Emergency vehicle lights have improved over the years due to technological advances, so let's compare technologies and note their strengths and weaknesses.
Emergency Vehicle Light Requirements
Emergency responders have three basic requirements for emergency lighting. First and foremost, the lighting has to be highly visible. It should grab the attention of everyone within viewing range to announce that the first responders are present. Approaching vehicles need to be aware of a stationary emergency vehicle in time to avoid a collision, and normal traffic needs to be aware of an approaching emergency vehicle so that they can pull over and allow the rescue team through. In this sense, high visibility could mean a flashing strobe, a rotating light, or a correctly positioned steady light.
The second requirement is brightness. Emergency lights have to have the output to provide superior visibility for all involved parties. A patrol car’s spotlight is useless if it doesn’t have the output to illuminate the object of the search, and first responders have to be able to see clearly all around the responding vehicle.
The third requirement is durability. Emergency vehicles must often travel rough roads at high speeds and in adverse weather. A ride that rattles your bones will wreak havoc with delicate light fixtures. Emergency vehicle lights have to function equally well in hot or cold conditions, in wet or dry environments, and they have to be dependable for long periods of time.
Cost is not listed as a requirement, but the reality is that all departments have budgets. Initial outlay must be considered along with operating and maintenance costs.
With those three requirements in mind, we will now look at three different emergency lighting technologies: Incandescent lights, strobe tube lights, and LED lighting.
Standard Incandescent Lights
Incandescent light bulbs heat a wire filament to incandescence with an electric current. The wire is enclosed in a glass or quartz bulb that is either evacuated or filled with an inert gas to prevent oxidation of the filament. The filament wire is made from tungsten. Over time, the tungsten sublimes from the filament and condenses on the inner surface of the glass bulb. This darkens the glass and causes standard incandescent lights to have decreased output as they age. Eventually, the filament thins to the point where it breaks and severs the electrical circuit.
Quartz-Halogen Incandescent Lights
Halogen lamps are a special type of incandescent bulb. Instead of vacuum or an inert gas, the bulb contains krypton or xenon gas mixed with a small amount of halogen gas. Iodine was used in the very early models, but bromine compounds are used almost exclusively today. The halogen gas initiates a chemical reaction with the tungsten that sublimes from the filament. A gaseous tungsten halide compound is formed. This is a reversible reaction. Unlike a regular bulb, where hot tungsten condenses on the cooler bulb surface, tungsten halides revert back to tungsten and halogen gas at hotter temperatures. The filament is the hottest part of the system and the reverse reaction deposits tungsten back onto the filament. The result is a longer-lasting filament with a nearly constant light output through the life of the halogen lamp.
To promote the reversible halogen reaction, halogen lights must operate at much higher temperatures than an ordinary incandescent bulb. Standard borosilicate glass cannot withstand these higher operating temperatures so aluminosilicate glass or quartz bulbs are used. Quartz bulbs are stronger than aluminosilicate glass bulbs and allow higher pressures of halogen gas to be used. Higher halogen gas pressures slow the sublimation of the tungsten filament and allow the bulb to operate at higher temperatures for the same useful life.
Strengths and Weaknesses of Incandescent Strobes
To create flashing with an incandescent light, curved mirrors or reflectors focus the light into a beam. They are mounted on a small, rotating turntable. The beam of light appears as a repeating flash to viewers as it spins. Triangular or diamond-shaped mirrors are often used between the lights in emergency light bars to create the appearance of additional lights. Rotator light bars often use quartz-halogen bulbs.
Incandescent light technology is the least efficient of available lighting types. Approximately 95 percent of the energy used by incandescent lights is wasted as heat, and only about five percent is converted to light energy. They have short lifetimes when compared with other lighting technology.
In their favor, incandescent lights don’t require any type of external regulator and work with AC or DC current. Their emission spectrum is similar to natural sunlight, although manufacturing methods can make the emissions warmer or cooler, and they have the lowest manufacturing cost of any type of light bulb. These factors made them the most common type of emergency lighting for many decades.
Strobe Tube Lights
In the 1990s, most incandescent emergency lights were replaced by strobe tube lights. Strobe tube lights are similar to the flash bulbs used in photography and create very brief, very bright flashes of light by discharging current through xenon gas contained in a tube. The tubes come in linear, U and M shapes. Linear tubes are the least expensive strobe tube lights and produce the weakest flash. M-shaped strobe tube lights produce the brightest flash.
