Ultraviolet Spectral-Sensitive Photoelectric Sensors

DECEMBER 15, 2011


A fire starts in a room, the faster it can be located and extinguished, the less the damage there will be. Smoke detectors are devices that detect smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector itself. Some advanced system turn on extinguishing systems to help prevent the spread of fire and help remove the fire altogether. Ultraviolet spectral-sensitive photoelectric sensors allow fire to be detected through the use of the photoelectric effect of metal and the gas multiplication effect. These sensors work great for detecting presence of fire faster than current systems in use due to not relying on the buildup of smoke for detection and for its large distance range. They are highly reliable, not affected by common sources of UV, have low power requirements, and are easy to integrate into various systems.


The most common and currently used systems to detect presence of a fire, contain either a semiconductor photo-detector or an ionization chamber. However these systems detect the smoke for a fire and not the fire itself. In some case, smoke is not always present in a fire, or by the time smoke has reached a detector, a large portion of the area has already set alight. A different type of detector is required for these cases, one that can detect the actual fire itself and not the byproduct.

Spectral-sensitive photoelectric sensors have a select set of wavelengths that they can detect. Some detect visible light (violet, blue, green, yellow, orange, red), some detect larger wavelengths (infrared, microwave, radio), and others detect smaller wavelengths (ultra-violet, x-ray, gamma ray). Fire emits a large spectrum of wavelengths; from infrared (300um), through the visible light spectrum, to ultraviolet (10nm). In most areas, visible light is always present; therefore infrared (IR) and ultraviolet (UV) wavelengths are the best to associate fire with. A large source of light that is present in most areas is sunlight, out of 1 kilowatt per square meter at sea level; 527 watts is IR radiation, 445 watts is visible light, and 32 watts is UV radiation. Therefore, an ultraviolet sensor is the best detector as it will be less affected by outside sources when detecting the presence of a flame.

Figure 1: Fire Robot

Prior work with UV detection was for a mechatronics project that was based on competition at RoboRAVE International in Albuquerque, NM. This challenge involves deploying a robot that is capable of detecting, navigating towards, and extinguishing a small flame. To accomplish this, a popular ultraviolet detector was used, the Hamamatsu UVTron, which is capable of detecting a cigarette lighter greater than 5 meters away.



Ultraviolet detectors make use of the photoelectric effect of metal and the gas multiplication effect. The photoelectric effect deals with electrons being emitted from matter as a result energy absorption from electromagnetic radiation of such as visible or ultraviolet light. The gas multiplication effect involves using electrons that collide with gas molecules to create ionization. The following steps demonstrate how the sensor works:

Theory of Operation - Step 1

When the cathode is exposed to ultraviolet light, photoelectrons are emitted from the cathode and then accelerated toward anode by the electric field. A diagram of this process can be seen in Figure 2.

Figure 2: Step 1

Theory of Operation - Step 2

When voltage is applied, the electric field becomes stronger causing the kinetic electrons to ionize the gas. A diagram of this process can be seen in Figure 3.

Figure 3: Step 2

Theory of Operation - Step 3

Because of the ionization, more electrons are generated. At the same time, positive ions accelerate and collide with the cathode, generating secondary electrons. This avalanche process causes a large current between anode and cathode, causing discharge to take place. The driving circuit looks for the discharge effect and outputs a signal. A diagram of this process can be seen in Figure 4.

Figure 4: Step 3


Figure 5: Wavelength Optimization
(Diagram courtesy of Hamamatsu Photonics)

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10nm to 400nm. The UVTron detects a narrow range of UV light, between 185nm and 260 nm, this allows the sensor to ignore sunlight and other sources that are not desirable in fire detection. The graph in Figure 5 shows that the sensor is able to detect a gas flame but cannot detect sunlight or a Tungsten bulb, even though they all produce a range of Ultraviolet light.

Figure 6: Field-of-View Optimization
(Diagram courtesy of Hamamatsu Photonics)

Depending on which type of UVTron is used, the range of sensitivity changes, as displayed in Figure 6, the R9454 has nearly a +/-45 degree angle in both the horizontal and vertical field-of-view, whereas the R9533 has closer to a +/-30 degree field-of-view; this is caused by how the internal electrodes are placed in the envelope. For both types of UVTron, the field-of-view only exists for the front facing part of the sensor, thus causing the out edges (+/-90 degrees) to result in zero sensitivity.

