I plan to get the 30 mW 405nm Mode-Hop Free laser module from Analog Technologies Inc (http://www.analogtechnologies.com/) some time soon. I estimate the senstivity of DCG at 405 nm to be in the region of 0.5 mW/cm*cm. For a 10 um thick DCG layer the dichromate concentration will have to be very low (~0.1 %) to prevent too much of the 405nm from being absorbed when using single-beam techniques. This low dichromate concentration has the advantage of lengthening the usable lifetime of a DCG plate to ~2 weeks.The disadvantage is that the developed DCG hologram will have to be 'swelled' from 400nm to ~540nm to bring it to the maximum sensitivity of the human eye.But then, DCG is renowned for its ability to reconstruct at much higher wavelengths given the right processing conditions. If you are interested the module is: "ATMF101-DL-LS5005-30-405" ($3,150, 30mW 405nm)
Interesting! This laser module also makes it possible to record transmission holograms in the Shipley S1800 photoresist. New vistas are opening up for the amateur holographer!
I am still wondering about those blue/violet diodes at:
www.photonicproducts.com/products/laser_diode_modules/ellipticaltec_laser.htm
They have a 4 mW diode at 440 nm.
"I estimate the senstivity of DCG at 405 nm to be in the region of 0.5 mW/cm*cm."
In "Topics in Applied Physics vol 20, Hologrpahic Recording Materials" there is a section on DCG by D. Meyehofer. In this there are absorption curves for DCG which he (Meyerhofer) references from a paper by Shankoff in Applied Optics going back to 1968. Anyway, in this curve the sensitivity at 405 seems to be about twice the sensitivity at 488. Obviously, this depends on the concentration of dichromate in the gelatin, and even the type (Potassium as opposed to Ammonium). I believe in those early days they all did their DCG by fixing out the silver from Kodak 649F (at least, all the early books going back to Collier and Lin say so). In addition, gelatin is very slightly birefringent so the density of gelatin will also have an effect.
There's a lot being said about the sensitivity of DCG. I hate to say this as a scientist of sorts (maybe a delusion of sorts!) but I basically look at it and say, "That looks like 120 secs". Then I tweak.
Although I've never used DCG at such low wavelengths, I have used resist. In that experience, I found that the extremely low sensitivity picked up a lot of noise. Baffling, or carding seems to be even more important.
"Wavelength (nm)Required exposure energy (mj/cm2)
514 nm 200 to 400
488 nm 40 to 80
442 nm 3 to 6"
The problem is that he doesn't say where this information is from. From my experience of shooting in 514 and 488, there is not an order of magnitude difference. Rather, both exposure are in the same order, about half. To change by two orders of magnitude in sensitivity over a 14% change in wavelength (514 to 442) implies an exponential change in the photo-reactivity rate. Perhaps this is possible but it would imply a steep resonance which seems to me to not be characteristic of a diffused emulsion.
Of course, part of the problem is what is meant by "sensitivity of DCG". I've made HOEs with exposures of up to 200 mJ/cm^2 and displays with exposures down to 20 mJ/cm^2, both using 488. I think the key is that the old 649F was a very hard emulsion with probably a fairly high bloom (I'm guessing here!).
The quantity of dichromate was probably also low. We can calculate this as follows: The emulsion was 8 microns thick, which means that the mass of gelatin in a plate of area A is,
m_g = 8*A*d_g (where d_g is the density of gelatin in gms/cubic micron)
If by immersing in water you assume that the water content absorbed is about 10% of the gelatin mass (which sounds high, but this is a seat-of-the-pants type of calculation!), the mass of water absorbed would be
m_w = 0.8*A*d_g
With a 7% solution of dichromate, and I'm assuming 7 gms/liter and not 0.7M, the mass of dichromate in the emulsion is therefore
m_dc = 0.07*m_w = 0.056*A*d_g
which is pretty small.
"The problem is that he doesn't say where this information is from."
Unfortunately, yes.
" From my experience of shooting in 514 and 488, there is not an order of magnitude difference. Rather, both exposure are in the same order, about half. To change by two orders of magnitude in sensitivity over a 14% change in wavelength (514 to 442) implies an exponential change in the photo-reactivity rate."
