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Perhaps you are wondering what makes this ability possible. The answer lies in infrared rays. All reflected light that we can see with the naked eye represents a fractional portion of the electromagnetic spectrum, which is infinite. We refer to this section as "visible light". All around us, light is reflected which the human retina cannot detect, such as ultraviolet and infra-red radiation.
The visible part of the spectrum falls between the wavelengths of 430nm~690nm. (1nm=10-9m) Infrared rays have much larger wavelengths than this. We divide them into "Near Infrared Rays" (690nm-4,000nm) and "Extreme Infrared Rays" (over 4,000nm).
Unlike ultraviolet and visible rays, infrared rays tend to penetrate any medium rather easily because of their large wavelengths. This also means that infrared rays are not refracted much at all when passing from one medium to another. When we shine sunlight through a prism, it is refracted at an angle according to its wavelength. The blue end of the visible spectrum has the shortest wavelength, so is refracted the most. At the other end of the spectrum, beyond the red, visible light, infrared rays are barely refracted at all because of their long wavelength.
The KAYA PF exploits this characteristic of infrared light. It only lets through these long-wavelength rays, which have low refractive rays, and not all the ultraviolet and visible rays. Here's how this relates to the mannequin experiment:
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Almost all of the ultraviolet and visible rays are unable to permeate the fiber and are reflected back instead. Conversely, almost all of the infrared rays can easily permeate the material due to its low refractive rate. Having passed through the cloth, the infrared rays fail to penetrate the mannequin's surface and are reflected back.
The PF is struck by ultraviolet & visible rays that are reflected from the cloth and also the infrared rays that are reflected from the mannequin's surface. But the PF only lets the infrared rays pass through. The infrared rays are then transformed into electrical signals by the CCD of a camcorder, which forms those signals into a visible-light image.
In conclusion, the observer will be able to view the scene as though infrared light has become visible.
Of course, if you were to simply look through the PF, you would not see anything at all. Remember, the PF lets through infrared light only, which is invisible to the human eye. Therefore there must be some media, or device, installed to detect and record, or convert, the image. As mentioned previously, camcorders and digital cameras are suitable devices for this purpose because they employ a CCD which responds to both visible and infrared light.
Strictly speaking, it would be more accurate to regard these "See-Through" pictures as "near-infrared images" rather than "infrared images" since almost all CCDs can only respond up to 1400nm. Thus from hereon we shall use the term "near-infrared", or "NIR" when talking about the images the PF allows.
Fluoresced "See-Through"
The principle of "Fluoresced See-Through" is very different to the "Reflected See-Through" principle above.
When some substances are illuminated by certain wavelengths they reflect back not only those same wavelengths but also they may transform some of these into other, usually longer, wavelengths. For example, some substances may transform the illuminating visible light energy into longer-wavelength infrared energy.
What causes this phenomenon? The answer can be found in atomic physics and quantum mechanics. Within atoms, electrons orbit about a central nucleus. If a packet of light energy (a "photon") is absorbed by the atom, it causes one of the electrons to move out to a wider orbit. As described by quantum mechanics, atoms will only ever absorb radiation which has the right amount of energy to make one of the electrons perform this "quantum leap" to the next electron "shell". A photon's energy is dependent on its wavelength (and therefore, its colour), with violet being higher energy than red. However, an atom with an electron out of place is not stable for long so the electron falls back, re-releasing a 'photon' of energy. Some energy is lost so the photon given off is shifted towards the red end of the spectrum compared with the one absorbed.
A Fluoresced See-Through image is slightly more difficult to achieve. Besides the PF filter, an Infrared Cut Filter ("ICF") is required to create one. This filter performs the exact opposite task to the PF - it lets through visible light but cuts all infrared light from passing through. To capture a Fluoresced See-Through image, the ICF is placed over the light source to limit the incident light and prevent any infrared rays emitted by the source from reaching the subject. Some of the visible rays, which remain, striking the subject are changed to longer, invisible infrared rays and then the PF filter allows only these newly created infrared rays to pass into the camcorder or digital camera.
This Fluoresced See-Through technique often reveals characteristics of a subject that are not readily apparent through other examination methods including Reflected See-Through. For example, chlorophyll in plants does not reflect infrared rays, but it does fluoresce. This may provide a means for studying certain plant diseases. Similarly, this technique can be used for the study of inks, hardwoods, forged documents or paintings and sometimes startling results can be obtained.
To take more effective Fluoresced See-Through pictures, note the suggestions below:
| Environment | If possible, total darkness. |
| Light source | Tungsten or Electronic Flash. |
| ICF | KAYA's ICF1 over the
light source. This is not required if there is another way to ensure the subject is only illuminated by visible light. For example, wavelength-tunable lasers or blue-green lasers are ideal. Wavelength-tunable lasers have the advantage of being able to easily select from a wide range of wavelengths. |
| PF | PF2 or PF4. |