Pseudocoloring method using two different spatial modulations.

Pseudocoloring o f gray level information is a technique for introducing false colore in a black-and-white transparency. The importance of this operation is based on the human eye’s ability to distinguish different colore better than gray levels. In the last few years several pseudocoloring optical methods have been proposed.1-10 Furthermore, the pseudocoloring technique has been extended to encode image spatial spectral bands11 13 and holographic interferometric fringe patterns.14 In a recent paper15 we presented a further generalization of pseudocoloring for assigning false colore to depth level contours of 3-D objects. In this Letter we propose a modification o f the pseudoco­ loring speckle method9 for storing two images in a single recording material in such a way that each recorded image has a different spatial modulation. Therefore, in a decoding step, on illuminating the processed píate with a white light source, there exist two wavelength valúes for which the spatial spectra o f both encoded images are centered at the same spatial frequency valué. Thus, an additive mixture o f both images is produced with a pseudocoloring effect which depends on the two mentioned wavelengths. The scheme of the optical arrangement utilized in the encoding step is shown in Fig. 1. The image o f a certain scene E \ is recorded by using as a lens pupil a double aperture of separation d i. This causes a spatial spectra shift A u i given by A u i = d i/XD, where A is the light wavelength, and D is the image distance. Afterward, a record of the image o f another scene E 2 is done in the same píate but now using as a lens pupil a double aperture of separation d2. Thus, the spatial spectra shift A l¿2 is given by A u2 = d2/XD. The decoding step is shown in Fig. 2. The developed píate H is illuminated with a collimated white light beam. Then, for a certain valué of the viewing angle d, there exists a superposition of the £ 1 and E 2 images in the colors given by the corresponding wavelengths Ai and Á2 such that, d2/d 1 = Á1/Á2 In this way, all the common parts of the E\ and E 2 images are pseudocolored by a color mixture which depends on Áj and X2. So far no relation exists between E\ and E 2. However, if E i and E 2 images are complementary (for object transpar­ entes, the positive and negative images), the image resulting from the superposition is a pseudocolored one of either E\ or E 2. The corresponding contrast reversed image can be obtained employing the method of Ref. 10, that is, recording the light scattered from the silver developed grains that form the original image transparency. An alternative approach con­ sista of employing the optical subtraction technique through Young’s fringes modulated speckle.16 In this case, a first record of the image of a speckle pattern is made. Then the image of the same speckle pattern is recorded but modulated by the object transparency E\, introducing a 7r-phase delay between both exposures. Thus, a contrast reversed image of E\ is obtained. Another case of interest arises when the image ofE 2, instead of being the complementary one of E\, is a modified versión of it. In this case, the third pseudocolor encodes the common regions o f both scenes. Some experiments have been done using as E i the isochromatic fringe pattern that appears in a loaded photoelastic model. The scene E-> corresponds to the complementary fringe pattern (obtained by rotating the polarization state o f the incident light by 90°) but with a d if­ ferent loading condition. Therefore, in the decoding step, the resulting moire pattern appears in a pseudocolor given by the mixture o f Xt and A2. Finally, another point of interest for this pseudocolor en­ coding consists of studying the spatial changes in one direction of an object transparency. In this case, the E 2 image is a shifted versión of the E 1 image. The amount o f shifting de­ pends on the size of the finest details to be studied. Thus, the sign of the gray level change is coded by a spectral color which corresponds to Ai or A2. In summary, we have presented a simple method for generalizing the speckle pseudocoloring technique to inelude other cases besides gray level pseudocoloring. An advantage of this method consists of employing colors, given by Ai and A2, that are spectral. When using green and red as false colors, as usual, it is possible with this method to obtain a third color which lies in a straight line in the chromatic diagram, very cióse to the locus of the spectral colors. In this way, false colors of high purity are obtained.