I guess those assumptions are related to the absorption curve of dichromates. Moreover, if we don't have any (?) first-hand holographic experiences for blue-violet DCG recordings, we may get some guesses regarding speed from the lithography/alt.photo area. And they seem to point to greatly increased speed under such conditions.
However with regards to holography I ignore how serious a problem light absorption becomes at those wavelengths (not to mention the mess with scatter). So it might well be that the expected speed increase will be countered by strong absorption, allowing for very low dichromate concentrations only. Having said that it would all the same be extremely interesting to make some practical tests with those diodes...
P.S. Not meant to make you correct your calculation but if I am not misled, the 649-F emulsion used to have a thickness of 16um...
"However with regards to holography I ignore how serious a problem light absorption becomes at those wavelengths (not to mention the mess with scatter). "
I remember answering an earlier post by mentioning that as you go deeper into the blue/violet area, so it's important to keep emulsion thickness low because of absorption. Scattering is an opposite volume effect, since the deeper the emulsion, the more scattering, so you have two competing effects that are very wavelength dependant. As far as I know, no-one has done a study of these effects to find an ideal emulsion thickness. Of course, there's a possibility that such an ideal thickness may not be the best thickness for HOEs since these require greater thickness for narrow-band replay.
I was also thinking last night that until fairly recently, no 405 lasers existed. It's only been very recently that even LED's at that wavelength became available. Any early ideas about holographic exposures at these wavelengths must have been extrapolated from either theory or determined by non-coherent sources (Mercury vapour lamps). I'm not sure how valid such results are since coherent light is inherently different, especially as regards scattering cross sections. The curve from the Meyerhofer article I referred to was a transmission plot of DCG, which is also not completely valid. In the end, I think that the availability of these low wavelength lasers will give us new knowledge of coherent light properties in deep blue/violet as regards holography.
"P.S. Not meant to make you correct your calculation but if I am not misled, the 649-F emulsion used to have a thickness of 16um."
oops! Well replace all the '8's' in the calculation by '16's'. I based the 8 on Agfa. Sorry, sloppy thinking!
"As far as I know, no-one has done a study of these effects to find an ideal
emulsion thickness. Of course, there's a possibility that such an ideal
thickness may not be the best thickness for HOEs since these require greater
thickness for narrow-band replay."
Yes, it's also my impression that the emulsion thickness needs to be discussed. I am still wondering (having bored Sergio and Kaveh with that subject already) what is happening if one made thinner emulsions for color holography (maybe 4um).
"I am still wondering (having bored Sergio and Kaveh with that subject already) what is happening if one made thinner emulsions for color holography (maybe 4um)."
In England we have a phrase - "Teaching your grandmother to suck eggs" This refers to trying to lecture a more experienced and knowledgeable person on something you know little about. Since I know little about color holography, let me indulge in a little egg-sucking excercise!
It seems the important factors in reflection color holography are color rendition, cross talk and shrinkage. I realise that color rendition is pretty gamut dependant, but assume there is a "perfect" gamut and the hologram needs to accurately render the colors of that gamut in the same color space. In this case, the bandwidth (or, rather, the spread of Bragg components) for any single color in the gamut must contain the appropriate distribution of sinusoidal components such that, on diffraction from that particular distribution of Bragg planes, the color of the gamut is reproduced, in the same color space. So, let's say the original object is pink with a hint of orange. Then the Bragg planes must have a distribution such tha most of them are spaced at around 300 to 340 nm (half lambda for 600 to 640) with a few planes at 295 to give the orange. This must diffract white light so that the diffracted light intensity vs wavelength peaks at pink-with-a-hint-of-orange. It seems to me that you'd need a fairly thick emulsion to accurately reconstruct such a wide (statistically speaking) distribution.
In terms of cross talk, it seems that it wouldn't matter, since cross talk is an angualr phenomenon. I think you'd need a new 'Q' factor to ensure that the emulsion is 'thick' enough that you can distinguish different angular seperations easily, ie d(theta)/d(lambda) needs to be large. Since the 'Q' factor is wavelength and fringe frequency dependant, it's tempting to simply define a "color Q" as the sum of three Q's,
Q_col=Q_red + Q_green + Q_blue
But Nature never makes things this easy for us! There are probably cross terms of the type Q_col1+Q_col2.
