• OPINION: This is a really bad paper. The idea seems problematic and not for practical use. The results are limited to filtering in the horizontal and vertical directions. They claim the setup is good for a cascading system, but they don't have experimental results to support that. In a practical sense, the setup with 2 beams overlapping to select spatial frequencies seems inexact; they don't talk about alignment of the beams to select spatial frequencies. The paper itself seems very elementary because it talks about basic concepts of spatial filtering that I thought were known already by 1990.

SIMILAR PAPER: Optical image processing by matched amplification

http://www.opticsinfobase.org/abstract.cfm?URI=ol-17-23-1694

• OBJECTIVE: amplify selected frequencies of an image
• HOW: Uses spatial amplification from previous paper. One beam contains an image while the other beam contains a selected part of the image. When the beams meet in the F. plane, the selected frequencies from the partial image are amplified. The result should be the original image with the selected part amplified.
• OPINION: This doesn't work. This paper is even worse than the original spatial amplification paper. The results are really bad. The idea concept is problematic. The spatial frequencies of the selected image region are amplified instead of the spatial region itself. Processing in the frequency domain will do that (obviously!). For example, they have an image with some lines of text. They use the letter 'w' as their selected image region on the second beam, and they expect the output to amplify all the w's. Of course, frequencies from the letter w are present in other letters, and those other letters will be amplified as well. They try to perform a spatial operation in the frequency domain, and then spend a whole column and a half explaining why it didn't work as they expected. I thought this was common knowledge about frequency domain filtering by 1992.

Optical image encryption based on input plane and Fourier plane random encoding

http://www.opticsinfobase.org/abstract.cfm?URI=ol-20-7-767

• OBJECTIVE: encrypt/decrypt images optically, convert images to/from stationary white noise
• HOW: To encrypt the image, multiply the image with a random noise phase mask, n, in the input plane. Then put another random noise phase mask, b, in the Fourier plane. The output is an encrypted complex image with amplitude and phase. To decrypt the image, put the encoded complex image in the input plane and the random noise phase mask, b, in the F. plane. The output when imaged onto a CCD is the decrypted image since the CCD will capture the squared magnitude of the output beam.
• OPINION: This paper is truly a significant and influential paper in this field. 89 forward references! It has become a standard in the field of optical encryption.