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Excerpt from Field Guide to Spectroscopy

A detector, or radiation transducer, is any device that converts an amount of radiation into some other measurable phenomenon. Ultimately, most of these other measurable phenomena will be tied to an electrical signal.

There are two main types of detectors: photon detectors and thermal detectors. All detectors have similar characteristics:

  • The output of a detector must respond to changes in the incident light intensity. The ability to respond is expressed by quantities such as responsivity, sensitivity, and dynamic range.

  • The output of a detector must respond quickly to quick changes in incident light intensity. This can be quantified by a detector time constant.

  • A detector must have a minimal dark signal (also called dark current if current is the measurable of the detector). The dark signal is the signal when no light impinges on the detector.

  • A detector must have an acceptable level of noise. Noise is unwanted signal from any source other than the signal of interest.

The earliest detectors were human eyes, followed by photographic methods. Eyes are extremely sensitive detectors; their dynamic range is 1010–1014, while photographic film and modern electronic detectors have a dynamic range of only 103–105. It is said that the human eye can detect up to 7 million different colors, meaning it can differentiate light that varies by as little as 0.000056 nm in wavelength! However, the eye is notoriously unreliable and its response is difficult to quantify. Certain forms of spectroscopy still use photographic methods, in part because photographic film can be sensitive to regions other than visible light.

D. W. Ball, Field Guide to Spectroscopy, SPIE Press, Bellingham, WA (2006).

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David W. Ball
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