: Optical Sensors Flashcards
- Quantal nature of light
o When a material is illuminated by photons, they may be either reflected or
absorbed. If they are absorbed, the energy of the photon may be
transformed into random motion as heat, or it
Thermal Light Sensors:
Indirect sensing Method
o Do not depend on the quantal nature of light
o Response of the material depends upon the radiant power, not on the
spectral content of the radiation (although the light absorbent material may
have some wavelength dependence)
o Absorbed radiation results in an increase in temperature of the material
(therefore want low thermal mass of absorber)
o Thermal effects generally occur on a millisecond time scale
o Energy gaps
§ In direct photodetection:
In metals and semiconductors:
In direct photodetection, the photons interact directly with electrons
in the material
§ In metals and semiconductors, electrons are bound to their atoms
by electrostatic force. The average strength of this force is described
by the material’s ionization energy, or work function. Only an
electron with energy greater than this can escape the atom.
§ Semiconductors have much lower forbidden energy gaps than the
work functions of metals, and can therefore detect low energy
photons.
§ Direct photodetection of visible light is almost always performed
using semiconductors
o Internal Direct Effects
o External Direct Effects
§ Those in which the electrons of holes stay within the material
§ Photon causes emission of an electron from the absorbing material
(known as the photocathode).
• External devices often charge-multiply this electron,
producing gain.
• Photoconductive
Photometer o A photon increases the concentration of free electrons or holes in the material, causing an increase in electrical conductivity
Intrinsic Photoconductivity
o Occurs if the photon has
enough energy to excite electrons across the band gap
to produce an electron/hole pair. The electron/hole
pairs result in an increase in conductivity
Extrinsic Photoconductivity
o Occurs when an incident photon
produces an excitation at an
impurity centre as either a free
electron/bound hole or a free
hole/bound electron
o Impurity levels that are able to
accept an electron excited from
the conduction band are called
acceptor levels, while impurity levels that can have an
electron excited from this into the conduction band are
called donor levels
o Photons with energy greater than acceptor level excite
an electron to the impurity level, leaving a hole in the
valence band, thereby giving rise to p-type
photoconductivity
o Photons with energy greater than donor level excite
an electron to the impurity level, leaving an electron
in the valence band, thereby giving rise to n-type
photoconductivity
o LDRs/Photoconductive Devices
§ I.e. increasing light intensity cause increase in
conductivity/decrease in resistance
Photovoltaic
• Operation
o Photoexcitation of electron/hole pairs occurs near a
junction when radiation of energy greater than the
band gap is incident on the junction region.
o The internal energy barrier of the junction causes the
photoexcited hole/electron pair to separate,
generating a voltage across the diode.
o A reverse-biased detector will generate a current and
give a much faster response
o The photovoltaic effect occurs at a simple p-n junction
but can also be observed in p-i-n diodes which have a
thin intrinsic (undoped) material between the two
doped regions
o Absorption of incident radiation in the intrinsic region
produces electron/hole pairs. Because of the high
collection voltage and small distance across the
intrinsic region, the electron/hole pairs drift rapidly
through this region. As a consequence, the frequency
response of a p-i-n photodiode is usually higher than
that of a comparable p-n photodiode.
• Photodiodes vs Phototransistors
Photodiodes - High Dynamic Range - High light bandwidth - Linear - Very reproducible - Inexpensive - High electrical bandwidth Phototransistors - Built in gain - Light bandwidth limited - Non linear - Large variations in sensitivity between devices - Electrical bandwidth limited
Avalanche
o Occurs in p-n diodes of moderate doping under
relatively large reverse bias. Photoexcited electrons or
BIOMENG341 – Section A Notes
Mairi Robertson
19
holes are accelerated in the high-field region of the
junction. As they are accelerated, they collide with the
structure and free more electrons, causing an
avalanche of electrons in the high-field region
o Avalanche photodiodes have high internal gain and
high speed, but are noisier than p-i-n photodiodes
o They may be used to count individual photons, but
require active drive circuits to rapidly quench the
avalanche response
Lateral Effect Photodiodes
o The photovoltaic effect can be used to measure
displacement by using a
LEP. The LEP consists of a
single-large area
photodiode, which has
uniform resistive sheets
and contacts on each of the two sheets. The photo
generated current carriers (electrons and holes) divide
between the contacts in proportion to the resistance
of the current paths between the illuminated region
and the contacts. The position of a light spot centroid
can be deduced from the currents of the contact pairs,
since the resistances are directly proportional to the
lengths of the current pairs.
o Photocurrent at each electrode is inversely
proportional to distance to spot centroid
• Split Photodiodes
o Photodiode in four quadrants
o Response in each quadrant can be used
to quantify light beam position
• In photoemissive devices, the incident radiation
causes
the emission of an electron from the
surface of the absorbing material, the
photocathode, into the surrounding space from whence it is
collected by an anode.
• Photoemissive effects are used in vacuum phototubes and
photomultiplier tubes.
• Photomultiplier’s (PMTs)
PMTs are used for low light level detection and
measurement
o Photons strike a photosensitive alkali metal
photocathode. If the energy of the photon is greater
than the work function of the photoemitter, a single
electron is ejected.
o Each electron is accelerated to the next dynode
through vacuum by an electric field.
o On striking the dynode, further electrons are released.
o These electrons are ejected.
o For each electron striking a dynode, approximately
two electrons are released.
o With 10 dynodes, approximately 1000 electrons are
produced per incident photon.
o PMTs have a typical dark current of about 10-7A
o Their response time is in the order of 10-9 to 10-8s
o The spectral sensitivity depends upon the properties
of the photoemitter and the transmittance of the
enclosing glass
Examples of optical measurements
Point measurements
Area measurements
