Optical or light related sensors and its principles are discussed. The use of the LDR, photocell, photodiodes, and many more transducers which are based on optical sensors are discussed with the applications related to it.

1. Compiled by: Prof. G B Rathod
EC Dept., BVM Engineering college.
Email: ghansyam.rathod@bvmengineering.ac.in
Optical Sensors

2. Outlines
 FUNDAMENTALS OF EM RADIATION
 PHOTODETECTORS
 PYROMETRY
 OPTICAL SOURSES

3. FUNDAMENTALS OF EM RADIATION
 Frequency and Wavelength Because we use the term
electromagnetic radiation to name this form of energy, it is no
surprise that it is intimately tied to electricity and
magnetism.
 The frequency represents the oscillation per second as the
radiation passes some fixed point in space. The wavelength is
the spatial distance between two successive maxima or
minima of the wave in the direction of propagation.
 Speed of Propagation EM radiation propagates through a
vacuum at a constant speed independent of both the
wavelength and frequency. In this case, the velocity is

4. FUNDAMENTALS OF EM RADIATION

5. FUNDAMENTALS OF EM RADIATION
 When such radiation moves through a nonvacuum
environment, the propagation velocity is reduced to a value
less than c.
 In general, the new velocity is indicated by the index of
refraction of the medium. The index of refraction is a ratio
defined by

6. FUNDAMENTALS OF EM RADIATION
 Example:

7. FUNDAMENTALS OF EM RADIATION
 Wavelength Units For many applications, specification of
EM radiation is made through the frequency of the radiation,
as in a 1-MHz radio signal or a 1-GHz microwave signal.
 Another unit often employed is the Angstrom ( Å), defined as
10^-10 m or 10^-10 m/ Å.
 EM Radiation Spectrum We have seen that EM radiation
is a type of energy that propagates through space at a
constant speed or velocity if we specify the direction.
 The oscillating nature of this radiation gives rise to a different
interpretation of this radiation in relation to our
environment, however.

8. FUNDAMENTALS OF EM RADIATION
 This one type of energy ranges from radio signals and visible
light to X-rays and penetrating cosmic rays and all through
the smooth variation of frequency. In process-control
instrumentation, we are particularly interested in two of the
bands, infrared and visible light.
 Example: Describe the wavelength (in m and Å ) for the
frequency of 5.4 X 10^13 Hz. (Solution)

9. FUNDAMENTALS OF EM RADIATION
 Photon: No description of EM radiation is complete
without a discussion of the photon.
 EM radiation at a particular frequency can propagate only in
discrete quantities of energy.
 Thus, if some source is emitting radiation of one frequency,
then in fact it is emitting this energy as a large number of
discrete units or quanta.These quanta are called photons.

10. FUNDAMENTALS OF EM RADIATION
 The actual energy of one photon is related to the frequency
by

11. FUNDAMENTALS OF EM RADIATION
 Example:

12. FUNDAMENTALS OF EM RADIATION
 Intensity A more complete picture of the radiation emerges
if we also specify the spatial distribution of the power
transverse to the direction of propagation.
 In general, the intensity is

13. FUNDAMENTALS OF EM RADIATION
 Divergence We still have not quite exhausted the necessary
descriptors of the energy because of the tendency of light to
travel in straight lines.
 Because the radiation travels in straight lines, it is possible for
the intensity of the light to change even though the power
remains constant.

14. FUNDAMENTALS OF EM RADIATION
 Example:

15. FUNDAMENTALS OF EM RADIATION

16. FUNDAMENTALS OF EM RADIATION

17. FUNDAMENTALS OF EM RADIATION
 Photometry
 The conventional units described previously would seem to
be satisfactory for a complete description of optical
processes. For designs in human engineering, however, these
traditional units are insufficient.
 This is because the human eye responds not only to the
intensity of light but also to its spectral content.
 Suppose it is known that a human needs an intensity of 1
W/m^2 to read. If an infrared source at that intensity were
used, the human would still not be able to read, because the
eye does not respond to infrared.

18. FUNDAMENTALS OF EM RADIATION
 Because of these problems and others, special sets of units for
EM radiation have been developed to be used in such human-
related design problems.The most basic unit is the SI candela
(cd).

19. PHOTODETECTORS
 An important part of any application of light to an
instrumentation problem is how to measure or detect
radiation.
 In most process-control-related applications, the radiation
lies in the range from IR through visible and sometimes UV
bands.
 The measurement sensors generally used are called
photodetectors to distinguish them from other spectral ranges
of radiation, such as RF detectors in radio frequency (RF)
applications.

20. PHOTODETECTORS
 Photoconductive Detectors:
 One of the most common photodetectors is based on the change
in conductivity of a semiconductor material with radiation intensity.
 Principle Earlier, we noted that a semiconductor is a material in
which an energy gap exists between conduction electrons and
valence electrons.
 In a semiconductor photodetector, a photon is absorbed and
thereby excites an electron from the valence to the conduction
band.
 As many electrons are excited into the conduction band, the
semiconductor resistance decreases, making the resistance an
inverse function of radiation intensity.

21. PHOTODETECTORS
 For the photon to provide such an excitation, it must carry at
least as much energy as the gap.

22. PHOTODETECTORS
 Example:

23. PHOTODETECTORS
 Cell Structure Two common photoconductive
semiconductor materials are cadmium sulfide (CdS), with a
band gap of 2.42 eV, and cadmium selenide (CdSe), with a
1.74-eV gap.
 Because of these large gap energies, both materials have a
very high resistivity at room temperature. This gives bulk
samples a resistance much too large for practical applications.
 To overcome this, a special structure is used, as shown in
upcoming Figure , that minimizes resistance geometrically
and provides maximum surface area for the detector.