As for shrinkage, I would think a small change in a 4 micron emulsion would have more effects for color than a similar change in thicker emulsions. A 10% change, for example, would shrink the 4u emulsion to 3.6u and an 8u emulsion to 7.2. Since fringe width is absolute, the differential shrinkage might affect color rendition worse in the former case.
Seriously, though, Martin was asking about the possibility of 4u for a color. Thinking about whether or not there was such a thing as the "right" thickness for color, it occured to me that in order to get good color reproduction, you can't have dispersion. This means fairly high Bragg selectivity. One measure of high Bragg selectivity, or "thckness" of an emulsion is the so-called Q factor. This is a measure of the ratio of the fringe, or plane, seperation to the emulsion thickness. However, the fringe seperation depends on the wavelength, which would obviously be different for the three different wavelengths (or 4 if you believe Hans). In this case, you can either "stack" emulsions on top of each other,treat them as independant 'emulsions' and then define a "thickness" for each of the seperate emulsions, or you can somehow define some combined "thickness parameter" which is a mixture of the three individual thickness parameters all mixed in into one emulsion. If you could define such a "Q-mixing" parameter, this would give a good indication of the thickness necessary for dispersion-free, full-color Bragg selectivity.
Dinesh:
"Thinking about whether or not there was such a thing as the "right"
thickness for color, it occured to me that in order to get good color
reproduction, you can't have dispersion. This means fairly high Bragg
selectivity."
To make complicated matters even more complicated, what happens if you had a
"full-color" holographic recording at three different wavelengths - say the
ones Kaveh advocated for: 450, 540 and 610 nm? Speaking of objects of mixed
colors, would there be an effect similar to certain photographic color
systems (additive/subtractive), that's to say, the joined recordings at 450,
540, 630 nm will be acting like bandpass filters upon each other? So under the regime
of three different wavelengths you may get less image blur under certain
recording conditions than you might have when dealing with one single
wavelength?
Several contradicting „guidelines“ seem to struggle here:
- generally speaking, broader bandwidth (hence thinner recording layers) leads to brighter image reconstruction (at the expense of increased blurring)
- depending on the index modulation for a particular recording material,
thinner layers may actually yield less bright images.
And of course, there is the case of Lippmann photography. I am tempted to consider Lippmann photography as a form of holography (a reflection hologram that has been recorded with an infinity of light sources of different wavelengths and of poor coherence).
>To make complicated matters even more complicated, what happens if you
>had a "full-color" holographic recording at three different wavelengths
>- say the ones Kaveh advocated for: 450, 540 and 610 nm? Speaking of
>objects of mixed colors, would there be an effect similar to certain
>photographic color systems (additive/subtractive), that's to say, the
>joined recordings at 450, 540, 630 nm will be acting like bandpass
>filters upon each other?
This is not more complicated actually, and exactly what happens. You are describing the recording of a color reflection hologram in a panchromatic emulsion. What you get is three sets of fringes, or 'Bragg planes' working independently. Each one will replay its own image, and all three images will be superimposed. So each set of fringes is indeed working as a bandpass filter, selecting its 'own' wavelength from the white light illumination beam.
All color holography is additive, all printing is subtractive. (If you need more reasoning, I will give it.)
>So under the regime of three different
>wavelengths you may get less image blur under certain recording
>conditions than you might have when dealing with one single wavelength?
I think this is an extremely good point and you are absolutely right, if I understand you correctly. Suppose you need a certain brightness for your image, and suppose, for simplicity, that you get 50% diffraction efficiency for each holographic image.
For a conventional monochromatic hologram, all the light comes from the single wavelength that the hologram 'chooses' from the white light reconstruction source. The brightness will depend on the bandwidth of the light that is diffracted. The broader the bandwidth, the brighter the image, and of course the less sharp too, because of chromatic dispersion.
Now the 3-wavelength holographic image is made of 3 peaks, in red, green and blue. So to get the same brightness in the image, each band need not be as wide as the monochromatic case.
