Choosing a light source. Choosing the best light source or lamp type for an application has an impact on security, maintenance, public acceptance and aesthetics. At night this is the component of the system that will receive the most attention, positive or negative. Some lamp types used in solar lighting are Compact Fluorescent (CFL), Induction Lamps, High Pressure Sodium (HPS), Metal Halide (MH) and Light Emitting Diodes (LED). LED sources will be discussed in more detail when we compare LED luminaires to luminaires using traditional lamping.

Key lamp characteristics for solar lighting are Lamp Efficacy, Correlated Color Temperature (CCT), Color Rendering Index (CRI), Rated Life and Lumen Maintenance. Lamp efficacy in basic terms is how many lumens are produced by a lamp divided by the wattage used to operate that lamp. Correlated Color Temperature is stated as degrees Kelvin and generally describes the hue of the light. Incandescent lamps have a CCT value of 2700K (warm reds-yellows-pinks) and natural daylight is more in the range of 6500K (cool blues-whites). CRI uses a scale 1 to 100 and rates the ability of a light source to render color of an object faithfully compared to a full spectrum natural light source. Rated Life is determined by testing a group of lamps under specified conditions until 50 percent of the lamps burn out. This is considered the average rated life of the lamp. Lumen Maintenance varies by source type; all light sources have depreciation in light output over time. By dividing the mean lumens of a lamp (output at mean life of lamp) by the initial lumens you can determine the lamp lumen depreciation for that source. Now that we know which lamp characteristics are most important to compare we can begin to analyze the different sources to see where they may be best applied. The information below is based on specific lamps and generally represents the performance of their group.

At the top of the lamp comparison chart we see information for a Metal Halide T6 lamp. T6 is a small, point source type of Metal Halide lamp. The small burner or "point source" of the T6 lamp is easier to control than a larger diffuse source which allows for a more precise distribution. Ranked highest for its superb CRI value the Metal Halide T6 lamp provides accurate color representation which can be an advantage when security is a concern or when crisp accurate color is your priority. Efficacy of this lamp is also something to note. It is producing more lumens per watt than any other lamp group, making it a great match for use with a solar lighting system when every watt counts. Metal Halide T6 lamps do not respond well to dimming. This lamp type also has the shortest life at about 3 years if operated for an average of 12 hours per day, per year.

The next lamp group in our chart is the Compact Fluorescent Lamp (CFL). Once scrutinized during early commercialization, Compact Fluorescent lamps have improved dramatically and are now well accepted by the public and design professionals alike. A good balance of lumen maintenance, color, efficacy and life make the CFL a good choice for a variety of applications. Compact Fluorescent lamps do go through a warm up period to reach full brightness and color stability. This will be more noticeable in cold climates.

 

Induction lamps are less commonly used but certainly have their place for specific applications. The large diffuse source and associated generator can provide an impressive 100,000 hours of operation. Best applications for induction lamps are areas difficult to access or areas that demand the longest possible operating life. Color, efficacy and lumen maintenance are all adequate for most exterior lighting environments. The size of the lamp and generator make these lamps difficult to integrate into small forms and precision optical systems.

High Pressure Sodium (HPS) lamps are commonly used in street lighting for their excellent lumen maintenance and long life. This lamp group is easy to identify at night due to the pinkish color of light and low color rendering abilities. These attributes can make it difficult to identify approaching people or even your car in a parking lot. For these reasons HPS lamps should not be used when security is the overriding factor of a project.