evaluated. Although the method is well-known, its potential for fast lens testing has not been assessed in the open literature. To optimize speed, the simplest possible instrument setup with minimum amount of mechanical movements during measurement execution is used. The setup includes only a random target, lens under test and a CCD camera with focus adjustment. The target consists of a random black and white pattern of a flat spectrum. The MTF of the lens is acquired by imaging the random target on the CCD using the lens, and then analyzing the spatial frequency content of the image using an ordinary PC. It was found out that a suitable compromise between speed and precision is achieved using a matrix of 128*128 samples per measured field point. This provides better than 2% precision and a few seconds total execution time per lens including best focus evaluation and the measurement of tangential and sagittal MTF curves of 5 field points. Using commercially available components, measurement frequencies up to 100 cycles/mm seem achievable using the simple instrument setup.