An optical cryptosystem based on the joint Fresnel transform correlator (JFTC) with double optical wedges is designed. The designed cryptosystem retains the two major advantages of JTC-based optical cryptosystems. First, the encrypted image is real-valued and therefore is easier to record and transmit. Second, the encryption process is simplified, since it doesn’t require accurate alignment of optical elements or the generation of the complex conjugate of the key. Also, the designed optical cryptosystem can produce a decrypted image with higher quality than a JTC-based optical cryptosystem, because the original encrypted image is divided by the Fresnel transform power distribution of the key mask to generate the new encrypted image, which significantly reduces the noise during the decryption process. Simulation results showed that the correlation coefficient of the decrypted image and the original image can reach as large as 0.9819 after denoising and adequately selecting half-central interval $a$ and encrypted image width $w$. Another improvement relative to JTC-based optical cryptosystems is that the attack resistibility gets enhanced due to the nonlinearity of the encryption process as well as the additional key parameter $a$, which enlarges the key space.

A simple method is proposed to calibrate the fisheye distortion center, which synthesizes the algorithms of the ellipse fitting and Gaussian surface fitting. Through analyzing the imaging projection of the equidistant fisheye lens, the calibration principle is introduced in detail. From the experimental results, we find that ellipse fitting error and the relative error of Gaussian surface fitting are less than 0.06 pixels and 1.5%, respectively, and the proposed method has a higher calibration precision and stability compared with the methods in the inferences. Besides, the proposed method does not rely on any particular distortion model and other fisheye parameters, and only the image information of the calibration board is utilized, which reduces the calibration complexity and is suitable for the center calibration of any equidistant fisheye camera.

A noiseless optical cryptosystem based on a joint Fresnel transform correlator (JFTC) with a Billet split lens (BSL) was designed. Benefitting from the adjustable structure of the BSL, we can control the size and recording position of the joint Fresnel transform power distribution by changing the split width of the BSL. To avoid the text being affected by noise, we converted the text into quick response (QR) code in advance, then used the QR code as the plaintext and utilized the noise tolerance ability of the QR code to realize noiseless recovery of the text. In this cryptosystem, we designed a complex secret key (CSK) to replace the secret key made from a random phase mask, and the CSK consists of a QR code filter and a spiral phase mask. Because the QR code and spiral phase can be determined by a few simple data, the transmission and preservation of the CSK is convenient. Finally, the validity and feasibility of the cryptosystem were demonstrated, and the sensitivity of the CSK was researched by simulation and experiment.