24. PHOTODETECTORS
 This result is based on the equation:

25. PHOTODETECTORS
 The area presented to light, A=WL, is large for maximum
exposure, whereas the electrical length l=L, is small, which
reduces the nominal resistance. Of course, such a structure is
unwieldy, so the strip is wound back and forth on an
insulating base, as shown in Figure(b)

26. PHOTODETECTORS

27. PHOTODETECTORS
 Cell Characteristics The characteristics of
photoconductive detectors vary considerably when different
semiconductor materials are used as the active element.
These characteristics are summarized for typical values in
Table 1.

28. PHOTODETECTORS
 Photovoltaic Detectors:
 Another important class of photodetectors generates a
voltage that is proportional to incident EM radiation
intensity. These devices are called photovoltaic cells because of
their voltage generating characteristics.
 They actually convert the EM energy into electrical energy.
Applications are found as both EM radiation detectors and
power sources converting solar radiation into electrical
power. The emphasis of our consideration is on
instrumentation type applications.

29. PHOTODETECTORS
 Principle Operating principles of the photovoltaic cell are
best described by Figure .We see that the cell is actually a
giant diode that is constructed using a pn junction between
appropriately doped semiconductors.

30. PHOTODETECTORS
 Photons striking the cell pass through the thin p-doped upper layer
and are absorbed by electrons in the n layer, which causes
formation of conduction electrons and holes.
 The depletion-zone potential of the pn junction then separates
these conduction electrons and holes, which causes a difference of
potential to develop across the junction.
 The upper terminal is positive, and the lower negative. It is also
possible to build a cell with a thin, n-doped layer on top so that all
polarities are opposite.
 Electrical characteristics of the photovoltaic cell can be
understood by reference to the pn junction diode IV
characteristics.

31. PHOTODETECTORS
 You will note that when the junction is illuminated, a voltage
is generated across the diode, as shown by the IV curve
crossing the zero current axis with a nonzero voltage. This is
the photovoltaic voltage.
 Photovoltaic cells also have a range of spectral response
within which a voltage will be produced. Clearly, if the
frequency is too small, the individual photons will have
insufficient energy to create an electron-hole pair, and no
voltage will be produced.

32. PHOTODETECTORS
 The IV curves of a pn junction diode vary with exposure to
EM radiation.

33. PHOTODETECTORS
 Since the photovoltaic cell is a battery, it can be modeled as
an ideal voltage source,Vc,in series with an internal
resistance, Rc,as shown in Figure.
 It turns out that the voltage source varies with light intensity
in an approximately logarithmic fashion:

34. PHOTODETECTORS
 The internal resistance of the cell also varies with light
intensity. At low intensity, the resistance may be thousands of
ohms, whereas at higher intensities it may drop to less than
50 ohms.
 This circuit converts the cell short-circuit current into a
proportional voltage.

35. PHOTODETECTORS
 Example

36. PHOTODETECTORS

37. PHOTODETECTORS

38. PHOTODETECTORS
 Cell Characteristics The properties of photovoltaic cells
depend on the materials employed for the cell and the nature
of the doping used to provide the n and p layers.
 Some cells are used only at low temperatures to prevent
thermal effects from obscuring radiation detection. The
silicon photovoltaic cell is probably the most common. Table
2 lists several types of cells and their typical specifications.

39. PHOTODETECTORS
 Photodiode Detectors
 The previous section showed one way that the pn junction of a
diode is sensitive to EM radiation, the photovoltaic effect.A pn diode
is sensitive to EM radiation in another way as well, which gives
rise to photodiodes as sensors.
 The photodiode effect refers to the fact that photons impinging on
the pn junction also alter the reverse current-versus-voltage
characteristic of the diode. In particular, the reverse current will
be increased almost linearly with light intensity.
 Thus, the photodiode is operated in the reverse-bias mode. Figure
a shows a basic reverse-bias connection of such a diode.

40. PHOTODETECTORS

41. PHOTODETECTORS
 Photodiodes are very small and often use an internal lens to focus light
on the junction.
 Phototransistor An extension of the photodiode concept is the
phototransistor.
 In this sensor, the intensity of EM radiation impinging on the collector-
base junction of the transistor acts much like a base current in producing
an amplified collector-emitter current.

42. PHOTODETECTORS
 Photoemissive Detectors:
 This type of photodetector was developed many years ago,
but it is still one of the most sensitive types. A wide variety of
spectral ranges and sensitivities can be selected from the
many types of photoemissive detectors available.

43. PYROMETRY
 One of the most significant applications of optoelectronic
transducers is in the noncontact measurement of
temperature. The early term pyrometry has been extended to
include any of several methods of temperature measurement
that rely on EM radiation.
 These methods depend on a direct relation between an
object’s temperature and the EM radiation emitted. In this
section, we consider the mechanism by which such radiation
and temperature are related and how it is used for
temperature measurement.

44. PYROMETRY
 Read the other types of pyrometry from the reference book.
 OPTICAL SOURCES: Read reference book for the
details of optical sources.

45. Reference
 Curtis D. Johnson. “Process Control Instrumentation
Technology”, Prentice Hall, 8/E, 2005 ISBN-10:
0131194577 • ISBN-13: 9780131194571

46. Thank You