Search

Colorimetric data can be readily computed from measured spectral data, however, as illustrated by metameric pairs, the mapping from spectral data to colorimetric values is many-to-one and therefore typically not invertible. In this paper, we investigate inversions of the spectrum-to-colorimetry mapping when the input is constrained to a single color reproduction medium. Under this constraint, accurate recovery of spectral data from colorimetric data is demonstrated for a number of different color reproduction processes. Applications of the spectrum reconstruction process are discussed and demonstrated through examples.

This paper introduces a moire-free color half toning algorithm. Moire patterns might appear in traditional halftone images. This could be caused by interference among halftone screens of different colors, or beating between he input image and the screens. In our algorithm, we adjust the halftone screens of different colors, or beating between the input image and the screens. In our algorithm, we adjust the halftone output in such a way that the low-frequency difference between the input and the appearance of the output is minimized. Due to non-linear color mixing, the appearance of a color print is not a simple combination of CMYK four channels, and is hence not easily predictable. We use the 2 by 2 printer model to estimate the pixel-level CIE-Lab values from the halftone output. Next we calculate the difference between the original CIE-Lab input image and the 2 by 2 estimation and run a low-pass filtering on the result. Then, we feed back the low-frequency difference into the input. The low-frequency-feedback process cancels the potential color moire patterns.

Invisible watermark images can be incorporated in printed halftone images using specially designed halftone screens. The watermark information is embedded into the image by varying the spatial correlation of the halftone texture. The halftone screen with embedded watermarks can be used exactly as a normal halftone screen, so there is no additional image processing required for processing individual images to embed watermarks. Once the binary output image is printed on the paper, the correlation of the binary image is converted into physical spatial correlation between neighboring areas of the printed image. This correlation relation is not visible to the eye but it can be detected by scanning the printed image on a desktop scanner and processing the scanned image. Printer and scanner distortions can interfere with the self-alignment of the scanned image, so localized adjustments are made to detect the embedded spatial correlation information in the watermarked image. The retrieval of this watermark is robust to copying and distortion and it can be detected in reproductions of the halftone image.

Introduction of 2 X 2 centering can significantly reduce the number of dot patterns required in color calibration. As a result, local calibration can be performed using direct measurement and fast table lookup. In this paper, 2 X 2 calibration is applied to error diffusion. A novel concept for error diffusion is introduced. It matches the color increment instead of color. The concept makes integration of error diffusion and 2 X 2 calibration natural and free of causality issue.

A method will be shown to incorporate digital watermarks in printed halftone images using stochastic screens. The watermark is not visible to the eye and introduces no loss in image quality. Although it cannot be seen, the watermark can be extracted at a later time with post processing. Watermarks of high contrast are incorporated in the image by varying the statistics of the stochastic screen. The watermark information can be made visible by comparing the relative changes in spatial correlation in the halftone texture of the image. Watermarking allows a printed image to be tested for the purposes of identifying the owner or the source of the image. Arbitrary customer information can be incorporated into the image, including variable information, such as the data or time of day. The technique is robust to copying of the printed image and the watermark can be detected in reproductions of the halftoned image.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Hexagons are widely observed as a packing or tiling geometry in nature, yet they appear to have been avoided in conventional halftone tiling. A goal of the present study is to understand the potential barriers that have prevented their use and present new halftone geometry options that overcome the issues while offering several potential benefits. While conventional halftone geometries often include the fourth screen (e.g., yellow) in a suboptimal manner, the hexagonal geometry presented here can include a clustered-dot fourth screen moire-free. Hexagonal screens can appear to have smoother texture. Due to differences in packing geometry and touch point geometry, hexagons have the potential to possess different tone reproduction characteristics, which may be favorable for some marking processes. We also present a parametrically controlled hexagonal halftone spot function that allows for optimization of dot touch points and provides compact growth. The controllable touch points can prevent a tone reproduction discontinuity, while the compact growth throughout the gray range ensures maximum stability. Additionally, we present a three-colorant dot-off-dot halftone configuration using hexagonal geometry. Examples are provided.

Particle measurements are of importance in the study of aerosols, combustion processes, and pharmaceuticals. In this report, first we briefly review a simple derivation of an inversion formula from basic principles. We describe a modification to the usual form based upon intensity superposition so that one has a form that is accurate at tiny scattering angles. Then, we present the results of a new study of the wavelength dependence of the optical transform of a particulate distribution. Starting with the idealized amplitude transmittance, p(x−x m ,y−y m )for a particle at spatial position (x m ,y m ) and taking the Fourier transform, one finds immediately the exponential term given by exp [−i2π(f x x m + fY m )] in which the transform frequencies are f x = ξν/(Fc) and f y = ην/(Fc). We show how this exponent contains the essence of the temporal frequency dependence of the speckle pattern for the particulate cloud. We calculate the second moment of the intensity in the optical transform. We prove theoretically that it is possible to measure the axial distribution of a particulate sample simply by gathering optical transform data by using illumination by a tunable laser. This novel technique is then demonstrated with (1) computer simulations and (2) masks with various axial distributions. Some concluding remarks will also be made about the use of neural network software in re mote sensing of particulate sizing and spatial distributions.

