physics

Stray light

Stray light 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 Stray light rather than just read about it. In short: Stray light is light in an optical system which was not intended in the design. The light may be from the intended source, but follow paths other than intended, or it may be from a source other than that intended.

Key takeaways

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

Reference excerpt

Stray light is light in an optical system which was not intended in the design. The light may be from the intended source, but follow paths other than intended, or it may be from a source other than that intended. This light will often set a working limit on the dynamic range of the system; it limits the signal-to-noise ratio or contrast ratio, by limiting how dark the system can be. Ocular straylight is stray light in the human eye.

Optical systems

Monochromatic light

Optical measuring instruments that work with monochromatic light, such as spectrophotometers, define stray light as light in the system at wavelengths (colors) other than the one intended. The stray light level is one of the most critical specifications of an instrument. For instance, intense, narrow absorption bands can easily appear to have a peak absorption less than the true absorption of the sample because the ability of the instrument to measure light transmission through the sample is limited by the stray light level. One method to reduce stray light in these systems is the use of double monochromators. The ratio of transmitted stray light to signal is reduced to the product of the ratio for each monochromator, so combining two monochromators in series with 10−3 stray light each produces a system with a stray light ratio of 10−6, allowing a much larger dynamic range for measurements. Methods have also been invented to measure and compensate for stray light in spectrophotometers. ASTM standard E387 describes methods of estimating stray light in spectrophotometers. The terms used are stray radiant power (SRP) and stray radiant power ratio (SRPR). There are also commercial sources of reference materials to help in testing the stray light level in spectrophotometers.

Astronomy In optical astronomy, stray light from sky glow can limit the ability to detect faint objects. In this sense stray light is light from other sources that is focused to the same place as the faint object. Stray light is a major issue in the design of a coronagraph, used for observing the Sun's corona.

Sources There are many sources of stray light. For example:

Ghost orders in diffraction gratings. These can be caused by periodic variations in the spacing of grooves in ruled gratings, for instance. Light scattered towards a telescope from particles along the optical path to a star. Light emitted by components of the optical system. Infrared optical systems are, obviously, especially susceptible due to thermal radiation. one way to reduce the effect of stray IR generated within the system is to move from working with DC signals to a narrow frequency band where the amplitude of the stray emissions is smaller. This can be done, for instance, by modulating the source light entering the system with an optical chopper, and isolating the detected source signal component from the detected stray component with a lock in amplifier synchronized to the chopper frequency. However, this approach is still limited by the dynamic range of the detector. That is, the stray component must not be so great that it saturates the detector. Reflections from lens surfaces. Anti-reflective coatings are used to reduce stray light. Narcissus effect - Specifically, thermal radiation from the infrared detector reflected back to itself from lens surfaces. Light scattered from the surfaces of supporting structures within the optical system. Diffuse reflection from imperfect mirror surfaces. Light leaks in the enclosure of the system.

Design tools A number of optical design programs can model stray light in an optical system, for instance:

ASAP FRED Synopsys LightTools TracePro Zemax Such models can be used to predict and minimize stray light in the final system.

See also Glare (vision) Lens flare Spectral power distribution Veiling glare

References

Illustrations

Worked examples

Example 1 — a first encounter with Stray light

Start with the simplest possible case. Write down what Stray light 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 Stray light 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 Stray light 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 Stray light

In research: Stray light 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 Stray light 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: Stray light is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical metrology, Photometry, so understanding it makes those chapters shorter.
In everyday life: Look for Stray light 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 Stray light in 20 minutes

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

Frequently asked questions

What is Stray light in simple terms?

Why does Stray light 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 Stray light?

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 Stray light.