Alliance for Lighting Information


There is More to Lighting Quality Than What Meets the Eye

by David M. Keith

The rising interest in quality outdoor lighting can bring many benefits to a community. Nighttime outdoor activities are - with some notable exceptions - improved by outdoor lighting, and in some cases made possible at all. With appropriate lighting comes a sense of personal safety and security, as nighttime outdoor activities contribute to community building and people's enjoyment of their opportunities. While the desire for outdoor lighting has grown, so has the need to regulate it, so a community doesn't have to put up with unacceptable solutions to the need for outdoor lighting. It is reasonable to expect people to be good neighbors, and this includes controlling their outdoor lighting appropriately.

With regulations come restrictions that either solve "the problem" or do not. Without a doubt and in every case there are also unintended consequences and additional costs created. The best way to evaluate these is to clearly define "the problem" and the intended solution(s). So what is quality lighting and how do we know when we have it?

It would seem this is a question that can be answered only by looking at the lighting produced by a lighting system - and this is one of the most important ways to evaluate lighting quality. But eyes are very individual, and situations change, and so the industry has defined some metrics that help keep evaluations even.

The first impression about outdoor lighting is usually the overall brightness. The amount of light is typically measured in illuminance, and - when lighting the same objects - as illuminance is increased brightness does too. But not at the same rate! Since our eyes are so adaptable, they only report an increase in brightness when the illuminance is doubled or tripled. This means that illuminance values can be confusing. One of the reasons is that our visual system does not see illuminance, the light incident on a surface. Our visual system responds to light reflected from surfaces, which is luminance, the measure of 'brightness', produced by the combination of the light onto a surface and the surface's reflectance. In this way, surfaces' reflectances directly effect the perception of the quantity of light, without any change in the actual quantity provided by the lighting system. There needs to be "enough" light or the lighting quality is always poor. How much is "enough" depends on the situation and circumstances, because lighting design is always contextual.

Most lighting systems are designed using horizontal illuminance as the criterion for lighting quantity. However this metric does not address all the issues that promote a feeling of safety and security. A lighting system's ability to provide vertical illuminance is very important. Without adequate vertical illuminance facial recognition is difficult and the sense of safety and security deteriorates.

This tells us that merely dumping the light out onto the ground is not enough - how it is spread around is the next significant aspect of lighting quality. Too much light in some spots or not enough in others reduces visibility in those locations. The metric for uniformity is defined as a ratio between either the average or the highest light level compared to the lowest. This means that lower uniformity ratios are better than high ones. While our eyes are very adaptable, good uniformity helps maintain visibility over the entire lighted area, and this is another important aspect of lighting quality.

After quantity and uniformity issues, the other important visual aspects of lighting quality are more qualitative themselves. The first is glare - the presence of a bright spot so intense it makes seeing other things more difficult. Glare makes the eye contract, so a reasonably bright scene appears too dark. Of course the perception of glare is as individual as people, and in particular varies with age. Along with many other changes, as they age our eyes become less tolerant of glare. Glare can be especially difficult in outdoor lighting because the sensation of glare is dependent not only on the brightness of the source but the darkness of its visual surroundings. This is demonstrated by the difference in the glare from automobile headlights seen in the daylight or the nighttime. Since outdoor lighting equipment is seen so often against a dark background, glare presents more of a problem. Controlling glare is one of the most important ways to insure lighting quality for outdoor lighting.

The metric used in North America for glare in roadway situations is veiling luminance, which relates the brightness of each light source to the brightness of the roadway, which is assumed to be the field of view. Veiling luminance is determined for the worst glare situation along the road and compared to the roadway's brightness to insure that the driver's visibility is not significantly impaired. This metric works reasonably well for roadway situations. Unfortunately no similar metric for glare has yet been established for other nighttime situations.

The other visual aspect of lighting quality is color rendering - the "trueness" of the colored surfaces. Color is a feature of light that may be critical in some cases and less important in others. For this issue, the quality of the lighting depends on the intent of the lighting. When appearance is important and when color recognition or appreciation matters, then the color aspects of the lighting become very significant.

