Hazard lights on Life jackets are designed to make people who have fallen overboard visible at night. But how easily can they be spotted under realistic conditions? Tests carried out in the North Sea show that a higher measured luminous intensity does not necessarily mean that a light at sea will be spotted sooner.
A small light with a big job to do
Anyone who falls overboard at night is difficult to spot even from a short distance away, amidst the waves and spray. Alongside high-visibility colours, reflective strips and a signal whistle, a flashing light is therefore an essential feature of a lifejacket. Its purpose is to alert search and rescue teams to the presence of a person adrift in the water. Lifejackets are not usually fitted with such lights as standard; customers can retrofit them and have a choice of different models, ranging from the classic incandescent bulb and individual LED lights to a light that illuminates the entire buoyancy chamber of the lifejacket from the inside.
But how far can such a lifejacket light actually be seen under realistic conditions? The Federal Maritime and Hydrographic Agency (BSH) has investigated this question in collaboration with the German Maritime Search and Rescue Service (DGzRS). The test compared approved Position-Indicating Lights (PILs) using two different light sources: a modern LED and a traditional incandescent bulb. Specific manufacturers and models of lifejackets and lights are not named in the test report.
In the laboratory, the LED is superior
First, the researchers measured the luminous intensity of both models. As expected, the LED light came out on top: at room temperature, it achieved an effective luminous intensity of 1.71 candelas, whilst the incandescent lamp model reached 1.27 candelas. Even at temperatures of six and 14 degrees – corresponding to the water temperatures during the experiments – the LED remained brighter.
Both lights thus exceeded the minimum luminous intensity of 0.75 candela required for approved lights on life jackets. According to the standard calculation models, the LED should therefore have been visible from a greater distance in all the tests.
Comparative tests carried out on land confirmed this expectation: there, the LED appeared brighter and was visible from a greater distance than the incandescent light.
A surprise on the North Sea
At sea, however, the situation was different. The two types of beacon were mounted on freely drifting test objects and fitted with AIS transponders. Vessels from the BSH and the DGzRS then approached the targets at night in a search formation. Several observers recorded the distance at which they were first able to make out the flashing lights – with the naked eye or, in some trials, using binoculars.
The surprising result: In three out of four trials, the incandescent lamp model – which is actually less bright – was spotted from a greater distance than the LED. Only in one trial did the order correspond to the laboratory measurements and the observations made on land.
In one of the tests, both lights were 445 metres away from the observers. In the photographs taken during the test, the light from the incandescent bulb appears larger and slightly brighter than that from the LED.
Do waves and spray dim the LED light?
There is as yet no definitive explanation for the effect. The researchers suspect that the different spectral compositions of the two light sources may play a role.
White LEDs and incandescent bulbs produce light in different ways. Consequently, the wavelengths that make up their light differ. Directly above the water’s surface – as on life jackets – moisture, spray, foam and waves could scatter or attenuate certain components of the light more than others. As a result, the LED light could become less visible, even though it achieves a higher luminous intensity in the laboratory.
The fact that this effect was not observed in the onshore trials supports this assumption. Furthermore, the only offshore test in which the LED was first sighted took place at a different location, closer to the coast, and under different conditions. However, this does not prove the hypothesis.
No evidence of a general weakness in LED lights
It cannot be concluded from the study that LED life jacket lights are, as a rule, more difficult to see. The sample size of four trials at sea is too small for this conclusion. Furthermore, the comparison was limited to two specific types of light. LEDs can vary considerably in terms of, amongst other things, their light colour, spectral composition and beam pattern.
The results therefore neither call into question the approval of LED lights nor do they justify a blanket recommendation in favour of older incandescent lamp models. Rather, they indicate that the luminous intensity measured in the laboratory alone may not be sufficient to assess the practical visibility of a life jacket light at sea.
Further trials are intended to provide clarity
The BSH intends to continue the investigations in collaboration with the DGzRS. Plans include trials with light sources of varying spectral composition, as well as measurements under changing weather and sea state conditions. Narrow-band light sources and spectrometers are to be used to help detect any possible wavelength-dependent attenuation immediately above the water’s surface.
The test is part of a larger Research collaboration between BSH and DGzRS to improve search and rescue at sea.
Ursula Meer
Redakteurin Panorama und Reise