Automatic recognition of faces is now a reality. In our experiments we seek to establish bounds on the information content required as a function of the configuration of the machine. In general with the direct image, excellent recognition is obtained for information capacity below Harmon’s (1973) benchmark value of 500. Our main data base is gathered from twenty-five people consisting of three views (front, 30° turn, 60° turn) and ten separate exposures for each view. Typically, these are grouped into a learning set of five and a testing set of five. In the first series of experiments, an optical Fourier transform configuration is used which includes a ring-wedge photodetector interface together with neural network software. Good recognition is obtained, and we report on the training using front and side views. In the second series of experiments, frame-grabbed images are used, again with the neural network software. Tests are made of the information capacity required as a function of the image coding. Comparative error rates are presented for recognition in the following cases: with 256 grey levels or using global binarization or using an error-diffusion halftoning algorithm. Particularly good results are obtained from the error-diffusion form of the imagery.

Color printer calibration is the process of deriving correction functions for device signals (e.g., CMYK), so that the device can be maintained with a fixed known characteristic color response. Since the colorimetric response of the printer can be a strong function of the halftone, the calibration process must be repeated for every halftone supported by the printer. The effort involved in the calibration process thus increases linearly with the number of halftoning methods. In the past few years, it has become common for high-end digital color printers to be equipped with a large number of halftones, thus making the calibration process onerous. We propose a halftone independent method for correcting color (CMY or CMYK) printer drift. Our corrections are derived by measuring a small number of halftone independent fundamental binary patterns based on the 22 binary printer model by Wang et al. Hence, the required measurements do not increase as more halftoning methods are added. First, we derive a halftone correction factor (HCF) that exploits the knowledge of the relationship between the true printer response and the 22-model predicted response for a given halftoning scheme. Therefore, the true color drift can be accurately predicted from halftone-independent measurements and corrected correspondingly. Further, we develop extensions of our proposed color correction framework to the case when the measurements of our fundamental binary patches are acquired by a common desktop scanner. Finally, we exploit the application of the HCF to correct color drift across different media (papers) and for halftoneindependent spatial nonuniformity correction.

Using halftone techniques and additive color mixture notions, we have implemented different ways to obtain colors matching CIE specifications from hard-copy color printing devices. We also report on our similar studies for matching colors to the Optical Society of America uniform color scales. The range of color reproduction is obtained by determining the tristimulus values of the color primaries. As long as the given color is within this color reproduction range, it can be reproduced by selecting the proper color primaries and the required amount of each primary. For colors outside the color reproduction range, we compare different ways to make the closest match. We performed experiments in which we used a thermal-wax-print color image printer that was calibrated by a precision spectrophotometer. We describe the error in matching colors quantitatively in units of "just-noticeable differences."

Using a pulsed, pump-probe experimental arrangement, we have investigated the speckle generated by a 100 μm path-length cell of smectic A liquid crystal of 4-cyano-4′- octylbiphenyl doped with 0.1% wt/wt dichroic dye. Upon irradiation, the guest-host system undergoes a phase transition from an initially transparent condition to a scattering state. From the statistical theory of speckle, we are seeking to establish the effective scales of turbulence in the liquid crystal, both transversely and longitudinally. A time-resolved analysis of the observed speckle during the relaxation of the system back to the ordered, homeotropic condition revealed the effective scale lengths of the liquid-crystaldye mixture in the irradiated region. Dynamic response as a function of molecular anchoring conditions was also investigated. The importance of this research to power-limiting applications will be discussed.

We describe the lens design for a twin-imaging white-light interferometer in which the interference pattern at the exit-pupil plane is the cosine transform of the spatial-intensity distribution of the object. The achromatic condition in terms of optical power is derived. The analysis of the transform aberration shows that the even aberrations, e.g., spherical aberration and field curvature, do not degrade the cosine transform and need not be corrected. This significant simplification permits us to design systems with good performance and uncomplicated lens structures. We present a lens design with three elements and a length of 320 mm. The system is capable of resolving more than 10(6) pixels with an operating spectral bandwidth of 100 nm. The results of an experiment with an early four-element design are also presented.

A theoretical study is presented comparing the performance of several correlation filters including the Fourier-based matched filter, the Hartley transform system, and the sine-filter-cosine configuration.1 In principle all these can be realized in systems using incoherent illumination, but the latter two possess the distinct advantage that the filter transmittance is a real number. Both a sine filter and a binarized sine filter are studied for the sine-cosine cascade correlator, and the latter is clearly shown to be superior. The evaluation is based on calculations of the ratio of peak output signal-to-noise as is customary. Additionally it is important to consider the ambient background signal in regions near the correlation peak and the spatial variation in this local background. We show that all four systems provide matched filter performance. However, the sine transform filter and its binary version are superior, since for these the mean value of the local background is zero. Computer simulations are provided, illustrating the two important applications of sorting pictorial scenes and words.

