physics

Position sensitive device

Position sensitive device 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 Position sensitive device rather than just read about it. In short: A position sensitive device and/or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure a position of a light spot in one or two-dimensions on a sensor surface. Principles PSDs can be divided into two classes which work according to different principles: In the first class, the sensors have an isotropic sensor surface that supplies continuous position data.

Key takeaways

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

Reference excerpt

A position sensitive device and/or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure a position of a light spot in one or two-dimensions on a sensor surface.

Principles PSDs can be divided into two classes which work according to different principles: In the first class, the sensors have an isotropic sensor surface that supplies continuous position data. The second class has discrete sensors in an raster-like structure on the sensor surface that supply local discrete data.

Isotropic Sensors

The technical term PSD was first used in a 1957 publication by J.T. Wallmark for lateral photoelectric effect used for local measurements. On a laminar semiconductor, a so-called PIN diode is exposed to a tiny spot of light. This exposure causes a change in local resistance and thus electron flow in four electrodes. From the currents I a {\displaystyle I_{a}} , I b {\displaystyle I_{b}} , I c {\displaystyle I_{c}} and I d {\displaystyle I_{d}} in the electrodes, the location of the light spot is computed using the following equations.

x = k x ⋅ I b − I d I b + I d {\displaystyle x=k_{x}\cdot {\frac {I_{b}-I_{d}}{I_{b}+I_{d}}}}

and

y = k y ⋅ I a − I c I a + I c {\displaystyle y=k_{y}\cdot {\frac {I_{a}-I_{c}}{I_{a}+I_{c}}}}

The k x {\displaystyle k_{x}} and k y {\displaystyle k_{y}} are simple scaling factors, which permit transformation into coordinates. An advantage of this process is the continuous measurement of the light spot position with measuring rates up to over 100 kHz. The dependence of local measurement on form and size of the light spot as well as the nonlinear connection are a disadvantage that can be partly compensated by special electrode shapes.

2-D tetra-lateral Position Sensitive Device (PSD)

A 2-D tetra-lateral PSD is capable of providing continuous position measurement of the incident light spot in 2-D. It consists of a single square PIN diode with a resistive layer. When there is an incident light on the active area of the sensor, photocurrents are generated and collected from four electrodes placed along each side of the square near the boundary. The incident light position can be estimated based on currents collected from the electrodes:

x = k x ⋅ I 4 − I 3 I 4 + I 3 {\displaystyle x=k_{x}\cdot {\frac {I_{4}-I_{3}}{I_{4}+I_{3}}}}

and

y = k y ⋅ I 2 − I 1 I 2 + I 1 {\displaystyle y=k_{y}\cdot {\frac {I_{2}-I_{1}}{I_{2}+I_{1}}}}

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Position sensitive device

Start with the simplest possible case. Write down what Position sensitive device 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 Position sensitive device 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 Position sensitive device 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 Position sensitive device

In research: Position sensitive device 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 Position sensitive device 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: Position sensitive device is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical diodes, Optoelectronics, Position sensors, so understanding it makes those chapters shorter.
In everyday life: Look for Position sensitive device 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 Position sensitive device in 20 minutes

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

Frequently asked questions

What is Position sensitive device in simple terms?

A position sensitive device and/or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure a position of a light spot in one or two-dimensions on a sensor surface. Principles PSDs can be divided into two classes which work according to different principles: In the fir…

Why does Position sensitive device 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 Position sensitive device?

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 Position sensitive device.