Light Fitting Materials

Light Fitting Materials for external Applications: what to look out for…

Does stainless steel really stain less? Light fitting materials…

December 1st, 2014 Dr. Walter Wendel

1. Housing materials for commercial exterior light fittings

Most common light fitting materials for the housings and other hardware are stainless steel, aluminum, galvanized steel and sometimes plastic. And this is also the ranking from best to rarely acceptable. Bronze is rarely used these days because of the high cost and reserved for special applications.
Let’s start with stainless steel (S/S):

Does it really not stain? It depends – of course, because there is not one grade S/S but several. The most common are 304, 316 and 904.

WIBRE commercial stainless steel pool light
WIBRE commercial pool light 72W LED, available in 2 different stainless steel grades and e-polished. A choice of light fitting materials.

The 304 is the most used S/S in the entire building industry and is a cost effective entry grade which will tarnish (“tea staining”) under certain conditions like hot weather and presence of salt or chlorine. In most cases not a structural problem but a cosmetic one, but can lead also to leak problems for IP67/68 rated fittings.
The 316 grade is a better grade which in most cases will not develop tee staining. It’s also called “marine grade” S/S. Very suitable for exterior and underwater light fittings. In hot climates in presence of sea water it still can develop tea staining.
The 904 grade is a high alloyed material suitable for all applications in extreme weather e. g. hot and ocean water or underwater lights in saline environment.

Courtesy of Anzor Fasteners Ltd
An additional protection for any corrosion is a surface treatment called e-polish (electro-polish) for stainless steel. It enhances the appearance and improves corrosion resistance.
Aluminum for housings and hardware and other light fitting materials
Aluminum is used as one of the light fitting materials either as casted aluminum, extrusions or sheet metal. Many alloys are commonly used. The corrosion resistance is good because aluminum spontaneously forms a thin but effective oxide layer. Especially copper free alloys (<0.3% copper) show a very good corrosion resistance. Marine grade aluminum (5000/6000 series) are suitable even in applications where salt water is present. Chlorine water, e g from chlorinated pools can create pitting corrosion especially at higher temperatures. Galvanic corrosion will occur if aluminum is in contact with another metal which is nobler, for example brass or if in contact with stainless steel and chlorine water is present. The lighting manufacture HESS offers a special undercoating for the paint for locations close to the ocean.

Urban pole lighting from HESS, Germany
HESS City Elements, versatile light poles made from marine grade aluminum with optional additional undercoating for salty air applications.

Galvanized steel is only acceptable without additional protection if there is no salt water or salty air condition. Also acid rain will slowly start corrosion. The best protection gives the hot dipped galvanized steel with thick layers of zinc. This is a material typical for brackets. Special paint primer with additional protection is available.
Plastics are used for some cheaper underwater light fittings, not so much for commercial outdoor applications. There are many different plastics – often they have a problem with the UV resistance and tend to get brittle over time or loose surface material. In combination with elevated temperatures from lamps or control gear the degradation is accelerated. Chlorinated salt water will also affect common plastic housings over time.

2. Lens materials: glass, acrylic, polycarbonate

Basically 3 different lens material are used for light fitting lenses: glass, acrylic and polycarbonate
Toughened glass is the oldest material for lenses, the advantages are scratch resistant, optical neutral, and longevity with nearly no degradation. Thick glass panels have a high impact resistance, thin ones are delicate.
Plastic materials are Acrylic (Trade names e. g. “Perspex”, “Plexiglass” or generic PMMA) and Polycarbonate (Trade names “Lexan”, “Macrolon”). Unfortunately not all advantages are combined in one plastic material, see table below.

Lighting Element  for pole lighting for area and street lighting from HESS, Germany with XICATO LED's
HESS City Element Module made of cast aluminum with glass lens and Xicato LED

3. Gaskets, cables, cable glands, electronics, screws

These days’ silicone rubber gaskets are used, preferable with heat treatment to avoid evaporation. They have excellent heat, saltwater and UV resistance and stay flexible. For outdoor and underwater application good quality cable glands made from brass or stainless steel are preferred over plastic glands because of the achievable compression of the rubber gasket inside.

Cables for under water fitting should be special underwater cables. For the fitting construction it is important that the cable connection inside the fitting must be protected against water intrusion and also the effect of water wicking thru damaged cables and getting into the fitting must be intercepted.
With the availability of electronic control gear for HID and the necessary use for LED potted gear is preferred over un-potted components. The potting material will not only help with the even heat dissipation but also eliminate corrosion problems possible coming from condensation. At least the electronic boards should be protected with a conformal coating.
For underwater fittings an external control gear is necessary for pool and spa applications and preferred for all other underwater application. Safety and service are the reasons for that.
Critical parts are suitable fasteners like screws and springs. These should be made of 316 stainless steel and in the case of screws either Teflon coated to prevent seizing in the screw holes or special stainless steel anti-galling lubricant has to be applied.

4. Connectors

Some outdoor and most underwater fittings cannot be opened to connect a cable. In these cases either the fitting has to be ordered with the required cable length or watertight cable connectors have to be used. These inline cable connectors are available in IP67 and IP68 ratings and also available in versions to connect three cables if there is a need for a loop through to the next fitting.

WIBRE connectors IP67 and IP68.

The IP68 rated versions for permanent underwater usage have to be filled with silicone or special soft resin.
Installations with RGB fittings require multi-pole connectors, which again are also available for loop thru applications.
An alternative are bronze, aluminum or stainless steel junction boxes. A good solution are flush mounted to the ground types for easy accessibility of the connectors. The IP68 rated versions for permanent underwater usage have to be filled with silicone or special soft resin.

WIBRE ground flush mounting watertight junction boxes

At the end of the day it is like with a chain: the weakest link gives way. All light fitting materials used for an exterior light fitting must be from the appropriate quality – if only one part fails the entire fitting fails as a result.