Credit-card security holograms ...not true holograms either. Right? They need no monochromatic illumination. And they don't employ any interference in order to produce their 3d images.
That's the whole trick, really. "Scratch holograms" employ exactly the same physics as Benton Rainbow holograms. Both store phase information, and both require coherent illumination, but neither one is based on diffraction (that's why they can still work, even with a white light source.) If you make a Benton white-light hologram, then instead make the "diffraction fringes" about 1mm apart, the hologram STILL WORKS. "Scratch holograms" are just Rainbow holograms where the fringe-spacing is enormous.
So, if credit-card Rainbow Holograms are real holograms, Benton white-light holograms are real holograms, then to be consistent, we're forced to admit that scratch-holograms are holograms. (Besides, they create unwanted pseudoscopic images, and required spatially-coherent illumination, same limits encountered with any true hologram.)
That's the whole trick, really. "Scratch holograms" employ exactly the same physics as Benton Rainbow holograms. Both store phase information, and both require coherent illumination, but neither one is based on diffraction (that's why they can still work, even with a white light source.) If you make a Benton white-light hologram, then instead make the "diffraction fringes" about 1mm apart, the hologram STILL WORKS. "Scratch holograms" are just Rainbow holograms where the fringe-spacing is enormous.
So, if credit-card Rainbow Holograms are real holograms, Benton white-light holograms are real holograms, then to be consistent, we're forced to admit that scratch-holograms are holograms. (Besides, they create unwanted pseudoscopic images, and required spatially-coherent illumination, same limits encountered with any true hologram.)