Dinesh wrote:kaveh1000 wrote:Of course color holography is additive by its nature anyway.
Is it?
This is so good question!
I am not an expert, but I have my own 'point of view', which may differ from others. And, this post is not by any means intended to offend anyone, All I need is just clarification of some terminology, for which I consider that is very important, especially in such 'tough' stuff as is hologram and it's properties (at least for beginners, like I am).
I will be rather careful calling it either 'additive' or 'subtractive'. I will rather prefer word 'selective' (I will explain why below).
This is something I have been thinking about for a while now. Consider that an additive system, such as a colour TV, emits R,G and B in appropriate quantities to achive a particular colour balance. However, the TV itself emits the appropriate "wavelengths". If you illuminate a printed sheet with a white light source, then any colour is achieved - "emitted" - by selective absorption of the "unwanted" colours.
I will be careful here calling it 'emitted' too. Maybe re-emitted, or 'selectively reflected' to be more precise. While CRT TV (or monitor) really emit light by its phosphors, which are excited by electron impact, light which goes through color pigment is 'filtered' out only. All reflection which come from surface of pigment is NOT related with pigment's 'filtering' ability. Consider printing on black paper - no image, or image looks faint and weird.
Color chalk on the other hand working on black and white paper, that is because it is white chalk with added pigment(s), so each pigment has its own reflecting surface.
So, it is more complex; transmission with filtering, reflecting, and again transmission by another filtering.
Also, I have problem with understanding 'subtractive' pigments.
While printing is done by pigments of various colors (each one has different ability to selectively filtering, including black one - filtering out all light), pigments which come on top of each other looks dark (what remains is 'subtracted', so yes - subtractive).
But how to call pigments which are next to each other? Each filtering its own color, but what remains gives us impression of different color than each pigment. For example, blue and yellow pigment gives me impression of green(ish) color, but there are no mutual 'subtraction' between each pigment. Then, how to define this effect? Is it 'addition by filtered light' ? Or, such 'printing scheme' should to be avoided?
Our brain is 'fooled' all the time, no matter is it additive or subtractive, or whatsoever it is.
This is a subtractive colour system, since it "subtracts" by absorption all the undesired "colours" (wavelengths). Now consider the colour of a coloured object. When an object is illuminated by white light , then again, certain "colours" are absorbed by scattering processes and whatever's left is emitted in order to give a colour to the real-life object. Is this additive or subtratctive? The coloured object does not emit it's own radiation, but radiates the illuminant after selective absorption. Consider now a colour hologram. In this case, there are two processes - the recording and the reconstruction.
So far, so good.
In recording the hologram, the illuminant is some mixture of RGB (and I'm ignoring the fact that the object in a colour hologram is illuminated by coherent radiation, since the coherence of the illumination is required only for fringe stability and not for the colour characteristics per se)
Wait!
We can't ignore coherent radiation. We can't ignore relationship between 'the illuminant' and the 'reference beam' whatsoever this means. All what we can ignore (maybe) is object if we have intention to make HOE (or, the 'object' is a mirror, or another HOE? ). Then, there is no object - just two beams in exactly defined (very important!) relationship, by means of angle(s) and by means of intensity.
By plural of angle, I mean situation where two beams are not with identical angles in respect to normal incidence. The result is diffraction grating with different properties (orders of diffraction are not 'symmetric' to the hologram's plane). I think the result will be gratings which may refract light at different angle differently (intentionally avoiding words 'transmit' and/or 'reflect'). Isn't it the case of shifting 'position' of the color(s) of the monochrome hologram when we changing angle of the light or when we changing the plate in respect to light or to us?
By means of intensity, if two beams has not the same intensity - the result may give us also different properties of such HOE, for example lower diffraction efficiency. Isn't it the case of bright and dark holograms or part of the holograms?
