Transmission holograms with relief structures

[Tidl]

Please help to understand...

I am trying to understand the structure of transmission hologram.
Everything looks clear with reflective - just patterned surface with mirror coating.
And what about transmission holograms? Do they have patterned surface, or they are some 3D structures inside of the film? As I see there are several kinds of transmission holograms, and I am trying to understand if there are transmission holograms that have only surface relief effect.

In other words - are there types of transmission holograms that can be replicated by just mechanical process?

The point is that I can do reliable replication of smallest nanopatterns in plastic, so I am wondering if it is possible reconstruct say EOTech holographic sight, or in-out-coupling holograms for smart glasses?

[jrburns47]

Are you attempting to commercially replicate the EOTech sight?

[Tidl]

No, I am not interested in commercial applications (at least so far). It is my hobby, not a job.
And if there are such holograms that I can replicate in plastic, I would better think in a direction of smart glasses.

[lobaz]

It might be better to talk about thin and thick holograms to avoid confusion - transmission/reflection geometry is somehow independent on hologram type.

In thick holograms, the entire volume of the holographic layer affects light due to the Bragg diffraction; you can imagine that each point (x,y,z) of the emulsion has different index of refraction. They are often created in the Denisyuk setup, i.e. the object wave and the reference wave illuminate the recording material from opposite sides, and they are often reconstructed in the reflection geometry. As far as I know, there is no way to replicate them by stamping or other mechanical process.

Thin hologram, on the other side, can be described as a 2-D function of the hologram properties. These can be transmittance, index of refraction or material thickness. Thus, if a hologram is placed in the xy plane, f(x, y) describes its properties there.

All types of thin hologram diffract light as described by the Raman-Nath diffraction. In the first type, the amplitude hologram, light is attenuated at each point of the hologram somehow. If the amplitude pattern is fine enough, significant light diffraction appears. The latter two types modify phase of light, one due to varying index of refraction, the second due to variable material thickness. Obviously, the relief hologram is the easiest to replicate by stamping.

Most security holograms are thin relief holograms. They are usually reconstructed in the reflection setup, but this is just because their back side is metalized and acts as a mirror. But a non-metalized version works perfectly in the transmission geometry.

Petr

[Tidl]

Dear Petr, many thanks for your answer, it is amazingly short and comprehensive.

Could you please correct me if I am wrong?
1) Thick holograms provide better quality images because of bigger volume of information can be stored - if one need good smart glasses, thick hologram should be used.
2) However if one wants to display something simple - like clock or speedometer, thin relief hologram may be just fine.
3) Security holograms (like Visas bird or Mastercams map) are relief type because of manufacturing simplicity. They can work as transmittance holograms if not equipped with metal foil.

One more thing is confusing me: security hologram is visible at white light coming from random angle. Holographic sight is visible only if exposed to laser beam at certain angle and wavelength. For smart glasses the second type needed. Is it possible to make it relief-type? (obviously stupid question if in Eotech they make it relief, however I cant find information about it)

And those commercially available kits for beginners (take laser pointer and a book, turn off the light, close the door etc.) - what type of hologram they usually are for? Is it relief or not?

Thank you again!

Serg

[lobaz]

You are very welcome, Serg.

1) Thick holograms provide better quality images because of bigger volume of information can be stored - if one need good smart glasses, thick hologram should be used.

I would not say "better quality". A thick hologram is usually more efficient than a thin hologram, i.e., more light is re-directed to make an image (if we talk about holograms that make images). If we talk about noise, aberrations and other parameters of image quality, I do not think there is a clear winner.

There is, however, a significant difference between thick and thin holograms. A thin hologram diffracts any light, which can cause chromatic aberration. Therefore, thin holograms are often designed for monochromatic (laser) illumination. A thick holograms usually picks just some wavelengths and diffracts them, while the others just pass through. Thus, thick holograms are often designed for broadband (e.g. white) illumination. I will write more about that in a while.

I am also not sure what do you mean by "smart glasses". I assume you mean an augmented reality system, such as Microsoft Hololens. Such a system contains thick holograms that do not form an image, they just redirect light from a microdisplay towards eye. In this case, the designers needed high diffraction efficiency and broadband illumination - which calls for a thick hologram.

2) However if one wants to display something simple - like clock or speedometer, thin relief hologram may be just fine.

Both hologram types can provide ultra realistic images. The hologram type is usually chosen according to intended illumination, manufacturing and so on. If we are talking about holograms that do not make an image, but just redirect light - then, it is a different story.

3) Security holograms (like Visas bird or Mastercams map) are relief type because of manufacturing simplicity. They can work as transmittance holograms if not equipped with metal foil.

Yes, they can work as transmittance holograms.

One more thing is confusing me: security hologram is visible at white light coming from random angle. Holographic sight is visible only if exposed to laser beam at certain angle and wavelength. For smart glasses the second type needed. Is it possible to make it relief-type? (obviously stupid question if in Eotech they make it relief, however I cant find information about it)

Security holograms are designed for certain illumination angle. If your illumination is not the deign one, the hologram still so-so works, because thin holograms diffract any light. If you need selectivity to a certain wavelength and angle, you need a thick hologram.

There are more things to consider. For example, many security holograms are created as "rainbow holograms", which means that they diffract light from design illumination to a specified position in space, usually the observer's eyes. If the illumination is not the design one, the hologram still diffracts light, but to another position in space, i.e. not to the observer's eyes. It means that the hologram is not selective, but for a fixed observer it looks like it is. This also means that a rainbow hologram efficiently reduces chromatic aberration - in a way.

And those commercially available kits for beginners (take laser pointer and a book, turn off the light, close the door etc.) - what type of hologram they usually are for? Is it relief or not?

Those holograms are, strictly speaking, thick. But depending on recording geometry, their selectivity can be very low, which means they more or less behave as thin holograms. In both cases, the kits are designed in such a way that the recording material has varying index of refraction after proper development. In short: no, they are not relief by default.

Petr