While diffraction pattern sampling has been widely used in the classification of patterns, its usage has been somewhat limited by the need to devise rather sophisticated algorithms. We describe sorting or classifying a variety of patterns using the ring-wedge photodetector and commercially available neural network software. With this combination, it is no longer necessary to write specialized software, and experiments which previously might have taken 4-6 months are now completed in 4 h or less. Three separate experiments are described quantitatively. First, thumbprints are sorted for eight individuals using six prints from each person. A learning set of four prints is shown to give excellent results and both ring and wedge data are used for the input neuron signals. Second, as an experiment in particulate analysis, colloidal suspensions of polyvinyltoluene spheres have been measured over a concentration of 256-1. Finally, automatic image quality assessment is described using a variety of input imagery. Widespread new uses are predicted for this hybrid system that combines the ring-wedge photodetector and the neural network.

Optical transforms of 2-D and 3-D objects taken in spatially incoherent broadband illumination are described. Two optical systems are compared: one is a lensless Fresnel-zone system and the other uses a Fourier achromat. Both are optoelectronic hybrids with a twin-imaging interferometer, a CCD array, and an output digital computer. Diffraction-limited performance is obtained and a variety of transforms including 2-D spatial sine, cosine, Fresnel, and Hartley are discussed. Also we describe a series of correlation experiments in which an optical transform and a computer-stored transform are multiplied and inverted. In this manner objects correlated include fingerprints on opaque cards, actual rough currency, and 3-D models. In a separate group of experiments image reconstructions were obtained. First, a computer method is used to invert the optical transform after bias removal and filtering, and the image reconstruction is displayed on a video terminal. This gives us an excellent means of measuring our overall space–bandwidth product. Second, image reconstruction using a cascade of two twin imaging interferometers is also described. Interesting optical processor applications for this two-stage system are briefly described.

A novel sine-filter-cosine correlator is described for coherent optical processing. The advantages of this system are that it uses filter masks that are real-valued and non-negative while still providing a maximum value of SNR in the output, i.e., matched filter performance. Additionally and quite importantly, this system also provides desirable control over the shape of the correlation signal. Theoretical justification is presented by calculations of the shape of the cross-correlation of two real-valued functions compared and contrasted with corresponding correlations of their even and odd parts separately.

In earlier work an optoelectronic hybrid consisting of a twin-imaging interferometer with crossed roof prisms, a Fourier transform achromat, a CCD array, and a digital computer was described. Spatial cosinusoidal optical transforms can be recorded for rough objects using bandlimited white light. We describe optical metrology and feature extraction using this system. If we consider an idealized amplitude transmittance t(x,y), the advantage in the white light system is that it measures the spatial transform of |t(x,y)|2. In cases where the phase portion contributes a signal that is of little or no interest, clearly the white light transform can be preferable, say, to a system that measures and transforms t(x,y). We describe two separate experiments. For optical metrology, we measure the film grain scattering from commercially available color negative film of various ISO ratings and at different densities. In a separate series of experiments we measured the optical transform pattern that results from printouts of various digital halftones, i.e., the analog of film grain scattering in the electronic printing industry. Finally, we conclude with a discussion of pattern recognition using this white light system combined with neural network software.

Theory and techniques of white light interferometry have been studied to devise an optoelectronic hybrid for diffraction pattern sampling. For an incoherently illuminated input scene, we obtain an output intensity that is the 2-D spatial cosine transform of the input plus a bias term. The bias can be subtracted electronically to within shot noise fluctuation limitations. The optical system consists of a double-imaging interferometer with a beam-splitter cube and crossed right-angle-prism reflectors followed by an achromatic-optical-transform lens pair. A photodiode array is placed in the optical transform plane, and this is coupled to a digital computer for further signal processing. Theoretical results are presented for the synthesis of the cosinusoidal transform configuration. From Fresnel-zone theory, a general form is derived for the impulse response of a cascade of ideal thin lenses, and this is used to design Fourier transform achromats that are well suited to this application. Experiments are presented showing cosine transforms for rough objects and for objects in reflection using illumination of negligible spatial coherence at the object. Excellent sensitivity is obtained with the bias subtraction, and a high space–bandwidth product is attained for the system.

A printing apparatus and method for duplex printing a document having a show-through watermark wherein the watermark is comprised of first and second images on both sides of the document. The images comprise stoclustic halftone screens having a substantially common registration wherein the second image is relatively shifted from the first image for forming an associated overlay pattern between the first and second images resulting in a recognizable pattern comprising the watermark upon illumination by a show-through light source. The watermark is unrecognizable from either image alone upon illumination from a front light source. The relative shifting can be implemented by either phase, frequency or angle shifting between the front and back side images.

Two-dimensional sine, cosine, and Fourier transforms are obtained for objects illuminated by spatially incoherent, bandlimited light. A lensless Fresnel zone processor consisting of a double-imaging interferometer, a CCD array, and a digital computer is described and analyzed. Excellent bias removal and shot-noise level performance are obtained using a 14-bit digitizer. Experimental optical transforms are presented for an offset circular aperture and for a binary zone plate transmittance. The space–bandwidth product is derived, and excellent comparison with experiment is obtained.

A novel sine-filter-cosine cascade is proposed that is realizable for either coherent or spatially incoherent illumination. Computer simulation is used to demonstrate effectively matched filter performance and elimination of deleterious background bias.