physics

Holographic optical element

Holographic optical element is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Holographic optical element rather than just read about it. In short: A holographic optical element (HOE) is an optical component (mirror, lens, directional diffuser, etc.) that produces holographic images using principles of diffraction. HOE is most commonly used in transparent displays, 3D imaging, and certain scanning technologies.

Key takeaways

Holographic optical element belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Holographic optical element to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Holographic optical element from memory before moving on to harder problems.

Reference excerpt

A holographic optical element (HOE) is an optical component (mirror, lens, directional diffuser, etc.) that produces holographic images using principles of diffraction. HOE is most commonly used in transparent displays, 3D imaging, and certain scanning technologies. The shape and structure of the HOE is dependent on the piece of hardware it is needed for, and the coupled wave theory is a common tool used to calculate the diffraction efficiency or grating volume that helps with the design of an HOE. Early concepts of the holographic optical element can be traced back to the mid-1900s, coinciding closely with the start of holography coined by Dennis Gabor. The application of 3D visualization and displays is ultimately the end goal of the HOE; however, the cost and complexity of the device has hindered the rapid development toward full 3D visualization. The HOE is also used in the development of augmented reality(AR) by companies such as Google with Google Glass or in research universities that look to utilize HOEs to create 3D imaging without the use of eye-wear or head-wear. Furthermore, the ability of the HOE to allow for transparent displays have caught the attention of the US military in its development of better head-up displays (HUD) which is used to display crucial information for aircraft pilots.

Early development of HOE The holographic optical element is closely linked to holography (science of making holograms), a term proposed by Dennis Gabor in 1948. Since the idea of holography came around, much has been done over the next few decades to try and create holograms. Around the 1960s, Yuri Nikolaevich Denisyuk, a graduate student from Leningrad, recognized that perhaps the wave front of light can be recorded as a standing wave in a photographic emulsion (light crystal) by using monochromatic light which can then reflect light back to reproduce the wave front. This essentially describes a holographic mirror (one of the first HOEs created) and fixed the issue of overlapping images. However, there was little practical use in Denisyuk's proposal, and his colleagues dismissed his results. It was not until around the mid-1960s that Denisyuk's proposals resurfaced after some development from Emmett Leith and Juris Upatnieks. These two associates encoded and reconstructed images with a two-step hologram process on photographic transparency. More experiments for holographic instruments, such as the holographic stereogram developed by Lloyd Cross in the 1970s, took the imaging process developed by Leith and Uptanieks and arranged them into vertical strips that were curved into a cylinder. These strips act as an aperture that light passes through, so when a viewer is to look through them, a 3D image can be seen. This demonstrates a very simple version of the diffraction concepts that are still utilized in the production of HOEs and a prototype for 3D glasses.

Classifications

Volume and thin HOEs HOEs differ from other optical devices since they do not bend light with curvature and shape. Instead, they use diffraction principles (the distribution of light as it passes through an aperture) to diffract light waves by reconstructing a new wavefront using a corresponding material profile, making HOEs a type of diffraction optical element (DOE). Two common types of HOEs that exist are volume HOEs and thin HOEs that are dependent. A thin HOE (one containing a thin layer of holographic grating) has a low diffraction efficiency, causing light beams to diffract in various directions. Conversely, volume HOE types (ones containing multiple layers of holographic gratings) are more efficient since there is more control on the direction of light due to a high diffractive efficiency. Most of the calculations done to create HOEs are usually the volume type HOEs.

Reflection-type and transmission-type HOEs In addition to being a thin or volume HOE, an HOE can also be affected by positioning, which determines whether it is a transmission type or reflection type. These types of HOE are determined by the position of the object beam and reference beam in relation to the recording material of those beams: being on the same side indicates a transmission HOE and otherwise a reflection HOE. Some materials that are most commonly used in manufacturing HOEs include silver halide emulsion and dichromate gelatin.

Applications

Aerospace industry In the early 2000s NASA conducted a test known as the Holographic Airborne Rotating Lidar Instrument Experiment(HARLIE) that utilized dichromate gelatin-based volume HOE sandwiched between float glass. The objective of the test was to find a new method of measuring surface and atmospheric parameters that could reduce the size, mass, and angular momentum of a spaceborne lidar systems. The ability of HOE to be made as curved or bendable allows it to be used in the construction of head up displays(HUD) or head mount displays(HMD). Additionally, transparency can be achieved due to the selectivity of the volume grating that is used to diffract light at a specific incident angle or wavelength. This allows for the development of transparent head-up displays that convey information to aircraft pilots and conserves cockpit space. The US military is currently running tests on these new aircraft displays.

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Worked examples

Example 1 — a first encounter with Holographic optical element

Start with the simplest possible case. Write down what Holographic optical element claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Holographic optical element before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Holographic optical element ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Holographic optical element

In research: Holographic optical element appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Holographic optical element in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Holographic optical element is common in secondary-school and first-year university syllabi. It links to neighbouring topics Holography, so understanding it makes those chapters shorter.
In everyday life: Look for Holographic optical element outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Holographic optical element in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Holographic optical element means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Holographic optical element out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Holographic optical element in simple terms?

Why does Holographic optical element matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Holographic optical element?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Holographic optical element.