When there is enough light reasonably distributed and controlled, the visibility within the area is sufficient, and the sense of lighting quality is high. As the lighting is further refined, the quality can be improved.

However, the visual aspects of a lighting system are not the only elements needed to make quality lighting. Simply evaluating lighting quality based on the visual presentation is like choosing a car solely on its appearance and a test drive. What about the sticker? What is the cost for what we are getting?

For lighting systems, the real overall price includes initial, operation and maintenance costs, just like a car. The initial costs of a lighting system include not only the equipment but also getting it installed properly - which is typically as expensive as the equipment itself. And of course a car needs fuel - in the case of lighting, the system needs electricity. The on-going cost of energy will usually be greater than the initial cost of the equipment and its installation before the time the equipment is replaced. Certainly when maintenance is considered, the initial costs are only a small part of the total over a handful of years.

Maintenance of outdoor lighting can be difficult and expensive, and poor or neglected maintenance can destroy the quality of the lighting. From replacing burned out lamps to cleaning dirty luminaires to avoiding using "a different lamp that works OK", maintenance plays a critical role in the visual aspects of quality lighting. Over the life of a lighting system, maintenance costs alone can exceed the initial equipment costs.

All of this is similar to a car - gasoline and oil and regular maintenance are expected parts of keeping a car going. Repairs are of course extra - for both cars and lighting systems.

There is a familiar way to take all this into account when evaluating quality - what is the cost? Not just the initial purchase, but the entire lifecycle of the entire system. Not just the components but the entire system with its energy use and replacement lamps must be considered. By reducing an entire system's initial and on-going costs to money, we make it easy to compare. Evaluating how much money each option takes makes the cost of any evident difference in quality clear - this much more for A instead of B. But this only works when everything involved in the system's purchase, installation and operation can be reduced to money, and even then the nature of money can be a problem.

Money can reflect all of the identifiable costs associated with lighting but fails entirely with some other consequences. There is no value clearly associated with light pollution, and it is difficult to put a price on the pollution associated with using electricity. The costs of filling landfills with mercury-tainted material cannot be defined in monetary terms. These are real but "hidden" costs associated with lighting. Sometimes money is not a sufficient metric by itself.

Also money can be elastic - prices vary and so does value. Electricity rates vary from place to place and time to time. A lighting system can have high costs based on ornamental aspects and produce low quality lighting, or have a great initial quality that degrades when maintenance costs are not paid. Money can be very slippery for making comparisons because of how many different variables are involved.

For streetlighting, a more practical metric of the system is the spacing between identical pieces of equipment. Poles with luminaires are spaced in a regular pattern, and each has a complete set of costs associated with it. When the poles are placed further apart, the costs go down. The relationship is very simple - each increase in spacing corresponds exactly to an inverse reduction in most of the costs. If every tenth pole is removed, most costs go down by 10/9 or 11% - almost every cost, including the "hidden" ones (but the wiring still needs to cover the entire project.)

However, spacing as a metric is not enough, because it doesn't include such aspects of a lighting system as the electrical components. Furthermore, the relationship between spacing and costs is clear but difficult to work with, because it is an inverse relationship.

Another metric for lighting systems is Unit Power Density, abbreviated as UPD. As a metric, UPD is similar to spacing, in that it changes as the distance between equipment changes - but UPD also reflects the performance of the electrical components. UPD is like money, in that it reflects the entire system - not just the equipment or the electricity use. And within certain limits, UPD is directly proportional to the changes in the costs of the lighting system. For similar type and wattage luminaires, as UPD increases, so does the quantity of lights, light bulbs, poles and arms, foundations, and even maintenance calls. Changes in UPD are always equal to changes in the resulting energy use and associated pollutions.

Unit Power Density is a simple metric to calculate - it is just the power used by the lighting system divided by the lighted area. The units of UPD are Watts per square foot. Unlike money, the calculation of UPD does not depend on factors outside the control of the lighting designer - it is entirely based on the characteristics of the lighting system. And UPD is not affected by fancy details or future price fluctuations.

