|The introduction of high-power LEDs is causing nothing short of a revolution in professional lighting. New product generations are coming out every six months and new products are coming out from different vendors every day. New claims in efficiency, energy savings and longevity are made every month. People and organizations are calling for adjustments in lighting standards and legislation to accommodate this new technology.
With so much going on, LEDs are taking up all the bandwidth and it is hard to keep track of what’s true and what’s real. It is all too easy to forget about conventional technologies which are also improving, in some cases faster than ever. Lighting designers need to carefully evaluate available technologies and application requirements to make the best outdoor lighting choices.
This article was published in the February 2011 issue of LEDs Magazine. To read the full version of this article, please visit our magazine page, where you can download FREE electronic PDF versions of all issues of LEDs Magazine.
The information below forms an appendix of supplementary information to the main Magazine article.
Lighting professionals face a daunting challenge today in choosing the best light source for a specific application. LED-based solid-state lighting (SSL) is garnering headlines with energy-efficiency claims while great improvements have also come to HID and fluorescent sources in terms of efficacy.
The attributes and characteristics of the various light sources need to be understood before undertaking a comparison. Below, we discuss the attributes of different light sources in detail.
Making the right choice
How do you decide what it is the best light source for your application? It is important to look at the main strengths of the dominant lighting technologies so you can make an educated choice. First you need to focus on the characteristics that are most important for applying the light sources, rather than focusing on in-depth technical attributes. In the end, it is all about choosing the right technology, for the right application and for the right reasons.
Basically, LEDs consist of a combination of two semiconductor materials that are joined together. The interface between the two materials is termed the junction. When the right voltage is applied, one of these materials gets an excess of electrons while the other becomes electron deficient, i.e. it has holes where electrons are missing.
At the junction the excess electrons fall into the holes, thus generating light and heat. The amount and wavelength of the light is controlled by selecting and fine-tuning the materials. The most efficient and therefore most popular method of generating broad-spectrum white light for general illumination is by using blue LEDs in combination with a yellow phosphor. A few important characteristics of LED systems that influence their application are:
- Heat is the enemy of LEDs. LED efficacy and lifetime are strongly dependent on the junction temperature. The industry has become so good at building semiconductors with highly efficient junctions that we can generate over 200 lm/W white light and expect lifetimes well of over 100,000 hrs. However, this is under ideal conditions where heat plays no role. When we mount such LEDs in something resembling a useful light source, i.e. multiple LEDs with electrical connections and running under realistic conditions during many hours, these numbers reduce to around 100 lm/W for top-line LEDs and lifetimes of 50,000 hrs or less. All heat needs to be conducted away from the chip. Any mechanical construction around LEDs therefore has to be optimized for conducting heat out of the LED chip (heat sinking) and preventing outside heat from entering the chip (heat shielding).
- LEDs are tiny and generate directional light. Each LED has its own small optics which allows for very accurate beam control. Each LED chip by itself does not generate a great many lumens. They need to be mounted in an array to create higher lumen packages. As a result, in many applications we need to consider LEDs as a surface source. The required lumen package determines the size of the surface and thus the weight and wind drag.
- The lifetime and performance of LED lamps is further determined by how well they are driven electrically and how well they are controlled optically.
The combination of these factors makes the creation of a good LED engine or lamp a non-trivial task. It requires a combination of mechanical, optical and electrical expertise as well as knowledge of lighting. Failing in one of these specialties will strongly deteriorate the (lifetime) performance of the LED system. This is the main reason why many LED products do not live up to their promises of efficiency and reliability, as shown in the Caliper reports published by DOE.
Assuming that the above mentioned factors are all well taken care of in the product design, we see the following strengths and status of LED technology:
- Useful product lifetimes in the 50,000 hrs range. When applied in confined spaces, such as indoor down-lights or incandescent retrofit bulbs, lifetimes are typically 20,000-30,000 hrs. In large open spaces, e.g. as in outdoor applications, longer lifetimes can be achieved.
- Efficacies are at around 100 lm/W, with the bulk of LED products in the 50-80 lm/W range.
- LEDs excel in controllability such as dimming, instant-on, hot restrike, color variation.
