What is real color

[Dinesh]

it has been used for describing the method of recording a rainbow hologram with multiple exposures at multiple angles and with multiple objects to get the desired colours for the composite at a given viewing angle

But all rainbow holograms are like this. All rainbow holograms (or at least any rainbow hologram with multiple H1's which is how all commercial rainbows are made) have multiple slits at multiple angles. If you look at any printed hologram, you'll see multiple colours that shift as you change your angle of view. Are we then to call all rainbow hologram 'pseudo-colour' ? If so, then the cohesion of any definition vanishes; if everything is black, why define white?

I am drawn towards

"Pseudo color" in holography is the color that is not related to the reflection spectrum of the object and is created solely by special processing using single laser wavelength. Much like painted b&w photograph.

"Real color" is the color caused by different reflection of different wavelengths by the object. Here we have to distinguish between "just real color" (using 3 or sometimes 2 lasers) and "true color" that represents the full spectrum (like Lippmann photography). Looks like "true color" is impossible in holography.

..with some reservations.I'm not sure how Gall differentiates between "just real colour" using two or three lasers and "true colour", presumably also using two or three lasers. If "true colour" means a faithful recording of a coloured object while "just real colour" means recording a coloured object and getting an unfaithful representation, then it seems to me that the difference lies in the technique (exposures, beam ratios, wavelength ratios) and the chemistry. If the above is true, then it is possible to get a fairly faithful representation of the colours in a coloured object (see below).

Consider a red cardboard with one green strip and one black strip.

Using two lasers in a "real color" process, we'll able to record that strip without any modification to the object.

But, if you record your box with one green stripe and one black stripe and the hologram shows one green and one black stripe, then isn't this also "true colour"? Is Gail saying that if I painted an entire spectrum on the box and then recorded it, only a subset of the stripes would show on the hologram, or that some colours would be faithfully recorded and some not so faithfully; and if the entire set showed on the hologram, then it'd be "true colour"?

Lets say you could record the exact same hologram with the exact same fringes with 3 exposures with 3 objects and exposure angles with one wavelength, and with 1 exposure with 1 object and three wavelengths. Would it make sense to call one pseudo colour and one real colour? Even though they are identical?

Doesn't this get to the heart of what is meant by "colour" in holography? It seems to me that the observer is left out of these discussions and the interaction of the observer and the creator seems to me to be the vital link. The technique whereby the colours end up on the finished piece is hidden from the observer, so it seems to me that 'pseudocolour' in holography is no different to a photographer using a set of filters to enhance the colour variations on a photograph. However, no photographer would call it "pseudo-colour photograph". Whoever invented the term (I thought it was Fred Unterseher, apologies to Jeff if I was wrong!) presumably did so historically because the colours in a scene could either be recorded with multiple lasers, or, if you didn't have multiple lasers, could be recorded with one laser. Historically, this way was an easy way to impose colours on the cheap and the artificial imposition of the colours were "pseudo" because the colours were recorded in some fake way - you didn't record the actual colours coming off the real object. However, the work of John Kaufman shows that the colours can be surreal and still be pseudocolour. Could those rocks be made into the colours that Kauffman made be done with three lasers. Yes. Simply colour the rocks first. I think that today terms like "pseudo-colour" have lost their meaning because you no longer need to artificially impose colours on a scene. In the end, it's up to the observer what the impact of the colours are - whatever technique was used in making them. Some of Dave Battin's work is pretty striking. I have no idea, at first glance, whether he uses swelling or multiple lasers. Personally, I don't care, I just appreciate the play of colours in the work.

In terms of the observer, let's say your three objects are a pink box, a yellow box and an orange box. If you recorded all three objects with three lasers and got a faithful image, that is: a hologram showing a pink box, an yellow box and an orange box. Now you show that hologram to a friend. Your friend will say, "That's a colour hologram". Now, still staying with the three lasers, let's say that the pink box shows red in the hologram, the yellow shows as blue and the orange shows as green. Then, your friend will say, "That's a colour hologram". If I shot each box individually with one laser, one shot at a time and swelled between each shot. Now I make my pink box swell to green, my yellow box swell to red and my orange box I leave alone. Your friend will still say, "That's a colour hologram" Your friend has no idea what the original scene looked like. So, to your friend (the observer) you've either made three colour holograms, a "true colour" hologram a la Gall, a "real colour hologram" - also a la Gall, or a pseudocolour hologram. Is the differentiation then only in the head of the holographer?