For these reasons and more, UPD is also used as a metric in energy codes and lighting ordinances across the world. It is the equivalent of gas mileage data for car buyers.

The usefulness of UPD is shown by its flexibility and proportionality. The simple practical aspect of its direct proportionality with initial equipment, installation costs, energy use, maintenance needs (to some extent) and associated consequences of outdoor lighting make UPD a very robust metric for evaluating lighting systems. UPD does not have the extreme flexibility of money and UPD is not appropriate for comparing very different systems. The more similar the systems, the more the comparisons between UPD's are valid. And comparisons are the key - absolute UPD values work for energy use restrictions, but for most cases it is the percentage difference in UPD that describes the overall cost difference between lighting systems - including such intangibles as ambient light pollution or even mercury into landfills.

UPD does not reflect all the costs of a lighting system either. Since different light sources have different hours for their rated life, the frequency of maintenance can differ substantially. UPD does not reflect such differences in maintenance costs associated with different sources. UPD does not tell the entire story by itself, but comparisons of UPD's for systems using different sources to light the same area can be very informative.

Research in roadway lighting indicates that lower UPDs can be produced by systems using several different techniques to refine the lighting system. One is to use more "effective" photometric distributions - spread the light that is being installed over the area effectively. This does not necessarily mean that the luminaire is "high efficiency" but that the system is. A bare bulb is extremely "efficient" - all the light gets out of the "luminaire" - but it probably does not make for an effective system.

This example of "luminaire efficiency" vs "system efficiency" is a clear demonstration of the need to use metrics that encompass the entire system and not just components. Component metrics can be extremely misleading about system performance.

Using lower wattage lamps is another way to reduce UPD. Smaller light fixtures can be more closely matched to the lighting task and area, so the system can be more effective. However, since UPD does not reflect the numbers of luminaires in each system, comparing UPD between systems with different wattage lamps may not indicate the difference in maintenance needed. Along with a better fit to the job, lower wattage lamps correspond to more maintenance points, and so more maintenance costs. In every case, differences in systems' UPD values indicate relative differences in electricity use and installed lumens.

Better maintenance is itself another way to reduce UPD. During the design of lighting systems, the anticipated maintenance program is used to establish the "worst case scenario" for the lighting system - how bad will it get just before it gets spruced up. The longer each lamp is left burning, the lower its lumen output - and the dirtier the luminaire from which those lumens are trying to escape. If the designer can be confident of better maintenance, then the system can be based on better lamp and luminaire performance at the "worst case scenario", and so there will be less equipment and lower UPD. Of course maintaining the initial lighting quality requires that the promised maintenance actually occurs.

Finally, research shows that as UPD is decreased, generally the amount of light contributing to ambient light pollution decreases. The metric used for evaluating ambient light pollution is Unit Uplight Density (UUD) - the amount of light escaping upward divided by the lighted area. The units of UUD are lumens per square foot. The relationship between UUD and ambient light pollution can be very complicated, but when "all else is held equal", as UUD changes ambient light pollution changes in direct proportion. This relationship between UPD and UUD allows for evaluating one of the "hidden costs" of outdoor lighting. In general, changes in UPD produce similar changes in UUD, but not exactly proportional. For very similar systems, when UPD is decreased, UUD goes down by about the same percentage.

And while UPD does not exactly reflect a lighting system's contribution of mercury - in the form of used light bulbs - to landfills, it has some proportionality. Since different light bulbs have different amounts of mercury in them, UPD comparisons do not entirely reflect the difference between different lamp types or wattages. However for the same lamps or source types, a change in UPD corresponds to a change in the mercury content of the system's used lamps.

This is similar to the relationship between gas mileage and air pollution - it's not a direct connection but it's there. And it's no surprise that both are dependent on maintenance.

So when evaluating lighting quality, there are issues about what meets the eye, and ways to measure them. And there are issues beyond what meets the eye - and metrics for measuring them too. Just like a smart car shopper wants to kick the tires, go for a test drive and also consider the sticker and the gas mileage, anyone who wants to evaluate lighting quality has to be aware of what's in plain view and also what is not.


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