Light sources and lamp characteristics
One of the first decisions in designing a good lighting system is the choice of a light source. A number of light sources are available, each with its own unique combination of operating characteristics.
Some characteristics of the lamp that a lighting designer must take into account when choosing a light source: efficiency, or lumens per watt; Color; the life of the lamp; and lamp light depreciation, or the percentage of efficiency that a lamp loses during its lifetime.
Although there are hundreds of lamps on the market today, they can be classified by construction and operating characteristics into three main groups: incandescent, fluorescent, and high intensity (HID) discharge. HID lamps can be grouped into three broad categories: high-pressure sodium, metal halide, and mercury vapor.
Another type of lamp, low-pressure sodium (LPS), shares some characteristics of HID lamps. Induction lamps are a special type of fluorescent
An incandescent lamp is the most common light source used in residential lighting. Light is produced in this source by a wire or filament heated to incandescence (light emission) by a current flow through it.
The short lifespan and low efficiency (lumens per watt) of this source limit its use primarily to residential and decorative commercial lighting.
Efficiency varies depending on wattage and filament type but typically ranges from 15 to 25 lumens per watt for standard lamps.
The incandescent source, however, does produce light in a well-accepted warm tone. It is more convenient than other light sources because it can be run directly on line current and therefore does not require a ballast. It can also be mitigated with relatively simple equipment. It is available in different sizes, shapes, and distributions of bulbs to add a decorative touch to an area.
The fluorescent lamp produces light by activating selected phosphors on the inner surface of the bulb with ultraviolet energy, which is generated by an arc of mercury. Due to the characteristics of a gas arc, a ballast is required to light and operate fluorescent lamps.
The advantages of the fluorescent light source include improved efficiency and longevity than incandescent lamps.
The efficiencies for fluorescent lamps range anywhere from 50 to 100 lumens per watt. Their low surface gloss and low heat release make them ideal for offices and schools where thermal and visual comfort is essential
The disadvantages of fluorescent lamps include their large size for the amount of light produced. This makes light control more difficult, resulting in a diffuse, shadow-free environment. Their use outdoors becomes less economical, as the light emission from a fluorescent source is reduced when the ambient temperature is low.
Induction lamps are fluorescent lamps without electrodes supplied by a high-frequency current, typically between 250 kHz and 2.65 MHz, usually via an external generator. They are available in limited powers and are known for their exceptionally long lifespan: up to 100,000 hours.
Lamp efficiencies typically range from 64 to 88 lumens per watt. The color rendering with induction lamps is very good.
Although it is difficult to optically control in a luminaire due to the large size of the lamp, induction lighting is often used in applications where luminaires can be very difficult to access or where maintenance costs are. an important factor in the design and installation of lighting. The initial purchase costs of the systems are high compared to the best HID or fluorescent systems.
High-Intensity Discharge (HID) and LPS
Sources of high-intensity discharge include:
- Metal halides, and
- High-pressure sodium lamps (HPS).
Light is produced by HID and low-pressure sodium (LPS) sources through a gas arc discharge using a variety of elements. Each HID lamp consists of an arc tube that contains certain elements or mixtures of elements which, when an arc is created between the electrodes at each end, gasifies and generates visible radiation.
The main advantages of HID sources are their high efficiency in lumens per watt, their long life, and their point characteristics for good light control.
The drawbacks include the need for ballast to regulate lamp current and voltage, as well as starting aid for SHP and some MHs, as well as the recovery delay after a momentary power outage.
Mercury vapor (MV)
The source of mercury vapor was the first HID lamp developed, fulfilling the need for a more efficient, yet compact, high output lamp. In its early days, the major drawback of this lamp was its poor color rendering. The color of the luxury white lamp is somewhat enhanced through the use of a phosphor-coated bulb wall.
Mercury vapor lamps have a good service life, averaging 24,000 hours for most larger wattage lamps. However, as the yield decreases dramatically over time, the economic life is often much shorter. The efficiency ranges from 30 to 60 lumens per watt, with higher wattages being more efficient than lower wattages.
As with other HID lamps, a mercury vapor lamp does not start immediately. The start time is short, however, taking 4-7 minutes to achieve maximum yield depending on the ambient temperature.
High-pressure sodium (HPS)
In the 1970s, as rising energy costs placed more emphasis on lighting efficiency, high-pressure sodium lamps (developed in the 1960s) became widespread. With efficiencies ranging from 80 to 140 lumens per watt, these lamps provide about 7 times as much light per watt as incandescent and about twice as much as some mercury or fluorescent.
The efficiency of this source is not its only advantage. An HPS lamp also offers the longest life (over 24,000 hours) and best light retention characteristics of an HID source.
The main objection to using the HPS system is its yellowish color and poor color rendering. It is ideal primarily for certain warehouse and outdoor applications.
Metal Halide (MH)
Metal halide lamps are similar in structure to mercury lamps, with the addition of various other metallic elements in the arc tube. The main benefits of this change are an increase in efficiency from 60 to 100 lumens per watt and improved color rendering as this source is suitable for commercial areas.
The light control of a metal halide lamp is also more precise than that of a luxury mercury lamp because the light emanates from the small arc tube, not the entire outer bulb of the coated lamp.
Pulse start metal halide lamps have several advantages over standard metal halide (probe start) lamps:
- Superior efficiency (110 lumens per watt),
- Longer life, and
- Better light retention.
A disadvantage of the metal halide lamp is its shorter lifespan (7,500 to 20,000 hours) compared to high-pressure mercury and sodium lamps.
The start-up time of the metal halide lamp is approximately the same as that of mercury lamps. Restarting after the lamp has turned off can take much longer, ranging from 4 to 12 minutes depending on how long it takes for the lamp to cool down.
Low-Pressure Sodium (LPS)
Low-Pressure Sodium offers the highest initial efficiency of any lamp on the market today, ranging from 100 to 180 lumens per watt. However, since all LPS outputs are in the yellow part of the visible spectrum, color rendering is extremely poor and unattractive.
Controlling this source is more difficult than with HID sources due to the large size of the arc tube.
The average lifespan of low-pressure sodium lamps is 18,000 hours. Although LPS maintains luminous flux well throughout life, there is a compensating increase in lamp wattage, which reduces the efficiency of this type of lamp in use.