Way more than cool
August 1, 1999
By Robert M. Cilic
Fluorescent light sources are members of a larger family known as gas discharge lamps, whereby an electric discharge inside a gas filled tube leads to light emission. In 1938 the first fluorescent lam...
Fluorescent light sources are members of a larger family known as gas discharge lamps, whereby an electric discharge inside a gas filled tube leads to light emission. In 1938 the first fluorescent lamp was introduced, the T12 (“T for tubular, 12 for the diameter in eighths of an inch”). These T12 lamps became the workhorse of the fluorescent lamp world, and were followed by more efficient versions such as the T8 lamp which saves 30 to 50 per cent energy.
The old T12 lamp is familiar, reliable and still widely used, but newer fluorescent lamp technologies offer many advantages. Research has resulted in improvements in the efficacy, size, colour and life of both the lamps and their ballasts. Today, especially exciting developments are taking place in lamps that minimize their impact on the environment, not only in terms of reduced energy consumption, but also in terms of the kinds of materials that go into the design of the lamps themselves.
One important advance in fluorescent lamp technology is in the colour and quality of the light they produce. These qualities depend on the chemical composition of the phosphor coating on the inside of the lamp. By combining different phosphors in varying proportions, it is possible to produce lamps in a wide variety of colour temperatures and colour rendering indices. With the introduction of rare earth phosphors, fluorescent lamps can now have both high light output and excellent colour properties.
In the older T12 lamp technology there were two common “colours” for fluorescent lamps: cool white and warm white. “Cool” lamps tended to be more blue, while “warm” lamps tended to be more red-yellow. Many people still visualize cool white lamps and the environment they produce when thinking about fluorescent lighting.
Today, lamps are not rated as cool or warm, but rather by correlated colour temperature (CCT) in Kelvin. The older cool white corresponds to a CCT of 4,100K and the warm white corresponds to a CCT of 3,000K. As a point of reference, an incandescent lamp has a CCT of 2,700K, halogen has 3,000K, and daylight can range from 5,000K and up. As the colour temperature increases, the energy in the blue part of the visible spectrum increases making the lamp appear “cooler.”
Colours viewed under daylight or incandescent light are considered “true.” This is because both sources can render all colours of the spectrum, producing “full spectrum,” or more appropriately, continuous spectrum light. All other discharge light sources, such as fluorescent, are rated according to their ability to render 14 standard colours in comparison to natural light. The scale, or colour rendering index (CRI), is 0 to 100, with incandescent and daylight approaching 100. Older cool and warm white fluorescent lamps had colour rendering indices of only 62 and 57 respectively. Newer high colour rendering T12, all T8 fluorescent and compact fluorescent types, however, use tri-phosphor coating technology and have colour rendering indices from 70 to 90.
Although manufacturers’ products vary slightly, T8 and compact fluorescent lamps are generally designated for example as “841,” meaning 4,100K colour temperature with a CRI colour rendering index of 80-89. In all cases, the first digit specifies the CRI range and the second and third digits specify the colour temperature.
The extended performance T8 lamp is the newest fluorescent phosphor type to be introduced. By combining increased lumens per watt (efficacy) and lumen maintenance (95% compared to 90%) with a longer rated life (24,000 hours compared to 20,000 hours for standard T8 lamps), T8 high performance phosphor lamps should make a big impact in the fluorescent lamp market. The longer life can add valuable savings to a “cost of light” calculation. When operated on an electronic ballast with low ballast factor, a high performance phosphor lamp will provide equivalent light levels to a standard T8 on a standard electronic ballast with an additional energy savings of 14%.
Getting smaller …
Research efforts have led to the reduction in diameter of the fluorescent tube. The advent of the T5 lamp, with it’s 5/8-inch diameter has opened up a new world of possibilities for sleek innovative fixture designs. The T5 lamp also has the benefit of increased lumen maintenance (95%) in both a standard and high output version.
The T5 is also a truly “global” lamp in that the same lamp type is used around the world. No matter where this lamp is specified or used, the lamp conforms to the same operating parameters.
