Visual warning signals, thanks to the different colours and degrees of brightness, create a language code that allows communication between machines and operators.

The degree of brightness varies according to:

– the distance between the luminous point and the observer

– the type of lens

– the colour of the dome

The light intensity of Sirena warning signals is measured in Cd (p) in a photometric chamber. The Cd (p) represents the peak luminous intensity using a transparent dome that allows 100% brightness. The distance, the colour and type of dome used must therefore be taken into consideration when installing a visual signal.

For example, if the viewing distance is doubled, the light intensity observed is reduced by a quarter and if the distance is quadrupled the light intensity is reduced by a sixteenth; a reduction of the brightness also depends on the colour of the dome. (See the LIGHT TRANSMISSION table). The light output is amplified if a Fresnel lens is used.

International standards regarding visual signals state that the light output of warning signals must be five times greater than that of the surrounding light level and emergency signals must be ten times brighter.

In order to ensure that the correct beacon is selected the ambient light level must be determined (measured in lux) and is obtained by means of a luxometer.
The choice of light signal – on the basis of European standards – depends therefore on the lux measurement and Cd (p) of the beacon:


Example: 10.000 Cd = 10.000 LUX at 1 m = 100 LUX at 10 m

There are three different types of visual signals:

– Flashing Beacon – cyclic ON/OFF of a filament bulb with greater light up times and less light intensity.
The effectiveness of the signal is attributed to the illumination of the whole surface during the light up time with emission at 360°.

– Rotating Beacon – The parabolic mirror revolves around the bulb emitting an intense beam of light.
Each point of observation is illuminated only when the mirror rotates in its direction.

– Xenon beacon – cyclic flash of a discharge bulb powered by an electronic circuit.
Differing from a flashing beacon the xenon discharge has an extremely high peak intensity in a short light up time. Visibility at 360° is guaranteed and can also be amplified by using a Fresnel lens. (See drawing below: THE LIGHT).