How Strobe Tube Lights Work
All strobe tube lights are significantly brighter than incandescent lights. They require no moving turntable to produce a flashing light because they actually do flash. Each strobe tube light is constructed of a sealed borosilicate glass or quartz tube that is filled with xenon. A cathode is sealed into one end of the tube, and an anode is sealed onto the other end. They are connected outside of the tube by a high-voltage capacitor with a rectifier and step-up transformer connected to a continuous power supply.
The xenon gas is non-conductive in its normal state, but it becomes conductive when it is ionized. A small amount of high-energy current flows through a trigger wire wound around the strobe tube when the capacitor is almost fully charged by the power supply. The current from the trigger wire ionizes the xenon gas in the strobe tube to make it conductive just as the capacitor becomes fully charged. The ionized xenon allows the capacitor to discharge, and the sudden discharge of current heats the xenon gas to an excited plasma state with extremely high conductivity and low resistance. It contains a very small percentage of electrons and positively charged ions.
Current flows through the strobe tube as positively charged ions move toward the cathode and electrons travel towards the anode. When these charged particles collide and recombine, they drop back to the non-conductive ground state and photons are emitted. This is the flash of light created by strobe tube lights. Strobe tubes can flash much more rapidly than rotating incandescent lights because the entire excitation and discharge cycle takes only a few thousandths of a second.
Strengths and Weaknesses of Strobe Tube Lights
Although they are much brighter than incandescent lights, strobe tube lights are directional. They give very little light to the sides and must be mounted correctly to have the desired effect. They are more expensive than quartz-halogen lights to manufacture, but the operating cost is essentially the same. They typically last longer than halogen lamps, and well-made strobe tubes can function for 10,000 hours.
In a standard or quartz-halogen incandescent lamp, the tungsten filament eventually breaks. Strobe tubes do not have filaments, but they won’t last forever. The useful life depends on the energy used to create the plasma. At low energy levels, the metal from the cathode evaporates and condenses on the walls of the tube in a process known as sputter. The coated tube walls block the light output, and sputter gradually ruins the strobe tube. The useful life cannot be accurately predicted at low energy levels.
At high energy levels, the inner wall of the tube forms microscopic cracks in a process known as ablation. Ablation causes the tube to have a frosted appearance and releases oxygen into the xenon gas. This increases the internal pressure of the tube beyond the point where triggering can reliably occur and causes the phenomenon of jitter. Most strobe tube lights are operated at high energies, however, because the useful life can be predicted with a high degree of accuracy at these levels.
Strobe Tube Purchasing Considerations
When purchasing a strobe tube emergency light, look for lamps with electron-rich cathodes. Barium alloys are electron-rich and result in less sputter. M or U tubes allow for a longer arc length and produce a more efficient flash. It is also easier to position the flash from these tube shapes at the focal point of the emergency light’s lens. Also look for lights with a separate timing circuit. A separate circuit for the flash trigger will create a flash rate that is independent of the charging power.
Most emergency responders feel that the explosive flash of a strobe tube commands attention better than flashing LED lights. They are often preferred for foggy or overcast climates. However, they can produce a harsh “flashback” into the driver’s eyes under foggy road conditions for this same reason.
LED lights are rapidly becoming the industry standard for emergency lighting. It’s almost inevitable that they will replace strobe tubes in the same way that strobe tubes have all but replaced incandescent lights. LED lights are very small, have extremely high efficiencies and last for more than 100,000 hours. They produce almost no heat and can be programmed into any flash pattern desired. Technical advances continue to make them brighter, and models are available now that rival even strobe tubes for luminous output. They are easily visible even in bright daylight.
How LED Lights Work
LED stands for Light Emitting Diode. Diodes are a very simple type of semiconductor. LEDs are simply p-n junction diodes that emit light. Semiconductors are materials that do not usually conduct electricity but can be made to do so under certain conditions. In the simplest terms, all atoms can be envisioned as electrons orbiting a nucleus like planets orbiting the sun. Strong nuclear attraction forces hold the electrons tightly in these orbits in the same way that gravity controls the orbits of the planets. Large atoms have more electrons and more orbital layers. As these orbits become more distant from the nucleus, the grip the nucleus holds on the electrons becomes weaker.
Each orbital can be thought of as an energy shell with an inner energy band, called a valence band, and an outer energy band called a conduction band. The physical distance between the valence and conduction energy bands is called the band gap, and the energy required to jump the band gap and move an electron from a lower valence band to a higher-energy conduction band is galled the activation energy.
