Invisible Imaging

The older diazotype printing uses stabilized diazonium salts such as (30) and diazo couplers such as (31). When a diazonium salt contacts the diazonium coupler, which is present in diazo paper or film, an azo dye is produced [24], 

The technologies described above produce visible images on a hard copy, usually paper or special media. However, there are important technologies which produce invisible images, usually for entertainment and publishing, using functional dyes. 

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Latent image Invisible image-precursor formed by the imaging light it makes a region of the material developable. 

The multicolor photochromism can be extended to near-infrared region by using Ag nanorods so that invisible images, which are viewable with infrared cameras, can be drawn and overlaid with a visible image [20]. Polarization-selective imaging is also possible with nanorods. 

There are several ways to deal with this problem. The simplest in concept is to use liquid imbibed particles that are uniform in structure, such as sugar cubes [12, 13] as already mentioned above. To my knowledge, this has never been done for structure studies. The second is to use NMR-invisible particles, for example, glass, and fill the spaces between the particles with a liquid such as water that can be imaged. The third is to use the usual radially heterogeneous particles and then use software to calculate the parameters. One first determines the centers of each particle and then decides whether the particles are in contact with each other or not by the distances between the centers [14]. 

The solution to access the invisible areas is readily copied from texture analysis tilt the sample by y and receive 1 data point on the meridian that corresponds to S3 = (2/A) sin y/. The result of the mapping is shown in Fig. 2.7. Thus by recording a series of images taken at different tilt angles of the fiber the blind area can be covered to a sufficient extent. Finally, the remnant blind spots may be covered by means of... 

For more information about the pigments themselves, try A History of Lettering, N. Gray, Phaidon, Oxford, 1986, or The Icon Image of the Invisible, Egon Sendler, Oakwood, 1988. Chapters 12-15 of the latter book are a mine of detail concerning the manufacture of pigments. 

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