In an electrical circuit, it may be useful to have a circuit component that can detect changes in light intensity. A photodetector provides a user with the ability to measure the light directed towards its sensing component through a change in output voltage. This is a very useful tool in light-based instrumentation. Watch the video below to learn how potentiometers work and how they may be used in biomedical instrumentation applications.
Self-Assessment Questions:
Click here to test your understanding of Photodetectors.
Discussion Questions:
- As mentioned in the video, pulse oximetry is a very important biomedical application for photodetectors, mainly photodiodes, which operate at two different wavelengths to get a measure of heart rate, oxygen saturation, and more. Why is measurement of light at two wavelengths necessary? How does a finger pulse oximeter achieve this? Use this article by Tsiakaka et al. for help.
- Photodetector measurements can often be affected by dark current, which is current that flows in the photodetector even in the absence of incident light. How do you think dark current affects measurement sensitivity and why?
(Note: review this article by Marozas et al. on strategies to minimize dark current and the implications of dark current on measurements.) - How are the two biasing modes (forward and reverse-bias) for photodetectors affected differently by dark current? Explain how you came to your conclusions on these differences in terms of voltage applied and circuit orientation.
- In which light detection cases would you use a photodetector over a photoresistor (think about different light properties such as frequency and wavelength!)? What are some applications in which this choice would be necessary? Can you think of applications where photoresistors would be preferred?
- An important application of photodetectors is in medical imaging, where photomultiplier tubes (similar to phototubes) are used heavily. Review this article by Moses that describes advances in these technologies, and discuss the pros and cons of the current advances in photomultiplier tubes. What disadvantages do you think are most pressing and need to be addressed in this technology?
Continue Reading:
- O. Tsiakaka, B. Gosselin, and S. Feruglio, “Source-detector spectral pairing-related inaccuracies in pulse oximetry: Evaluation of the wavelength shift,” Sensors (Basel), vol. 20, no. 11, p. 3302, 2020. Read the article here.
- This article provides the electrical and physiological background needed for understanding the methodology of noninvasive pulse oximetry. The authors then go on to discuss the effects of wavelength shifts in accuracy of this technology.
- B. T. Marozas, W. D. Hughes, X. Du, D. E. Sidor, G. R. Savich, and G. W. Wicks, “Surface dark current mechanisms in III-V infrared photodetectors,” Opt. Mater. Express, vol. 8, no. 6, p. 1419, 2018. Read the article here.
- This article explains the causes and driving factors of dark current in photodetector applications. The differences and effects of surface dark current versus bulk dark current are also covered.
- W. W. Moses, “Photodetectors for nuclear medical imaging,” Nucl. Instrum. Methods Phys. Res. A, vol. 610, no. 1, pp. 11–15, 2009. Read the article here.
- This article compares photodetector technologies used in nuclear medical imaging, including photomultiplier tubes, hybrid photodetectors, and silicon-based Geiger-mode photodetectors. The advantages and disadvantages of these technologies are explained in the context of imaging. This article is more advanced in its discussion of photodetector methodology.
About the Creator:
This video was created by a student in the Rice Bioengineering Class of 2023 (used with permission).
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