3. Introduction
• Sensors can be used to measure or detect a vast variety of physical,
chemical, and biological quantities.
• Sensor measurements are converted by a transducer into a signal
that represents the quantity of interest to an observer or to the
external world.
The sensing process
23.3.20183
4. Introduction
• An optical sensor is a device that detects a change in light intensity.
• The type of light and method by which the target is detected varies
depending on the sensor.
optical sensor device components
23.3.20184
5. Taypes of Sensing
• Diffused Mode Sensing:
In diffused mode sensing the transmitter and receiver are in the same
where.
Light from the transmitter strikes the target, which reflects light at
arbitrary angles and Some of the reflected light returns to the receiver,
and the target is detected.
Diffused Mode Sensing
23.3.20185
6. Taypes of Sensing
• Retro-reflective mode:
the transmitter and receiver are in the same where, but a reflector is
used to reflect the light from the transmitter back to the receiver.
The target is detected when it blocks the beam from the photoelectric
sensor to the reflector.
Retro-reflective mode 23.3.20186
7. Taypes of Sensing
Thru-beam mode:
• This mode uses two separate housings, one for the transmitter and
one for the receiver.
• The light from the transmitter is aimed at the receiver and when a
target breaks this light beam, the output on the receiver is activated.
This mode is the most efficient of the three, and allows the longest
possible sensing ranges for photoelectric sensors.
Thru-beam mode
23.3.20187
9. Light Sources
Every day light sources such as tungsten filament and arc lamps are
suitable, but there exists two types of devices, which are widely used
in optical communication systems:
Light Emitting Diode (LED).
Semiconductor Laser Diode (SLD or LD).
23.3.20189
10. Light Sources
• Light Emitting Diode:
A light-emitting diode (LED) is a semiconductor device that emits visible
light under the influence of an electric field.
The LEDs transmit at different voltage values according to the properties
of the additives used, and at the moment of transmission, the LED emits
various colors of light.
23.3.201810 Common LEDs
11. Light Sources
• Power Light Emitting Diode (power LED):
Normal LEDs spend about milliWatt (mW), while Power LEDs consume
about Watt power.
23.3.201811
The development of LED technology in recent years has allowed
these elements to be used in cars, home and street lighting .....
Physical Appearance of Various Power LEDs
12. Light Sources
Semiconductor Laser Diode (SLD or LD).
• The materials used in semiconductor lasers are aggregate of many
atoms, and large numbers of electrons revolve around these atomic
nuclei. When electrons with high energy return to a low-energy state,
they emit energy as light. In addition, other electrons are stimulated
by this light and also return to a low-energy state and emit light
(stimulated emission).
23.3.201812
13. Light Sources
Semiconductor Laser Diode (SLD or LD):
23.3.201813
In applications where extremely high power density within a small area is required, a laser is
almost always required.
Property Multimode Single Mode
< 0.21-5Spectral width (nm)
10-1001-10Output power (mW)
100-25050-150Drive current (mA)
6000-40,0002000Modulation bandwidth (MHz)
14. Photodetectors
Photodetector sensors are based on the principle of photoconductivity.
Sensors are sensitive for a given spectral region (range of optical wavelengths)
from ultra violet to infrared. Examples include:
• Active pixel sensors: such as those found in smartphone cameras and web
cams.
• Light-dependent resistors (LDRs): such as those found in street lighting
systems.
• Photodiodes: such as those used in room lighting-level control systems or in
UV measurement systems.
• Phototransistors: such as those used in optoisolators for a variety of
applications, including health care equipment, to provide electrical isolation
between the patient and equipment.
• Photomultipliers: such as those found in spectrophotometers detectors.
Photomultipliers are also used in flow cytometers for blood analysis applications
23.3.201814
15. Photodetectors
photodiode :
It converts light energy into electrical energy by applying reverse bias
voltage .
The photodiode generates voltage in proportion to the light falling on
it.
23.3.201815 reverse current-versus-voltage characteristic
16. Photodetectors
photodiode :
23.3.201816
Physical Views of Various Photo Diodes
It is used as a light sensor in the automatic night lightings , burglar
alarm systems, TV, music set etc. in the remote control devices with
automatic opening and closing door systems.
17. Photodetectors
• EXAMPLE:
The photodiode is used in this circuit shown in this Figure. What range of output voltage will result from
light intensities from 100 to 400 W/m2 ?
• A load line is drawn on the curves , extending from the supply
voltage of 20 V to the current with no voltage drop across the
diode -20 V/15 kΩ = -1.33 mA.
• This load line shows that the reverse current will range from
-200 to -800 µA as the intensity ranges from 100 to 400 W/m2.
• The op amp circuit is a simple current-to-voltage converter with
an output of Vout = 1000 I .
• Therefore, the range of output voltage will be from
1000(200µA) = 0.2 V to 1000(800 µA) = 0.8 V .
23.3.201817
18. Infrared sensors
• Infrared (IR) sensors are normally used to estimate the distance of an
object and we can also use them to detect the presence of objects.
• The transmitter outputs pulses of infrared radiation, while at the
same time, the receiver detects any reflections.
23.3.201818
Physical View of Infrared LED
19. passive Infrared Sensor
• Every object that has a temperature above absolute zero emit heat
energy in the form of radiation radiating at infrared wavelengths
which is invisible to the human eye, but can be detected by special
purpose electronic devices such as PIR motion detectors.
• The PIR sensor itself has two slots in it, each slot is made of a special
material that is sensitive to IR.
23.3.201819
20. optical sensors applications
They are widely used in industrial applications for measuring liquids
and material levels in tanks or in factory production lines to detect the
presence of objects.
• Movement Sensed Automatic Door Opening System:
These systems are used to open the door when a person comes near to
the entrance of the door and close it after he moves away from the
door or after entered into the door .
23.3.201820
21. optical sensors applications
23.3.201821
If a body movement is present near the
door
infrared radiation emitted from the body
will cause the sensor to produce
sensing signal which is fed to
microcontroller
The door motor is then controlled and
operated by the microcontroller through
driver IC
22. optical sensors applications
• Optical Smoke Alarms Work:
An optical smoke alarm works using the light scatter principle. The alarm
contains a pulsed Infra red LED which pulses a beam of light into the sensor
chamber every 10 seconds to check for smoke particles.
23.3.201822
23. optical sensors applications
when the smoke enters the optical chamber, its particles cause the Infra red
light to be scattered onto the photodiode light receptor.
Once the scattered light hits the photodiode light receptor a signal is sent to
the integrated circuit which causes the alarm to sound alerting the occupants
to the fire.
23.3.201823
