# The need of the reference beam in Holography

For a project, we're investigating how a transmission hologram works. I understand how the process of making this hologram works, but I can't get my head around just one thing. Why would a transmission hologram require a reference beam to illuminate the photographic plate? Why can a single light source be used to illuminate the object, which is then reflected on the photographic plate.

A hologram can be recorded with a single beam, if the beam is spread out enough so that it illuminates both the object and the recording plate (and light scatters off the object onto the plate). However, that is completely equivalent to having separate object and reference beams.

Imagine what would happen if the object were just a tiny piece of a mirror, so it throws a narrow beam of light onto a spot on the plate. With no reference beam, the plate would be exposed at that spot and that's all there is to it. But with a reference beam added, that narrow beam of light from the object would interfere with the reference beam at that spot; and thereby a diffraction grating would be formed at the spot. Look up the principle of a diffraction grating, and you will be able to understand that the diffraction grating formed there is exactly what is needed to deflect (diffract) the reference beam into the direction in which the object light was traveling when the grating was formed. That is the fundamental principle of holography.

I am not clear what the question is. The basic principle of holography is the interference of a reference beam (which is usually diverging or collimated, i.e. parallel) and an object beam. The object beam is the light that is reflected, scattered or refracted by the object. This beam is normally complex.

The result is a complex interference pattern on the plate or film. When this is reilluminated by a "reconstruction" beam, which is normally identical to the reference beam, part of the beam is diffracted and the image of the original object is reconstructed.

Let me know which part is confusing.

• The part that is confusing to me, is why this reference beam and the interference is required to make the hologram? Why can't the object beam be the only light that's 'saved' on the holographic film? Commented Sep 8, 2016 at 18:08
• This is the fundamental principle of holography: you need a beam (called reference beam) to encode every detail of the object beam, i.e. every ray of light that comes from the object beam, its direction, and its intensity, and save that information in microscopic interference patterns. During reconstruction, a beam identical to the reference beam diffracts from those patterns and "decodes" the information to reconstruct the original optical information coming from the object beam. Without a reference beam the photographic plate will just record a continuous grey "fog" on the plate. Commented Sep 9, 2016 at 7:19
• Okay, I understand that part. But why will the record then just be a grey "fog"? Why is this encoding and decoding needed, how does this change the photographic plate in such a way that it does show the image? Commented Sep 9, 2016 at 15:16
• Object beam by itself is like exposing a photographic film to light (without a lens). It just gets darker uniformly. But an object and reference beam that are coherent, and come from the same laser source, will interfere and produce interference patterns, thus recording all the optical information emanating from the object. Commented Sep 10, 2016 at 11:29

The hologram is effectively capturing path lengths of photons, which vary based on the geometry of the object being captured. It does so by capturing an interference pattern between the reference beam (which always comes from a single direction and has a fixed path length), and the objective beam (which has to bounce off of the object). If you had no reference beam, you would not be able to get the constructive and destructive interference patterns that are key to capturing the image. You'd merely illuminate the holographic material.

I believe the reference beam is needed to record the phase of the light as it reaches the holographic film.

The purpose of a hologram is to reproduce the light field that is emitted from an object. Light is a wave, so it has the properties of frequency (color), amplitude (intensity) and phase (I think this is perceived as depth).

The differences in phase between the reference beam and the object beam is recorded as interference. Frequency is constant.

I'm not a trained physicist, so I am not certain, but this is what makes sense to me.

• We cannot directly perceive the phase of light. We can only perceive it indirectly, when two light rays interfere with each other. Generally, that requires the 2 rays to come from the same source. Commented Jan 14, 2021 at 23:47

According to this page it appears that the object's reflection interferes with the laser beam to create the interference (diffraction) pattern needed to recreate the source point per photon.

A hologram consists of diffractive patterns recorded from each point source of an object. Diffraction because reflective/refractive Fresnel lenses also focus other wavelengths at different (not the original) points.

Nontransparent things like chocolate use diffraction via ridges, and photosensitive media use diffraction by sinusoidal/Fresnel zone plate patterns.

Optical illusions like the Mirascope produces are not recordings ("-grams") without reproducing the original object shape and color, which by itself would be more whole ("holo-") than its realistic full-color and reflective 3D effect, which is more whole than a dictionary hologram:

Definition of hologram : a three-dimensional image reproduced from a pattern of interference produced by a split coherent beam of radiation (such as a laser) also : the pattern of interference itself

• The comment about Fresnel patterns is interesting. But it stands as a comment, not an answer, unless you explain what you mean and how it arises. Commented Jan 14, 2021 at 12:21
• @garyp If you know what a Fresnel lens is and does, as could be assumed on a physics site like this, then it is obvious to me that the light cones it creates (burning an object at its focal point) are the inverse of those the interference pattern of the laser creates (burning the lens). Commented Jan 14, 2021 at 12:46
• Keep in mind that this is a site for education. If someone knows the answer, they wouldn't be here. One often has to infer the questioners level of physics knowledge from hints in the question, and it only makes sense to respond at an appropriate level. In this case, uncertainty about the need for a reference beam suggests that the OP might not know about Fresnel "patterns", and in fact, there could be confusion with the Fresnel lens. Commented Jan 14, 2021 at 14:00
• I came here because i had the same question as OP yet knew about Fresnel lenses because those are easy compared to holograms which turn out to contain many more of them, even in transparency grate zone plate form. Commented Jan 14, 2021 at 14:33
• The Fresnel "patterns" impressed in holography operate be diffraction, while Fresnel lenses operate by refraction. They are different phenomena. Hence the possibility of confusion. Commented Jan 14, 2021 at 16:30