[dannybee]

"It depends on what the meaning of the word 'is' is.

if the word mean what is defined in the dictionary then (real) means real, not ultered.. not fake, but then if you make up your own defintions like "thats bad" meaning thats good. but what the hey maybe you white is black and your green is red

[Dinesh]

These are three images from a colour holography shot. The first one is the H2, the second one is the real object and the third one is the H1. The point is that I think the "real" colours are represented, but I had to carefully choose the ratios, processing etc. I didn't just point and click. I also could have chosen to make these colours anything I liked and made a conscious decision to get the colours as real to the original as possible, this is not a choice have for a colour photograph. So Is this "real colour", "true colour", or" pseudo colour" ?

[Gall]

If a hologram is meant to replay with multiple colours in a single view but the colour components during replay are not the same as the wavelength(s) used for recording, call the recording method pseudo colour.

Correct, exactly what I meant. However, I'd like to mention that small wavelenght shift is not considered "pseudo-color" as long as it does not affect overall perception (i.e., is not turning red into gold like monochrome PFG-03 processing). It still is "real color" if red laser causes red replay and green causes green, even if red was 670 and turned into 640.

Another point is, many dyes present in natural products or plastics are more or less dichroic. Some of them are even hard to photograph, especially under fluorescent light, which has strong narrow green mercury lines in its spectrum. Such objects definitely will cause problems even in real-colour holography. This probably can be solved using digital color correction only.

[Gall]

Talking about the resulting image, one may have 3 situations. The difference in color between the original and the image can be:
1 - easily viewed with bare eye
2 - (almost) invisible for bare eye but can be found using spectral measurements
3 - is (almost) invisible even through a spectroscope.

If we consider the process, there are also 3 situations:
1 - each color of the resulting image is recorded from the reflection of light of similar wavelength from the object
2 - each color of the resulting image is recorded from the reflection of light of significantly different wavelength. Manual color correction methods are applied to achieve the correct color representation if it is correct at all.
3 - there is no object at all, it's a digital 3d model

[Dinesh]

PinkysBrain wrote:If a hologram is meant to replay with multiple colours in a single view but the colour components during replay are not the same as the wavelength(s) used for recording, call the recording method pseudo colour.

Correct, exactly what I meant. However, I'd like to mention that small wavelenght shift is not considered "pseudo-color" as long as it does not affect overall perception (i.e., is not turning red into gold like monochrome PFG-03 processing). It still is "real color" if red laser causes red replay and green causes green, even if red was 670 and turned into 640.

But, for a colour hologram recorded with 3 lasers, there is colour mixing. A yellow object giving a yellow image recorded with a red and a green laser reconstructs in neither red nor green, so " but the colour components during replay are not the same as the wavelength(s) used for recording" is true, but apparently pseudocolour under this definition: " call the recording method pseudo colour."

I believe that John Pecora asked this question a while back, but I'll repeat it anyway:
If I make a yellow image with green and red lasers, is the diffracted image wave a mixture of red waves and green waves, or is it a yellow wave? Have you created a new wave, or have you simply diffracted the recorded waves?

I think there are actually two questions here"
1. What is pseudocolour
2. What is the meaning of colour in holography?

I believe the second question is much more difficult to answer because all colour reproductions methods to date are concerned with either projected colour (if you allow that the TV/lcd gamut is 'projected colour', but generated by fluorescence or by a colour separation method that relies on the bandwidth of the separation method) or with printed colour. So, is diffracted colour RGB, CMYK, something else?

