Reduce the load

With increasing demands for higher energy efficiency, lower cost of ownership and better quality lighting, it’s becoming increasingly important to look at all the options when selecting light sources. Very often, these will be options that have only recently become available, thanks to new technologies.

A case in point is the choice of sources for low and high bay lighting – typically high intensity discharge (HID) lamps such as metal halide and high pressure sodium. Here, there are often established design traditions that have been used to determine the choice of light source for many years. However, as light sources change, the design options need to change with them if the end user is to get maximum value.
And it’s certainly true that light source technologies have been developing rapidly in recent years. Not only are there new sources, such as LEDs, but there have also been significant advances in the underlying technologies of established light sources, such as metal halide.
For example, improvements in the design of the lamp, combined with improvements to control gear, can now enable the use of a lower wattage lamp without compromising on light output. Thus, there are 200W metal halide lamps that can directly replace older style 250W metal halide lamps to save 66 circuit watts, or 28% energy consumption per fitting (including control gear losses, which are discussed in more detail below). Similarly, newer 350W metal halide lamps can replace 400W metal halide to save 13% for each fitting. In each case, a comparable light output is maintained.
Furthermore, when metal halide replaces another type of light source, the savings can be even greater. When an energy saving 350W metal halide lamp replaces a 400W high pressure sodium lamp, energy consumption is reduced by 16% per fitting. If 400W mercury vapour lamps are being upgraded to metal halide, it only requires a 200W metal halide lamp to give the same light output – resulting in a massive 49.6% saving.
Staying in control
Of course, substantially higher savings can be achieved when efficient light sources are combined with effective control. Traditionally, HID light sources have not benefited as much from this, simply because their control has been limited to switching on and off. Again, though, newer technologies have changed the situation.
For example, electronic ballasts designed specifically for HID light sources now allow metal halide lamps to be dimmed as and when necessary. Consequently, they can now be combined with photocells for daylight linking and even used with occupancy detectors. In the latter case, because of the re-strike time of all HID light sources, the lamps are dimmed to minimum when a space is unoccupied rather than being switched off.
Cutting your losses
When comparing lamp wattages most people think in terms of the ‘headline’ wattage and don’t take factors such as the efficiency of the control gear into account. However, these losses can vary considerably so it’s important to see the full picture. A ballast for a 400W metal halide lamp, for example, can add anything from 30W to 85W to the circuit.
Very often the control gear choice is driven by price but the efficiency of a ballast is directly related to the materials used in its manufacture. With steel and copper prices rising rapidly, the only way to keep the cost down is to skimp on the materials – which in turn impacts on efficiency. As a result, you could save 20-30p on a ballast and add 55W to each light fitting – making this quite a costly ‘cheap’ option.
When the lamp performance is taken into account as well, the contrast is even more dramatic because a higher performance lamp can often make it possible to achieve the same light output with a lower wattage.
Consider a theoretical example, where a light output of 35,000 lumens is sufficient to provide the required light levels. One option is to buy the cheapest products, which will require a 400W lamp using lower performance control gear to give a total of 485W per light source. At the other end of the spectrum, 35,000 lumens can be achieved by using a high efficiency 350W lamp with efficient control gear that gives a total of around 375W per lamp circuit. So, in this worst case versus best case scenario, that’s a total saving of 110W per lamp for the same light output!
And, on top of all these considerations, there’s the life of the lamp and control gear to consider as these also have an impact on cost of ownership. Higher quality lamps will last longer, and better control gear will assist them in doing that through integral protection features. As a result, re-lamping cycles can be extended so that maintenance costs are reduced, and there are fewer lamps to dispose of through specialist contractors, in compliance with the WEEE Directive and Hazardous Waste regulations.
This longer life is the result of the same high level of engineering that helps to improve efficiency. This is particularly true of metal halide lamps because they are among the most complex of light sources available.
For example, very high temperatures and pressures are generated inside the lamp, so the integrity of the chamber that surrounds the electrode, and the bulb that encompasses the entire structure, are very important. Not only do they have to be robust, they should also be very precisely shaped to control the discharge cycle within the lamp.

Higher quality metal halide lamps are manufactured using a quartz sculpting process, so that the shape of the arc chamber is contoured to follow the shape of the arc. It’s also important that the arc chamber created by this process has a uniform wall thickness, as this gives a uniform temperature at all points on the arc chamber wall. The result is higher light output, better colour uniformity and greater reliability.
Because of the pressures generated within higher wattage HID lamps there is usually a requirement for the luminaire to incorporate safety glass, so that if the arc tube ruptures its components are contained within the luminaire. An alternative is to use a metal halide lamp that incorporates a protective shroud around the arc tube, eliminating the need for safety glass in the luminaire. Not only is this solution more cost-effective, it also makes it easier to access the lamp for maintenance and eliminates the build up of dirt and dust on the luminaire glass, proving to be a very popular option in a wide range of applications.
Attention to detail
As it becomes more challenging to meet the various needs imposed on lighting installations it also becomes more important to look at the whole picture. The few examples given in this article serve to illustrate the fact that attention to detail in selecting light sources and control gear can make a significant difference to the overall performance of the installation.
As new technologies come on stream, those options are changing all the time, so it makes sense to keep up to speed with the latest developments from those lamp manufacturers that are breaking new ground in light source engineering.

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