In HID lamps, a plasma is created between two electrodes inside a discharge chamber filled with a gas mixture at moderately high pressures (up to several tens of times atmospheric pressure). The plasma generates intense light and heat. The properties of this light are manipulated by modifying the gas composition in the discharge. We will limit our discussion to white light systems.
A few important characteristics of HID systems that influence their application are:
- HID lamps deliver the highest lumens-per-volume, especially in the latest compact ceramic metal halide products. This allows for very compact luminaire designs and optimization of optical performance. The latest generations of compact metal halide lamps allow for substantially higher fixture efficiencies, wider pole spacing and sharp beam control to reduce spill light. The compactness of the light source also produces a sparkle effect similar to halogen lamps.
- HID lamps run at high temperatures. This allows them to shed much of their heat by radiation. It also means that gas pressures can rise high in the discharge chamber. As a result, high voltages (typically 10 kV or higher) are required for hot restrike. In order to start the lamps at lower voltages, they need to cool down for 10-15 minutes.
- Controllability of HID lamps is limited compared to other light sources. Most HID lamps are dimmable nowadays to 70% power, in some cases down to 50% power. Typically, HID lamps shift in color when dimmed because dimming reduces the operating temperature. There are very few exceptions to this behavior.
- The lifetime of conventional lamps is specified as the 50% failure point. Traditionally, HID lamps are known to lose much of their initial lumens over their life and the 70% lumen-level point occurs well before the electrical end of life. With the latest generations of white light HID lamps in combination with electronic ballasts, this picture has changed remarkably. There are now ceramic HID systems that have an electrical lifetime of 30,000 hours while still producing 70% of the initial lumen levels at that lifetime.
Some of the main strengths and status of HID technology are:
- Efficiency. The bulk of HID products produce around 100 lm/W. Top-line products are around 120 lm/W. Several companies have announced initiatives to bring this up to 150 lm/W in combination with lifetimes of 40,000 hrs.
- Lifetimes of HID systems are typically around 15,000 hrs for low wattage lamps (below 100W) and to 30,000 hrs for higher wattage versions.
- Relatively low cost and therefore short payback times of 1-3 years. There are retrofit solutions that offer 15-20% energy savings with a simple lamp replacement as well as system retrofit packages for various luminaires that allow higher energy savings of up to 50%.
Like in HID lamps, a plasma is created inside a discharge chamber filled with a gas. The main difference with HID is that in fluorescent lamps, the temperature and pressure of the gas is much lower. As a result, the gas itself radiates mostly in the UV spectrum. This radiation is converted to light by phosphor coatings on the lamp. The discharge chamber of fluorescent lamps consists of a long thin tube which can be bent in various shapes to create a more compact form factor.
A few important characteristics of fluorescent systems that influence their application are:
- Fluorescent sources are temperature dependent. The lumen output and efficacy of fluorescent lamps depends on the ambient temperature. When ambient temperatures are outside the optimal range, say 60-70°, the lumen output and efficacy drops quickly. This limits the usefulness of fluorescent sources to indoor applications.
- Fluorescent sources are relatively large. As a result, optical control is limited. An exception is made for T5HO lamps, which are long and thin. As a result, the optical control in radial direction is very good for these light sources.
- Fluorescent sources are controllable. Fluorescent lamps are instant-on, are dimmable and come in various colors. This makes them almost on-par with LED in terms of controllability.
Induction lighting can be viewed as a special form of fluorescent lighting. Here, the electrodes are located outside the discharge chamber. At the cost of some efficiency for the overall system, this allows for very long lifetimes of up to 100,000 hours with very little lumen loss. Overall system efficacies are typically in the 80 lm/W range. The lamps are relatively bulky which makes optical control difficult.
Some of the main strengths and status of fluorescent technology are:
- Lamp efficacies of 100-110 lm/W are common. Small incremental improvements in efficacy are continuously introduced. Induction systems are less efficient with 70-80 lm/W.
- Lamp lifetimes above 40,000 hrs are available, and up to 100,000 hours for induction.
- Fluorescent systems are offered at the lowest initial cost of all high-efficacy products.