I suspect a lot of people are going to simply shrug their shoulders with a "who cares" response. But, I believe that colour holography in general will not progress unless people study the meaning of colour in holography and not just take a for-granted attitude that "red is red and green is green, right?", just as the colour printing process did not progress until the advent of the 1931 CIE chart. I'm fairly sure that a lot of people had a similar "who cares, I'm making money and that's what's important"" view when the idea of creating a standardised theory of colour was proposed in the 1920's!

[PinkysBrain]

But, for a colour hologram recorded with 3 lasers, there is colour mixing. A yellow object giving a yellow image recorded with a red and a green laser reconstructs in neither red nor green, so " but the colour components during replay are not the same as the wavelength(s) used for recording" is true, but apparently pseudocolour under this definition: " call the recording method pseudo colour."

I said colour components instead of colours for that exact reason of course. That the colours are meant to be seen with the human trichromatic visual system is a base assumption really (lets leave the small percentage of tetra-chromatic mutants out of it ).

I believe the second question is much more difficult to answer because all colour reproductions methods to date are concerned with either projected colour (if you allow that the TV/lcd gamut is 'projected colour', but generated by fluorescence or by a colour separation method that relies on the bandwidth of the separation method) or with printed colour. So, is diffracted colour RGB, CMYK, something else?

Generally RGB, occasionally some other combination ... but only with RGB can you cover the visible colour spectrum well.

CMYK is a printing process, it's still meant to reproduce the RGB spectrally isolated colour components our eyes perceive.

[Dinesh]

Generally RGB, occasionally some other combination ... but only with RGB can you cover the visible colour spectrum well.

CMYK is a printing process, it's still meant to reproduce the RGB spectrally isolated colour components our eyes perceive.

CMYK reproduces the components our eyes need by isolating them from the white light that hits the printed page, hence the CMYK components are the inverse of the RGB components. As you know, to get red, you print its inverse, thus the white light hitting the printed page has all but red removed, leaving red. However, in holography, you hit the plate with a red beam to record, but reconstruct it with white light from which all but the red components are removed; what's left - red - is then received by the eye. Isn't this reconstruction in CMYK? It seems that in holography the recording process is RGB, but the reconstruction process is CMYK. Again, if you do not illuminate a printed page, ie look at it in the dark, then you receive black. If you do not illuminate a hologram, you also receive black. Another interesting fact: RGB sources are self-luminous, but CMYK "sources" require illumination.

(lets leave the small percentage of tetra-chromatic mutants out of it ).

So how did you know about the experiments done on my parents in that government lab???

[PinkysBrain]

However, in holography, you hit the plate with a red beam to record, but reconstruct it with white light from which all but the red components are removed; what's left - red - is then received by the eye. Isn't this reconstruction in CMYK?

No, at best it's reconstruction by subtractive White-R filtering. It's more useful to talk about it as a form of selection than subtraction though, it makes what happens when you use multiple wavelengths more naturally follow the language. Whereas if you say the hologram subtracts the rest of the spectrum trying to describe what happens with multiple wavelengths becomes needlessly complex.

The hologram selects the red wavelengths near the recording wavelength to be reflected, if you also use a green laser during recording then it also selects green wavelengths near the recording wavelength to be reflected.

[Ed Wesly]

As you know, to get red, you print its inverse, thus the white light hitting the printed page has all but red removed, leaving red.

Maybe that’s what you know, but anyone who really knows how RGB and CMYK are related, to get red, you print the inverses of green and blue, magenta and yellow, to absorb their complements, green and blue, thus leaving only the red component reflected from the white light. If you print the inverse of red, which is cyan, you get the other two of the color trinity reflected, which are green and blue.

Isn't this reconstruction in CMYK? It seems that in holography the recording process is RGB, but the reconstruction process is CMYK.

It looks like you’re missing the basic principle of the two modes of color reproduction: either add up to white with R, G, & B light sources, or start with white and subtract down to black using layers of C, M, Y, and sometimes K. (Chemical color films and prints do not use black dyes; printing presses and inkjet printers do use it.)

Since the hologram can be construed as altering the replay light, it acts like a light source, and its mode of color reproduction uses the additive primaries of RGB, like a monitor. That might explain why your image above is so hard to see!

Put that in your Kogelnik and smoke it!