Comparison of traditional and semiconductor traffic light sources in signalling


Our product is made from semiconductor light emitting diodes (shortly LEDs), it is moulded with synthetic resin and put into a metal housing. Proper light dispersion into different directions is set by directing each LED individually to achieve an evenly lighted surface.

(There is no lens, colour filter, light bulb, or bulb-holder.)

In traditional traffic light sources and in other traffic signals incandescent lamps (light bulbs) generate the light, which is directed by lenses or mirrors and the right colour is produced by colour filters.

Glass fibre is not widely spread (though it has been available since the 1990s) because it is expensive and the only advantage of it is that the light bulb can be put within easy rich (lower part of the mast, far from high-voltage wires).

Fluorescent lamps at the same power and even of the same size are completely new and can be proper substitutes for light bulbs because of their efficiency and much longer lifespan.

Our experience (on which the comparison was based) was gathered in Hungary during a five-year period of 1996-2001, but this experience is easily adaptable to other European countries because both the law of these countries and the light sources are conformed to international conventions.

Users required fewer types of different sizes than in case of a traditional light source (light bulb), so currently the three most frequent sizes are:

The most commonly manufactured and installed light source is the type ’LSzR-150-xx’ (trade name), with Ø 150 mm nominal diameter.

The comparison is made upon the users’ and operators’ experience and demands.


The scope of comparison

It mainly contains light-sources of 0–200­W power because these are the most common ones. Traffic control (both at public roads and at railways) uses 20–75W light bulbs for signalling.


Aspects of the comparison:

1.        Standards - legal aspect

2.        Reliability - technical, operating aspect

3.        Fitting to the current devices - designing and operating aspect

4.        Visibility - user’s aspect

5.        Economy



1.        Standards

(According to the Hungarian Law. As mentioned earlier it is almost the same in other countries.)

The Hungarian Standard nominates and determines five optical quantities, such as

The required brightness is easily obtained, usually all of the light sources can provide it. In case of an incandescent lamp even if it is older than three–four months and in flashing mode, brightness can decrease to its one-third, which is not enough as the minimum brightness must be the half of the maximum.

Brightness of a LED light source decreases by 25% after 100,000 hours (more than 10 years).

Light dispersion of lens or mirror systems can be hardly kept (even in the case of the narrow wave band radiators) as filament positioning should be done after every single light bulb change which is practically nearly impossible, so it is not done.

Direction of the LED light source does not change, because it is moulded in synthetic resin.

The colour of a light bulb changes a lot as it gets older. A light source with a yellow filter can become pink.

The colour of a LED is unchanging because it is determined by the structure of the crystal. The currently available LEDs are of every standard colour (red, yellow, green, blue and moon white), meeting the C.I.E. standards.

The unevenness of the light depends on how successful the positioning of the filament into the focus of the mirror or the lens was. Like in the case of light dispersion it is not correct after changing the light bulb.

Dispersion of the LEDs is unchanging.

The so called phantom light is dramatically strong particularly in the mirror systems. (The phantom light is the light that is reflected by the device.) The mirror and filter systems reflect sunshine in the very same colour as of the signal, that is why it is not perceptible whether it is its own light or the reflected sunshine.

The LED light source has practically no phantom light, the little reflected light is colourless, so it can not be mixed up with the signal. In Hungary this is the only light source that completely meets the standard.


2.        Reliability

In traffic control and in signalling systems long-life light bulbs are used, their lifespan is 3,000-4,000 hours. The lifespan in flashing mode is not given because it has such a bad value.

In practice the light bulbs that work in flashing mode must be changed every three months.

Life span of a LED is unlimited, it only has probability of failure. A LED is much more reliable than a light bulb. Probability of failure is not influenced by flashing mode. The light source consists of approx. 150 LEDs, arranged in mixed parallel and cascade connection, thus the chance of falling out of the allowed 50% can be measured in astronomical figures. Failure of a lamp referred to the LEDs is expected after a thousand million years! So the failure of a lamp is generated by the different parts like cables, plugs, etc. In a five-year period (1996-2001) of installation of approx. 700 pieces of our lamps at the MÁV (Hungarian Railway Company) not a single lamp has failed. Four of the lamps had to be replaced, one of them served as a target of a PA pistol shot (only the front glass got broken), the light source worked very well as the resin jammed the bullet which was found at the regular check. Two lamps were hit by lorries, one of them remained undamaged – just a little cleaning was needed – the front glass of the other one got broken. The fourth lamp was smashed by a stone but only the front glass needed a change. Front glass resists air rifle shot. (It is tested by vandals!)


3.        Fitting to the current devices

Traditional optics and light bulb can be replaced by LSzR 150 (name of the LED lamp) in about 8 minutes as it is fixed with four bolts. Voltage of light bulbs are applied which means both direct and alternating 12V, 24V and 35V in Hungary. In case of alternative requirements devices of other voltages can be produced. Input current is lower than that of a light bulb, so in some cases the coil of safety sensor has to be changed but no more change is needed. Driver circuits are disturbed by enormous turn-on current of the light bulb. At LED lamps this ‘kick’ of the turn-on current never occurs, thus driver electronics can be cheaper and more reliable.


4.        Visibility and perceptibility

Visibility compared with traditional light sources is much better even at the same brightness. The reason of that is LEDs emit quite pure colours which are particularly remarkable for the human eye and these colours – unlike of the traditional light sources - does not fade away with the distance and dyschromatope people do not mix pure colours up.

According to amazing statistics of two Hungarian lines there was not a single accident in those level crossings where these type of warning lights were installed. Looking back to the three-year period before five human life had been gone and 200 million HUF (about 800,000 euro) damage had been done in these crossings. (Data are from the MÁV.)


5.        Economy

Statistical data from the previous paragraph refer to this one too, but some more data are available.

A railroad signal or warning light can be placed several kilometres farer without intermediate switching unit which means almost half a million HUF (about 2,000 euro) saving.

With installed LED lamps at ten level crossing 4 million HUF (about 16,000 euro) can be saved annually by eliminating the light bulb changes.