Abstract

In Part I, a classification system for radiation detectors is proposed. The system is based upon the manner in which the noise equivalent power depends upon the time constant and the sensitive area, when the sensitivity is limited either by radiation fluctuations or by internally generated noise.

The major part of Part I is devoted to establishing the reference condition for the measuring of the noise equivalent power. In establishing the reference condition of measurement (Section 6), the output of the detector in the absence of a signal is first equalized so that the noise power per unit frequency band width is constant. The reference time constant is then defined in terms of the relative response as a function of signal modulation frequency (Section 7). The equalization is then modified by the addition of an RC low pass filter with a time constant equal to the reference time constant, and the noise equivalent power is measured. The power so obtained is termed the noise equivalent power in reference condition A, denoted by Pm. This procedure has the property that the noise equivalent power of the detector is measured in the presence of noise whose band width is equal to the band width of the detector.

In order to establish the reference condition of measurement it is necessary to consider the various sources of noise which are involved in detectors (Section 2), and the various types of time constants (Section 3). The important concept of responsivity-to-noise ratio is defined in Section 4. A general theorem involving the sensitive area and the responsivity-to-noise ratio is established in Section 5.

After the question of detectors with non-uniform spectral sensitivity is discussed in Section 8, the classification system is defined in Section 9: A detector is a Type I detector if its noise equivalent power depends upon its sensitive area A and its reference time constant τ in accordance with

Pm=A12/k1τ12,
where k1 is independent of A and τ. A Type II detector is defined by
Pm=A12/k2τ,
and more generally, a Type n detector is defined by
Pm=A12/knτ12n.

The usefulness of the proposed classification is illustrated in Section 10 by its application to sequential scanning systems.

In Part II the classification system proposed in Part I is used to determine the type number of each of eight different kinds of detectors. By a detailed analysis of each kind of detector, it is found that all of the detectors studied are either Type I or Type II detectors. The results of the analysis are summarized in Table I.

The detectors studied include bolometers (Section 2), thermocouples and thermopiles (Section 3), the Golay detector (Section 4), photographic plates and films (Section 5), vacuum and gas photo-tubes and photo-multiplier tubes (Section 6), and dipole antennas (Section 7). In the case of the dipole antenna it is shown that Johnson noise becomes equivalent to the noise produced by radiation fluctuations.

© 1949 Optical Society of America

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References

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Figures (6)

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Tables (1)

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Equations (81)

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