I am still in 'monochrome mode', but still getting full color spectra of refracted white light - only lacking of proper 'alignment' of such color(s) with the ' object's ' color(s). By alignment, I mean not getting exact color of the object where it was, but rather a 'false colors'. And, I am getting blue hologram, but laser was green. Or variation of colors, depending where hologram is overexposed or underexposed, and also where emulsion is thicker or thinner.
whose ratio is given by some appropriate colour map - usually the CIE 1931. If the object re-emits this with no absorption, then the object is, in effect, a mirror, which brings up the point: Is a mirror illuminated by coloured light an additive or subtractive system?
So tough question! And good one!
I think a mirror is a 'selective' by nature. And, mirror can be either additive and/or subtractive (depend of type).
Gold mirror, for example, reflect part of the visible spectra(green, yellow, red, IR...), while another part transmit (blue, violet, etc.). Also, depending of the thickness of the gold. Very thin layer of the gold on the substrate absorb light, while transmit whole visible spectra.
While trying to deposit silver onto glass, I observed at fist black deposit - until certain thickness is not achieved, then it reflect most of the visible spectra. By looking through such mirror, blue and violet light is transmitted.
Dielectric mirrors are very different from metallic mirrors - it can selectively transmit or reflect light. How much, depend of thickness of 'colorless' layers, number of such layers, it's difference in refractive indexes, and so on.
All in all, I will avoid using words 'additive' or 'subtractive' in holographic world. That is because light itself is additive and/subtractive itself. This phenomena is actually responsible to our dark and bright fringes, and essential for holograms as well.
And, it may be confusing too - addition we call constructive, while subtraction we call destructive interference, while the phenomena is not strictly related by any perceptive 'color'.
We actually dealing with two different phenomena: perception of the colors, and properties of the light. And, yet it is very closely related when our perception is in question.
Also, what gives some substance its properties at atomic/molecular level is responsible for its appearance at macro level. If anyone look at the pigment under the microscope with really good magnification, may be surprised that there is no color at some point of the magnification - just structure. Yet, colorless substances may act as a good filter/reflector if properly 'stacked'.
Now consider the reconstruction of a colour hologram. In this case, the hologram is illuminated by white light and emits colour by Bragg selection. This means that unwanted colours - wavelengths - are absorbed, and what's left is emitted to give a specific colour. In other words, the colour of a reconstructed colour hologram depends on the colours in the illuminant that are absorbed. This sounds like an subtractive system.
I think I am covered this subject already above, but I will try to add some of my 'weird' point of view. I think that holograms doesn't require absorption, nor reflection - but rather 're-direction' of the light.
Yes, AgX hologram may absorb some light if silver grains remained (as is it case of not completely 'silvered' mirror), and produce image by diffraction of the light. But, if bleached, such hologram may work on different principle - instead blocking part of the light and causing diffraction, it rather refract light (but not exclusively).
And, reflection hologram doesn't require any 'mirror' at all. Diffracted/refracted light may experience multiple diffraction and refraction until reach 'critical point', where part (if not whole) of the spectra is reflected by total internal reflection.
//Edit: Now I found myself in great doubt about statement above. By observing one of the poor looking reflection hologram, one 'patch' of the hologram showing some reflection looks completely transparent at first glance. But, careful re-inspection showing faint 'suppression' of the green color, exactly where hologram showing greenish reflection. So, what remains: various thing happening 'inside' hologram, including diffraction, refraction and reflexion (and maybe something else, for which I am not aware at this moment).//
For this reason, I am thinking of hologram NOT by means of small mirrors and/or lenses, but rather of small diffractive gratings (with 'repeating', refractive elements).
Therefore, I would argue that the recording of a colour hologram may be an additive system (what colour is a chameleon on a mirror?),
Depend of its mood, may be green or orange/red (if threatened):
http://www.youtube.com/watch?v=JaKk5dDW ... =endscreen
but the reconstruction of a colour hologram by selective absorption of not-colours is a negative system.
I think I am answered above to that issue too. Maybe I am not so clear, maybe my English is not the best, and my understanding of light and optic is not the clearest as well. But, I feel that some of the terms in holography are not properly used most of the time. For that reason there are confusions from time to time.
Best wishes--
milan--
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