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10.10.6 Spectral Aliasing Error
Spectral aliasing is the folding over of all signal and noise within the
bandpass of the system into the spectral free range, which spans from
0 to the Nyquist frequency (half the sampling frequency). To avoid
the overlapping of spectral features, it is important that any spectral
information outside the spectral range of interest be removed. This
can be done electrically with a low-pass filter, but this method does
not provide a sharp cut-off over the upper limit of the frequency
range or attenuation below the lower limit. A more appropriate
method is to use an optical filter, which provides both a sharp cut-off
and very high attenuation outside the spectral range of interest. This
method also reduces photon noise.

Further Reading
1. B. K. Yap, W.A.M. Blumberg, and R.E. Murphy, Applied Optics, Vol. 21, 4176
(1982).
2. J. B. Johnson, Nature 119, 50 (1927).
3. J. B. Johnson, Phys. Rev. 32, 97 (1928).
4. D. R. White et al., Metrologia, Vol. 33, pages 325–335 (1996).
5. D. D. Laporte, William L. Smith, and L.A. Sromosky, Applied Optics, Vol. 27,
3210 (1988).
6. H. Nyquist, Phys. Rev. 32, 110 (1928).
7. Hugh Lippincott, Notes on Johnson Noise, given on 19 February (2007).
8. L. B. Kish, Phys. Lett. A 352, 178–182 (2006) /physics/0509136; A Cho. Science
309, 5744, and 2148 (2005).
9. Peter R. Griffiths and James A. de Haseth, Fourier Transform Infrared Spectrometry,
Wiley-Inter-science.
10. Sabrie Soloman, Sensors Handbook, McGraw-Hill Publishing Company, New
York, 2009.
11. Sabrie Soloman, Sensors and Control Systems in Manufacturing, McGraw-Hill
Publishing Company, New York, 2009.
12. Sabrie Soloman, Non-destructive Identification of Tablet and Tablet Dissolution
by Means of Infrared Spectroscopy. U.S. Patent Number 5,679,954.
13. Sabrie Soloman, Real Time—On Line Analysis of Organic Compounds for Food,
Fertilizers, and Pharmaceuticals Products. U.S. Patent Number 5,900,634.
14. Sabrie Soloman, Sabrie’s Index of NIR-Light Energy, Sensors Symposia, Seoul,
Korea, 2007.