It's Not Easy Being Green

What it takes to achieve a truly green lighting design

Low-voltage transformers used with MR16 lamps have a power factor between 0.95 and 0.98. The 25W CDMI has a power factor of 0.57, and an input current of 0.36 amps at 120 volts. Assume that the input current remains constant, and the CDMI has a power factor of 1.0. In this situation, it would actually operate at 43.2W. The real power in watts would equal the complex power in volt-amperes, as in the case of an incandescent filament lamp that is strictly a resistive load or a typical CFL with a 1.0 pf corrected ballast. Because the actual power factor of the 25W CDMI is 0.57 and not 1.0, essentially 18.2W of delivered power is lost due to a displacement or lag between voltage and current. The loss is not accounted for in a watt-per-square-foot energy summary, because a watt-hour meter does not measure complex power.

Like the CFL, the CDMI contains mercury and an integral electronic ballast, which may contain additional environmentally toxic materials. Whether the ballast does or does not contain toxic materials remains to be documented by the manufacturer. In either case, the CDMI contains an added level of embodied energy due to disposal of the contained mercury and production of the integral electronic ballast that is thrown away with each lamp change. The MR16 contains neither mercury nor electronics, making it a better choice with regard to environmental waste streams and embodied energy.

Other lighting issues to consider when specifying the 25W CDMI are color rendering, dimming, restrike time, bulb size, and system cost. The CDMI has a rated correlated color temperature (CCT) of 3,000K (± 200K) and a rated color-rendering index (CRI) of 87. It is true that the color quality of ceramic metal halide is superior to that of standard metal halide and, in most applications, provides a brilliant color that some designers consider better than halogen. Ceramic metal lamps — and this lamp in particular — have a spectral color spike between 525 nanometers and 625 nanometers, which renders skin tones in the yellow range. This may be a detriment in clothing retail, where looking good in a mirror is critical to a purchase.

As with all M-H lamps, the CDMI has a restrike period estimated at about 5 minutes from power fluctuation or power loss to restrike and an additional 3 to 4 minutes before the lamp reaches 80% of full light output. Unlike halogen and fluorescent lamps, the CDMI cannot be dimmed or controlled via occupant sensors. In rooms or spaces that are largely unoccupied during much of the normal building operation, CDMI lamps will likely remain on unless building occupants learn to accept the warm-up and restrike times associated with switching them off.

Weighing your options

Aesthetics and construction costs are not green design issues per se; however, they are integral to the success of every project. Evaluating the aesthetic appropriateness of the PAR38 CDMI lamp versus the MR16 is a question of scale within a space. The MR16 may be lost in a large room whereas the PAR38 may take up all of the space in a small room. Finally, there's the question of cost. The price of lamp holders and track for each system will be very similar, but the lamp cost is significantly different. Transformers for the MR16 low-voltage lamp holders may cost more initially than the total cost of the CDMI, but will only be purchased one time and will typically operate up to 15 years. The cost of each CDMI lamp includes the cost of the integral ballast, which can only operate for the life of the lamp — about 2½ years operating at 12 hours per day.

Because the decisions required for sustainable or green designs are numerous and complicated, they must be carefully weighed based on environmental stewardship, overall performance, and building economics. In the end, the operational requirements of the lighting system, the architectural program and details, and the project sustainability goals will guide designers to make appropriate system choices for each design application. Although the MR16/IR lamp may not be the best design solution for every downlight or accent light application, it should be carefully considered as an option — not automatically dismissed simply because it is incandescent.

Gallina is a lighting designer based in Minneapolis. She can be reached at [email protected].

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.