So not only do you get a color hologram, but it is sharper too.
"This is not more complicated actually, and exactly what happens. You are describing the recording of a color reflection hologram in a panchromatic emulsion. What you get is three sets of fringes,or'Bragg planes' working independently. Each one will replay its own image, and all three images will be superimposed. So each set of fringes is indeed working as a bandpass
filter, selecting its 'own' wavelength from the white light illumination beam."
When it comes to the actual layer thickness, what would be a good value to rely on? Based on a continous spectrum light source for reconstruction, would it be 6um or rather 20um?
Regarding silver halide/DCG emulsions we must not forget to address an extra difficulty: processing might have a strong impact on bandwidth (not to mention color shifts).
Frankly, I don't quite see how this relates to the layer thickness issue. Is here something like grating integrity vs. chirped gratings at stake?
> When it comes to the actual layer thickness, what would be a good value to
>rely on? Based on a continous spectrum light source for reconstruction,
>would it be 6um or rather 20um?
Both are fine. The thicker emulsion will give you a narrower bandwidth, a sharper image, and more saturated colors. The thinner one will be brighter.
> Regarding silver halide/DCG emulsions we must not forget to address an
>extra difficulty: processing might have a strong impact on bandwidth (not
>to mention color shifts).
Yes, it is essential that there is no shrinkage. Or if there is, it is somehow compensated by a change in refractive index. I think that if you get that bit right, and you have three clean holograms, then you will get a great hologram whatever the bandwidth.
Don't forget that using just two colors, say green and red, you will get more than 2/3 of the way to full color holograms.
The best way to preview what colors the final hologram will contain, is simply to illuminate the object with the two laser beams, and to switch off all other lighting. I was surprised when I did this with two colors.
> Frankly, I don't quite see how this relates to the layer thickness issue.
>Is here something like grating integrity vs. chirped gratings at stake?
Maybe I was thinking of something different. I had in mind an optical multilayer interference filter, which selects a wavelength range to reflect, depending on the thickness of the layers. The more layers you have, then sharper the reflection peak. In reflection holograms, the more fringes you have, the narrower the bandwidth.
> When it comes to the actual layer thickness, what would be a good value to
>rely on? Based on a continous spectrum light source for reconstruction,
>would it be 6um or rather 20um?
Both are fine. The thicker emulsion will give you a narrower bandwidth, a sharper image, and more saturated colors. The thinner one will be brighter.
> Regarding silver halide/DCG emulsions we must not forget to address an
>extra difficulty: processing might have a strong impact on bandwidth (not
>to mention color shifts).
Yes, it is essential that there is no shrinkage. Or if there is, it is somehow compensated by a change in refractive index. I think that if you get that bit right, and you have three clean holograms, then you will get a great hologram whatever the bandwidth.
Don't forget that using just two colors, say green and red, you will get more than 2/3 of the way to full color holograms.
The best way to preview what colors the final hologram will contain, is simply to illuminate the object with the two laser beams, and to switch off all other lighting. I was surprised when I did this with two colors.
> Frankly, I don't quite see how this relates to the layer thickness issue.
>Is here something like grating integrity vs. chirped gratings at stake?
Maybe I was thinking of something different. I had in mind an optical multilayer interference filter, which selects a wavelength range to reflect, depending on the thickness of the layers. The more layers you have, then sharper the reflection peak. In reflection holograms, the more fringes you have, the narrower the bandwidth.
"What you get is three sets of fringes,or'Bragg planes' working independently."
It would also seem that the distribution of the fringes would be important in color holography.The color of an object, ie it's reflectivity at different wavelengths, is not just limited to the wavelengths but also to the independant reflectivity at each wavelength, ie the S value in an HSV color space. If this reflectivity is not mapped onto the Bragg planes, it seems you won't get an accurate color rendition. If the efficency of each fringe component has to accurately mirror the reflectivity of each wavelength, there needs to be a linear relationship between the holographic fringes and the object reflectivity, for each component. However, in a phase hologram, the DE is not a linear function of the beam intensity so that there will be a shift of the S value, even if the H value is not affected. In addition, the higher frequency fringes would probably not remain sinusoidal in any processing scheme that ensures the sinusoidal fringe structure of the lower frequencies. If you processed carefully enough to keep the 'red' fringes linear, you may not simultaneously keep the 'blue' fringes linear. You might then create higher order 'blue' diffraction that may diffract into the 'red' image. This is not exactly the same as cross talk, where each component has only a single order but diffracts into the 'wrong' angle. This is each component diffracting into the 'right' angle, but the higher order diffraction due to non-sinusoidal fringes.