Compact fluorescent lamps have replaced incandescent lamps in a wide variety of commercial applications. Advances in glass and lamp chemistry technology have resulted in extremely small lamps that yield high light output with long life (typically 10,000 hours). Most notably, the 32-watt and 42-watt lamps are so compact and powerful they permit fixture manufacturers to produce highly efficient, small fixtures that often outperform their incandescent or mercury vapour predecessors. In fact, the 42-watt lamp produces 3,200 lumens (equivalent to a 4-foot fluorescent) from a lamp only 15 centimetres long. This high light output coupled with high colour rendering (85 CRI) and colour temperatures from an incandescent-matching 2,700K to a cool 4,100K, make compact fluorescent lamps very versatile.
Even smaller is the T2 fluorescent lamp. It measures only 5 millimetres in diameter and is suited to low profile applications such as showcase or under-cabinet lighting. The small size of the lamp allows excellent light control from the reflector to give an even spread of light. These lamps measure from 218 to 523 millimetres long, with wattages from 6 to 13.
For the environment
With environmental considerations becoming increasingly important, research has been going on to find ways to reduce the amount of mercury used in fluorescent lamps. Minute quantities of liquid mercury are placed in the bulb to give mercury vapour which is essential to the lamp operation. Research has already made possible the T8 and T12 lamps with a reduced mercury content, and these are readily available. Presently, however, it is not economically feasible to produce a mercury-free fluorescent light source that has the same efficacy as today’s lamps. However, by combining phosphor technology with reduced mercury content, the T8 fluorescent lamp not only has increased luminous efficacy and a longer life, but also a reduced environmental impact.
A current example of a mercury-free fluorescent lamp is the commercially available flat fluorescent lamp. Using Xenon gas and high frequency electronic ballast technology, this specialized lamp has a luminous efficacy of 50 lumens per watt. Although it also boasts a long life of 50,000 hours, the mercury-free technology must still overcome efficacy hurdles to become a viable source for general lighting.
A remarkable emerging technology is the electrodeless fluorescent lamp. It features a sealed fluorescent tube that has no internal cathodes but uses an externally applied electromagnetic field to stimulate the mercury atoms. Standard fluorescent lamp life is determined by the life of the cathodes, typically 20,000 hours, whereas the electrodeless lamp life is rated at 100,000 hours. This extremely long lamp life, coupled with efficient fixture designs, will change the basic “cost of light” calculation parameters.
When used as a system with its electronic ballast, the lamp has a high efficacy of 80 lumens per watt. It also achieves a sustained, high luminous flux. In other words, the lamp and ballast system features instant flicker-free starting with a short “run-up” to full light output. The electrodeless lamp has a CRI of 80, equivalent to that of standard T8 lamps, and exists in different colour temperatures.
The extremely long life of the electrodeless lamp makes it ideal for applications where relamping is awkward and expensive, or where safety and reliability is of primary importance. The long service life reduces the high cost of relamping, while the instant re-strike capab
ility guarantees an uninterrupted source of high efficacy light without the need for auxiliary equipment. Also, the lamp’s dimensions allow it to fit specialized low-profile fixture applications.
Standard magnetic ballasts have long been the mainstay of the fluorescent lighting industry. These relatively simple units are passive devices designed to start the lamp, limit its operating current and provide power factor correction. Energy efficient versions use optimally designed magnetic materials and construction techniques to reduce stray losses and thus increase efficiency.
High frequency electronic ballasts operate lamps with higher efficacy, making it possible to reduce energy consumption without sacrificing light output. Electronic circuits also make it easier to start the lamp and ensure proper operating conditions. Light output control features such as dimming, or increased light output, are easily achieved through electronic ballast technology.
Microprocessor-based chip technology and high voltage semi-conductors are being used to develop the ballasts of the future. These ballasts start lamps better, and they provide improved colour control, longer lamp life and greater efficiency. They have new functions such as sensing end-of-lamp life, preventing excessive inrush current and regulating against the effects of input voltage fluctuations. These ballasts will have considerably less impact on the lamp during starting, thereby improving lamp life, especially in applications with occupancy sensors and short operating cycles.
As integrated circuit technology advances, applications for addressable control of the ballast, either individually or together as part of a building management system, will also become easier to implement regardless of the control system employed.
In summary, fluorescent lamps and ballasts are in a never-ending state of evolution. The future looks brighter indeed.
Robert Cilic, C.E.T. is technical services manager with Osram Sylvania Ltd., Mississauga, Ontario.