When a semiconductor receives activation energy, electrons in the outermost shell jump from the valence band to a conduction band and are able to move to neighboring atoms. As they move from one atom to another, they leave behind what can be thought of as energy holes. Valence electrons from neighboring atoms drop into these energy holes and create new holes in their previous positions. The electrons release energy in the form of a photon when they drop from an excited orbital to a lower one. Greater energy drops result in photons of higher energy and shorter wavelengths. By constructing LEDs of materials with specific band gaps, the energy level of the emitted photons, and thus the wavelength and color of the light, can be controlled. For this reason, LED emergency lights typically use a colorless dome or light bar.
Advantages of LED Lights for Emergency Vehicles
Tactical light bars using LED lights can be very thin because LEDs are so small. Thin light bars can reduce wind resistance by as much as 10 percent and contribute to fuel savings. Since they are switched on entirely by electronic signals, flash patterns can be customized in innumerable ways for code three pursuits or motorist alerts during traffic stops. The LEDs are so small that deck lights and dash lights are almost invisible when not in use. Some models install along the top of the windshield. Many inside installations draw power from a simple lighter socket plug. They are also easily concealed in the front grill of vehicles. When grill-mounted, care must be taken to angle the installation appropriately.
There are many different types of LED lights, and selecting the best lamp for a particular application can be confusing. The top priority for emergency lighting use is that the light has to be highly visible when activated. First responders are naturally drawn to lights with the description “high power” attached to them, but there is no standard definition of “high power” for LED lights. Any single LED greater than 0.5 watt can be called high power. Low power LEDs are generally 0.1W and have an operating current of 20 milliamps. High power LEDs are currently available in 1W, 5W, and even tens of watts sizes with operating currents as high as several hundred milliamps.
Generations of LED Lights
LED lights are also classified as Generation I, II and III lights. The size and power requirements of LEDs have changed as LED technology evolved, and the generation nomenclature indicates this.
Gen I LED lights are 5 millimeter diodes that operate on 20 mA. These lights were somewhat of a disappointment to emergency responders when they first appeared. The small size, high efficiency and low power consumption were positive traits, but the lights simply weren’t bright enough.
Gen II LED lights typically operate on 80 mA of power. These were the first LEDs to be called high power LEDs. Although they are no longer the top of the line in output, Gen II LEDs are very bright. Cost certainly has its place when considering emergency equipment purchases, and Gen II LEDs are bright enough that the relative low cost, when compared to Gen III LEDs, may make them the best choice for volunteers or cash-strapped departments.
Gen III LED lights typically operate on 350 mA to 1 A. A 1W LED is the same thing as a 1 A Gen III LED. Gen III LEDs are more expensive than Gen I or Gen II LEDs, but they operate at higher energy levels, are much brighter and can operate for longer periods of time. Emergency flashers must often be left on for extended periods when emergency responders are working accident scenes.
LEDs are also available with a “TIR” designation. TIR LEDs are Gen III LEDs with the addition of a Total Internal Reflection lens to focus the light in a forward direction. They are sometimes called TIR 3 LEDs or Gen IV LEDs. The light output from a TIR LED is brighter in the forward direction but dimmer in lateral directions. Cree is the largest producer of Gen IV LEDs, and many people call them Cree lights.
LEDs are now available in Gen V models. These powerful lights operate on 3W, 5W, or even 10W circuits. They are so tremendously bright that they are mainly used only for spotlight and floodlight applications. Gen V LED lights are capable of producing up to 128 lumens per watt of power.
Choosing The Right Emergency Lights
The right light for a particular application hinges on a variety of factors, but LED lights of one generation or another are almost always the best choice. LEDs dominate the emergency lighting market, and with good reason:
On the negative side, LED lights are expensive. Initial outlay for LEDs is significantly higher than all other emergency lighting options. This cost should be viewed over the entire life cycle of the light, factoring in maintenance and replacement costs for quartz-halogen or strobe tube technologies, to provide an accurate cost analysis. It should also be noted that the most advanced – and most expensive – LED technologies are not always the best lights for a particular application. Gen III lights are sufficient for many needs and are still superior to strobe tubes when budgetary constraints limit purchasing options.
In some locations, the efficiency of LEDs can also be a problem. They are highly efficient because they convert nearly all of their input power to light and produce very little heat. In cold climates, this can be an issue. A buildup of frost or snow on external LED fixtures can potentially block their illumination, and the lights themselves do not produce enough heat to melt the snow or ice away.
Now that you understand how emergency vehicle lights have changed over the years, you can appreciate these advances which have led to better safety for first responders and civilians alike