physics

Optical chopper

Optical chopper 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 Optical chopper rather than just read about it. In short: An optical chopper is a device which periodically interrupts a light beam. Three types are available: variable frequency rotating disc choppers, fixed frequency tuning fork choppers, and optical shutters.

Key takeaways

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

Reference excerpt

An optical chopper is a device which periodically interrupts a light beam. Three types are available: variable frequency rotating disc choppers, fixed frequency tuning fork choppers, and optical shutters. A rotating disc chopper was famously used in 1849 by Hippolyte Fizeau in the first non-astronomical measurement of the speed of light.

Use in science laboratories

Optical choppers, usually rotating disc mechanical shutters, are widely used in science labs in combination with lock-in amplifiers. The chopper is used to modulate the intensity of a light beam, and a lock-in amplifier is used to improve the signal-to-noise ratio. To be effective, an optical chopper should have a stable rotating speed. In cases where the 1/f noise is the main problem, one would like to select the maximum chopping frequency possible. This is limited by the motor speed and the number of slots in the rotating disc, which is, in turn, limited by the disc radius and the beam diameter.

Use in guidance systems Choppers were widely used in early missile guidance systems, and in this role are sometimes known as "reticle seekers". The earliest uses were on air-to-air missiles. A photocell sensitive to infrared light is positioned behind a chopper driven by a synchronous motor. As the chopper rotates, it periodically blocks the photocell's view of the target aircraft, creating a series of pulses of output. This signal is then smoothed to make a sinusoidal output which is then compared to the signal driving the motor. The difference in phase between these two signals reveals the angle of the target compared to a given point in the motor signal. Sampling the signal at two different points directly produces X and Y error signals that can drive the missile's flight controls. The same basic system has been used in a number of other roles. Early ICBMs used a similar chopper system connected to a small visible-light telescope to produce a star tracker that was used to improve accuracy by measuring the angles to one or more stars once they climbed above the atmosphere. Anti-tank missiles used an infrared photocell on the launcher that tracked a flare on the missile, using the X and Y error signals to drive the missile into the line of sight of the operator's aiming telescope. These systems are no longer in use in modern weapons, as they have generally been replaced by imaging systems that provide much more information.

Examples

Optical incremental rotary encoders are a form of choppers. These are used in many industrial machines. Some early anti-lock braking systems used rotary encoders for wheel speed sensors. Late 20th century opto-mechanical computer mice used two encoders for X-Y position measurement. Optical linear encoders also exist. LCD televisions use millions of LCD shutters paired with red, green or blue filters to control the color of the pixels on the screen. Movie cameras use an optical shutter to record individual frames of the movie. Movie projectors use an optical shutter synchronized with the movie frames to produce the effect of apparent motion on the movie screen. Liquid crystal shutter glasses are used in conjunction with a synchronized display screen to create the illusion of a three-dimensional image. Light signals are sent at sea and at airports using a signal lamp with a hand-operated shutter.

Illustrations

Worked examples

Example 1 — a first encounter with Optical chopper

Start with the simplest possible case. Write down what Optical chopper 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 Optical chopper 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 Optical chopper 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 Optical chopper

In research: Optical chopper 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 Optical chopper 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: Optical chopper is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical metrology, so understanding it makes those chapters shorter.
In everyday life: Look for Optical chopper 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Optical chopper in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Optical chopper 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 Optical chopper out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Optical chopper in simple terms?

Why does Optical chopper 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 Optical chopper?

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 Optical chopper.