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Introduction
Daylight is despite its ever-changing nature, also used as the reference for all other types of lighting. When there is insufficient or no daylight, we have to rely on alternatives to illuminate our world. As far back as 1417, lanterns with candles inside were used on the streets of London during winter nights. The subsequent invention of gas, oil and finally electric street lighting helped bring urban areas out of the dark ages. Today, billions of people take street lighting for granted.
Historical context of yellow light
High-pressure sodium lamps have often been first choice in street lighting for a number of years, basically because they produce high levels of illumination for a given amount of energy and have a long, reliable lifespan. However, their distinctive yellow/orange light makes it difficult to distinguish colours. The unnatural tint of our urban streets at night time is something we have mainly got used to through necessity rather than choice.
High Pressure Sodium Lamps [1]
As the name implies, high pressure sodium lamps are based on a high pressure sodium discharge. The lamps emit yellow-white light. The correlated colour temperature is fixed at ~2000K. While the colour rendering properties of HPS lamps are better than low pressure sodium lamps, they are still quite poor ( Colour Rendering Index (CRI [2]) ~25).
The changing role of outdoor lighting
The role of outdoor (street) lighting has evolved over the years. Its function in the 1930s was to make driving safer. Three decades later it also provided visual comfort for motorists. By the 1980s street lighting had an additional role; increasing pedestrians’ feeling of safety. Nowadays, lighting is often central in helping create more liveable and inviting streets with a better ambience. Today, outdoor lighting solutions no longer have to rely only on yellow light like High Pressure Sodium. There is an alternative which is much more successful in combining the qualities of daylight with the energy efficiency associated with high-pressure sodium. This alternative is high quality white light.
Ceramic Metal Halide Lamps [3]
The burner of ceramic metal halide lamps is made from ceramic instead of quartz. As a result, compounds can be added to the high pressure mercury discharge that cannot be used in quartz lamps since they would attack the quartz . Consequently, the colour properties, colour stability and luminous efficacy of the lamps can be increased further. Furthermore, ceramic metal halide lamps are compacter than their corresponding quartz metal halide analogues. As a result, ceramic metal halide lamps enable even better optical control for a given luminaire dimension. CosmoWhite is a new type of ceramic metal halide lamp with higher efficacy and lumen maintenance.
Benefits of High Quality White Light
White light offers many clear benefits when compared to yellow light. For a start, the ambience is perceived as being brighter and natural. Various tests have shown that a considerable majority of people find it preferable and more pleasant. ([4]) This greater clarity also gives a general feeling of improved security. Easier recognition of people’s faces and other details can act as a deterrent to crime, and also help generate sharper CCTV (Closed Circuit Television [5], for example security cameras) images. By increasing visibility for motorists, pedestrians and cyclists, the roads become safer as well. Research has shown that white light enables drivers to see movement at the roadside from a greater distance, giving them more time to brake. ([6]) And that’s not all. Modern white light sources have comparable or even better energy efficiency than high-pressure sodium lamps. In addition, the latest independent research shows that white light sources are visually more effective than yellow sources at the typical lighting levels used outdoors at night. That means you can dim lamps or even use lower wattage alternatives - both of which equate to lower energy consumption - while producing the same perceivable result. A CIE [7] technical committee is currently working on recommendations for white light, which takes the spectral sensitivity of the eye at low lighting levels into account. ([8])
The four main benefits of white light include:
- Aesthetic enhancement
- Security
- Accident prevention
- Energy efficiency
Aesthetic enhancement
White light is not the only choice for lighting up the urban landscape at night. There are also other options available, and invariably a combination of different lamp types is used. However, few - if any - offer the flexibility of white light. It is suitable for general ambience, floodlighting, illuminating facades and many other applications. It is equally effective complementing modern construction materials like glass, steel and polished stone as it is highlighting classical structures. It even gives (landscaped) greenery a healthy, verdant look.
Yet it is also perfect for functional lighting of streets and communal areas, producing a natural ambience that is generally preferred by many to the traditional yellow glow of sodium; almost 90% of respondents in one recent survey perceived it as being more authentic. (1)
And when taking other benefits into consideration, such as improved safety, heightened feeling of security among pedestrians and low energy use, it becomes clear that using white light is the undisputed choice for making the streets more liveable and enjoyable.
Security
White light is crucial in helping people feel safer when being outdoors at night. Some recent investigations indicate that facial recognition is easier, even from a distance, which goes a long way towards removing anxiety. ([9])
White light creates what you could call a ‘virtuous circle’. If city dwellers feel safer about their streets, they will use them more often. Having greater numbers of people walking, cycling and playing makes the streets welcoming and less desolate, and should discourage acts of vandalism and crime.
Safe Movement
White light improves safe movement at low light levels. To enable safe movement and reduce the chance of tripping when pedestrians are walking outdoors at night, it is important that they detect obstacles and other irregularities in the pavement. Recent experiments by Fotios and Cheal have shown tha t at low light levels, the ability to detect obstacles is improved under white ceramic metal halide sources compared to yellowish high pressure sodium sources. ([10])
Accident prevention
Improved visibility is a major contributor to road safety. Tests have shown that drivers can detect movement at the roadside faster and from a greater distance with white light (can link again to Driver test at LRC. Crucially, this can give them more time to stop if a child, adult, cyclist or animal is about to cross their path, or if another car approaches unexpectedly. ([11])
Energy efficiency
In the past, one of the main justifications for using high-pressure sodium lighting was energy efficiency. However, ongoing performance improvements mean the latest white light sources (compact ceramic metal halide) are more energy efficient than their sodium counterparts. These sources are therefore the sustainable solution for outdoor installations, allowing you to realise significant savings in a number of ways.
For instance, it’s possible to specify a greater distance between luminaires in new installations, reduce the mounting height during refurbishment, or install lower wattage lamps in upgrades. The result is reduced running costs – and CO2 emissions at a level lower than was previously considered possible – as well as superior light quality.
The distinct advantage of white light is its proven higher perceived brightness. ([12]) Because white light is experienced as being brighter than yellow light at low light levels, it becomes feasible to actually reduce light output while still giving people what they expect. The savings associated with this are enormous. ([13])
This advantage has already been recognised in British lighting standards. In the UK, the level of illumination required by law on subsidiary roads and paths may be reduced by as much as 30% when the light source used has a colour rendering of 60 or more, which is the case with white light (but not with high-pressure sodium). ([14])
