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Description
A LED (Light Emitting Diode) based directional lighting has been designed to indicate the best evacuation direction for applications like the LHC tunnel. The design includes constraints for redundancy required by safety systems and for components selection by radiation effects. Prototype lighting units were irradiated in CERN’s CHARM facility and were operational up to a Total Integrated Dose (TID) of 870 Gy. This paper describes the basic design and the irradiation effects.
Summary
Directional lighting that guide the personnel present in confined spaces is used in some safety systems, but this type of equipment is not qualified to operate in a radiation environment. The use of directional lighting was investigated during the tests performed in 2014 simulating an accidental liquid helium release into the 27 km circumference LHC tunnel that lies about 100 m below ground and has 9 access shafts. The LHC safety system provides, in its present status, excellent protection to the personnel present in the tunnel although a directional lightning system would be a useful addition to avoid personnel going initially towards the wrong exit shaft when an evacuation alarm is present.
The LHC tunnel curvature imposes a maximum distance between directional lights of 320 m in order to indicate the personnel whether to go to the left or right in case of an evacuation procedure. The light design shall be visible from up to at least the line of sight while limiting the amount of glare to which the personnel is exposed when working in close proximity.
The light emitting elements are red and green LEDs XLamp® XP-E2 model manufactured by Cree®. The LED light is concentrated by using a lens model IRIS-M manufactured by Ledil made of PMMA material and with a viewing angle of 29o.
At the nominal LED current of 350 mA, the red and green LEDs produce an approximate luminous flux of respectively 60 lm and 100 lm. These luminous flux through the IRIS lens would be far too bright resulting in glare that exceed the upper vision tolerance threshold (about 300’000 cd/m2) and may present a hazard to the personnel working in close proximity to the light source.
To address glare, the XLamp LEDs are used with a very strong derating (current of 5 mA and 2.5 mA respectively for the red and green LED) and by using per color up to five Ledil IRIS lenses in parallel to increase the light emittance area to about 57 cm2.
The relatively low currents permit easily to design a robust radiation scheme for which both the dc power supply and the safety control equipment stays in a radiation-protected area and only the passive components (resistors and diodes) are located in the hazardous area.
Per color, a cluster of ten LEDs and its associated IRIS lens was irradiated in the CHARM test facility. During the irradiation, five samples for each color were kept respectively ON and OFF; and their luminance was measured during the six CHARM technical stops. The total accumulated dose was 870 Gy and no failure occurred. No darkening was observed on the PMMA material of the lenses.
The luminance of the red LEDs decreased by about 80% after the irradiation, however from a physiological point of view the luminance stayed constant as the human eye compensated this variation. The luminance of the green LEDs decreased by less than 40%.
