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Photometric system

Photometric system is a science 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 Photometric system rather than just read about it. In short: In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used.

Key takeaways

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

Reference excerpt

In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided. A commonly adopted standardized photometric system is the Johnson-Morgan or UBV photometric system (1953). At present, there are more than 200 photometric systems. Photometric systems are usually characterized according to the widths of their passbands:

broadband (passbands wider than 30 nm, of which the most widely used is Johnson-Morgan UBV system) intermediate band (passbands between 10 and 30 nm wide) narrow band (passbands less than 10 nm wide)

Photometric letters Each letter designates a section of light of the electromagnetic spectrum; these cover well the consecutive major groups, near-ultraviolet (NUV), visible light (centered on the V band), near-infrared (NIR) and part of mid-infrared (MIR). The letters are not standards, but are recognized by common agreement among astronomers and astrophysicists. The use of U,B,V,R,I bands dates from the 1950s, being single-letter abbreviations. With the advent of infrared detectors in the next decade, the J to N bands were labelled following on from near-infrared's closest-to-red band, I. Later the H band was inserted, then Z in the 1990s and finally Y, without changing earlier definitions. Hence, H is out of alphabetical order from its neighbours, while Z,Y are reversed from the alphabetical – higher-wavelength – sub-series which dominates current photometric bands.

Note: colors are only approximate and based on wavelength to sRGB representation (when possible). Combinations of these letters are frequently used; for example the combination JHK has been used more or less as a synonym of "near-infrared", and appears in the title of many papers.

Filters used The filters currently being used by other telescopes or organizations. Units of measurements:

Å = Ångström nm = nanometre μm = micrometre

Note: colors are only approximate and based on wavelength to sRGB representation (when possible).

See also Photometry AB magnitude

References and footnotes

External links Johnson, H. L.; Morgan, W. W. (1953), Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas, The Astrophysical Journal, vol. 117, pp. 313–352 [2] The Asiago Database on Photometric Systems Michael S. Bessell (2005), STANDARD PHOTOMETRIC SYSTEMS, Annual Review of Astronomy and Astrophysics vol. 43, pp. 293–336 Infrared portrait of the nearby massive star-forming region IRAS 09002-4732, Apai, D.; Linz, H.; Henning, Th.; Stecklum, B., 2005

Worked examples

Example 1 — a first encounter with Photometric system

Start with the simplest possible case. Write down what Photometric system claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Photometric system 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 Photometric system 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 Photometric system

In research
Photometric system appears in science 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 Photometric system 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
Photometric system is common in secondary-school and first-year university syllabi. It links to neighbouring topics Photometric systems, so understanding it makes those chapters shorter.
In everyday life
Look for Photometric system 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 Photometric system in 20 minutes

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

Frequently asked questions

What is Photometric system in simple terms?

In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used.

Why does Photometric system matter?

Because it connects several science 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 Photometric system?

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 Photometric system.

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