The UVTron is capable of detecting very weak rays, anywhere from 1pW of light, the sensor is highly reliable and has a long service life of approximately 10,000 hours of continuous discharge use. This particular sensor has a high speed response (few milliseconds), Low current operation, and is compact and lightweight (approximately 1.5g).


Figure 7: UVTron Driving Circuit and Sensor Bulb
(Photo courtesy of Hamamatsu Photonics)

The UVTron operates at a voltage of 325+/-25V(DC) with a peak current of 30 mA. Since most applications cannot provide the 325V required, the UVTron bulb can be attached to a driving circuit (also made by Hamamatsu). The circuit allows the flame sensor to operate at a low input voltage between 6 to 30V. The driving circuit and bulb can be seen in Figure 7 (above).


The UVTron measures UV light and outputs a voltage, this output is either High or Low; and is therefore a binary system with a square waveform. The driving circuit in Figure 7 has an output of 0 or 5V and allows for either impedance, 5V as detection or 5V as no detection. All computation is done by driving circuit, and background cancellation filter can be set by an onboard jumper. This system however, is not analog as it cannot distinguish how big the fire it is.


Figure 8: Integration Diagram
(Diagram courtesy of Hamamatsu Photonics)

The above schematic diagram (Figure 8) shows the layout of the driving circuit shown in Figure 7, it allows for voltage stability, power conversion, and signal processing. A background cancellation filter can be set using a jumper to tell how many pulses during a 2 second interval should be detected before a signal is sent.

There are numerous considerations and precautions to take when using this type of sensor:
  • During discharge, the sensor emits ultraviolet radiation which may cause interference with similar sensors in close proximity.
  • Humidity around the sensor leads can generate leakage, resulting in the anode voltage dropping and causing the tube to stop operating.
  • Since the sensor operates at high voltage, static electricity causes dust to build up, thus lowering ultraviolet transmissivity and sensitivity.
  • The leads of the sensor must be soldered quickly (350 degrees C for less than 5 seconds), as the glass can crack or cause the measurement characteristics to deteriorate.
  • The UVTron has been tested to survive the vibration and shock tests in compliance with IEC 60068-2-6 (sinusoidal vibration test) and IEC 60068-2-27 (shock test); however, despite these tests, strong mechanical shocks my crack the glass envelope or cause deformation of internal electrodes.
  • Reversing the polarity of the system will cause damage to the sensor and possibly stopping the UVTron from ever working again.

Figure 9: Recommended Operating Circuit
(Diagram courtesy of Hamamatsu Photonics)

Figure 9 shows the recommended operating circuit for the UVTron, a capacitor is used to hold the charge for a signal pulse, and a 4.7 kΩ resistor must be connected within 2.5 cm from tip of anode lead to allow the bulb to receive the full power necessary for the sensor to function correctly.


The following are examples UV Spectral-sensitive photoelectric sensors:
  • Flame detectors for gas/oil lighters and matches
  • Fire alarms
  • Combustion monitors for burners
  • Inspection of UV leakage
  • Detection of corona discharge
  • UV switches

Most uses for this type of sensor, particularly the UVTron, are used in robots for various firefighting competitions. However since the UVTron has a large distance range and high sensitivity, the best application might be for use in arson monitors, for instance, in a parking lot or warehouse.


These UV sensors have a wide area of use in both small and large applications. They would be great for areas that want no fire at all such as a paper manufacturing plant and are not affected by their environment as sunlight does not influence its reading.

To help improve this type of sensor, Dynodes (middle points between the anode and cathode) could be added to increase sensitivity, a less fragile glass or plastic could be used to increase durability. A variable wavelength range may help fine-tune its detection and the addition of a cover may help with directionality.


Ultraviolet Spectral-Sensitive Photoelectric Sensors work great for detecting presence of fire faster than current systems in use due to not relying on the buildup of smoke for detection and for its large distance range. These sensors are highly reliable, not affected by common sources of UV, have low power requirements, and are easy to integrate into various systems.


Would like to thank RoboRAVE International (TM) for donating the sensor and their continuous support with these projects.


Wilson, J.S. (editor), (2004) "Sensor Technology Handbook," Newnes, 1st edition.

"C3704 Hanamasu ." Data sheets. N.p., n.d. Web. 7 May 2011.

"UVTRON R2868." Acroname Robotics. N.p., n.d. Web. 10 May 2011.