A very interesting and informative article from Michael Netherton of Responder PSE in the United States.
T.I.R. vs Linear – what does this mean to you?
Published at Nov 12, 2014
When it comes to emergency vehicle warning lights, they will almost all fall into one of two classes. TIR or Linear. Whether you’re dealing with surface mount lights, commonly referred to as “grille lights”, lightbars, minibars, arrowsticks or most other warning products except beacons, this is very reliable rule you can live by. What does this mean, you’re asking yourself. These two optical designs each have their strengths and weaknesses and they exhibit very different output characteristics.
TIR, which stands for “total internal reflector” can be understood by thinking of a typical flashlight. Each light source, in this case a light emitting diode (LED), is centered at the end of a conical reflector. The reflector captures a significant amount of the scattered light and focuses it forward. The beam this creates is round and narrow just like you expect to see when using a flashlight to see in the dark. In order to spread this beam of light out over a wider area, manufacturers include optics seen as vertical ridges in the outside of the lens. This gives the light its characteristic appearance. These ‘ridges’ work to spread the light along the horizontal axis. In general, this spread is designed with a 30 to 50 degree area of maximum intensity. As you pass beyond this ‘sweet spot’, the intensity of the signal drops precipitously. The signal is still visible but the effectiveness of the warning light is significantly diminished. What you end up with is a very powerful signal spread out in a narrow band facing the front of the light with poor off axis visibility. Depending on your application, a TIR style light may be just the ticket. But if you’re wanting high performance off angle, then you need to consider a product with linear style optics.
A warning light with linear optics produces a very wide angle warning signal and can easily be seen from almost any point where the face of the light is visible. This is achieved by surrounding all of the LEDs in the lighthead with an optical reflector that captures the scattered light from all of the LEDs rather than from each LED as in the TIR design. In addition, there is usually a collimator placed over the LEDs to align the light emitted from each LED. A collimator is a device that narrows a beam of particles or waves. To “narrow” can mean either to cause the directions of motion to become more aligned in a specific direction.1 When you look at a lighthead with linear optics, it will appear as a smooth lens on the surface with an oval shaped reflector around all of the available LEDs. So what you end up with is a design whereby the light is aligned to focus on a narrow horizontal plane at a very wide angle. Some linear optics can achieve more than 180 degrees of spread, making them very effective in off angle applications. This is their strong suit. The downside is that the lighthead is spreading it’s energy over a wider area than that of the TIR. So this means the strength of the warning signal’s appearance will be somewhat lower than a TIR lighthead with an equal number of the exact same LEDs.
What does this mean to you? It means that just like a mechanic or a carpenter will use the right tool for the job, so should you use the right light for your application. So before you buy that blinky light that looks cool online, consider what you want the light to do? How will it be used? Are you wanting a piercing high strength signal facing the front of your vehicle to help move traffic and get attention while running code? Or are you in need of a warning light that will cover a wide area for intersection warning or for providing coverage in the rear when your police car is parked at an angle during a traffic stop? All of these factors need to be taken into consideration when choosing the best warning lighting for your vehicle. Whether it’s a set of grille lights or a rooftop lightbar.
And that’s where we come in. Our job is to help you create an optimal and complete warning system that will provide a high level of conspicuity and safety for you and for those around you. Our staff here at Responder Public Safety Equipment are highly trained and eager to help you design a system that will provide the highest level of effectiveness within your available budget. Please give us a call before you’re ready to outfit your next vehicle and let us show you how we can help.