I would imagine that swelling will also affect the relative brightness of each component differently, since swelling would affect the Bragg selectivity properties of each component differently.
I am finding it hard to understand much of this post. I like to think of
one aspect of the problem at a time, so I get confused when there are too
many topics at the same time. To be honest, I think you are thinking too
hard, and inadvertently making a simple topic more complex than it is.
> It would also seem that the distribution of the fringes would be
>important in color holography.
I don't see what you mean by 'distribution'.
>The color of an object, ie it's reflectivity
>at different wavelengths, is not just limited to the wavelengths but also
>to the independant reflectivity at each wavelength, ie the S value in an
>HSV color space.
When illuminating the object with three wavelengths, the color is
determined by the reflectivity of the object in each of those wavelengths.
A part of the object which is purely red, will have very low reflectivity
in blue and green, but high reflectivity in red, so it will look red, etc.
>If this reflectivity is not mapped onto the Bragg planes,
>it seems you won't get an accurate color rendition. If the efficency of
>each fringe component has to accurately mirror the reflectivity of each
>wavelength, there needs to be a linear relationship between the
>holographic fringes and the object reflectivity, for each component.
>However, in a phase hologram, the DE is not a linear function of the beam
>intensity so that there will be a shift of the S value, even if the H
>value is not affected.
I don't think the color rendition will be affected by nonlinearity in the
film. If a part of the object is reflecting half the amount of red, say,
than another part, it will still do so in the hologram. The fringes may be
nonlinear, i.e. non-sinusoidal in profile, but I think that the relative
brightness of different parts of the image are restored in the holographic
image.
>In addition, the higher frequency fringes would
>probably not remain sinusoidal in any processing scheme that ensures the
>sinusoidal fringe structure of the lower frequencies. If you processed
>carefully enough to keep the 'red' fringes linear, you may not
>simultaneously keep the 'blue' fringes linear. You might then create
>higher order 'blue' diffraction that may diffract into the 'red' image.
>This is not exactly the same as cross talk, where each component has only
>a single order but diffracts into the 'wrong' angle. This is each
>component diffracting into the 'right' angle, but the higher order
>diffraction due to non-sinusoidal fringes.
Not sure about the above, but if we are talking about reflection holograms,
there won't be any higher order images, even if the recording is nonlinear.
> I would imagine that swelling will also affect the relative brightness of
>each component differently, since swelling would affect the Bragg
>selectivity properties of each component differently.
Again, can't get my head round this, but why should the Bragg selectivity
properties be affected differently for each component?
I would say the main effect of the dispersion is the same as in monochromatic holograms, namely blurring of the image. And the contradictory requirements of high image brighness and sharpness remain.
My intuition is that the magnitude of the dispersion is not enough to interfere with the color rendering, as it is still a small fraction of the total white light bandwidth. The effect of very broad bandwidth reconsctruction would be to reduce the color purity, or saturation, of the image.
A lot of work still to be done in color holography, for sure.
The 4µm value seems interesting for coating emulsions, as DCG or polymers, but one thing to consider in reflection holograms is the refractive index modulation achieved in the 4µm emulsion (suppose as strong as DCG)that could deviate the "parameters" to Bragg reconstruction introducing more saturation colour loss, so you achieve more diffraction efficiency and image brightess?
Yes, it's also my impression that the emulsion thickness needs to be discussed. I am still wondering (having bored Sergio and Kaveh with that subject already) what is happening if one made thinner emulsions for color holography (maybe 4um).
You're not even beginning to bore me. Try harder.
If you can make monochromatic holograms on a 4 micon emulsion, then you should also be able to make a multicolor one. In general, you will get a broader bandwidth and hence more chromatic blurring.
