An LED produces light in a narrow spectrum. In order to produce white light from a single LED module, the module is phosphor-coated. Another method of producing white light is combining several different color LEDs. Color rendering, and color rendering indexes, are harder to apply to LED luminaires than standard luminaires (see Chapter 1). Not all LED lamps available in the current market have good color rendering properties. Higher color temperature LED modules are more efficacious, encouraging their application. But the cooler color (bluish tint) associated with them may not be as readily acceptable to users (Clanton and Associates and VTTI, 2009; Clanton and Associates, VTTI, ETA, 2010). SSL luminaires need to increase the efficacy for color temperatures below 3,800 K in order to satisfy user preference for the lower correlated color temperature (CCT) (Figure 5.4).
One significant issue is the consistency of SSL luminaires’ color appearance. If LEDs from different bins are used in the same luminaire or in similar luminaires side by side, the light outputs will not match in color appearance (i.e., the CCT). This is an issue not only during initial installation, but also in deployment because of aging and replacement of luminaires. SSLs need to be consistent between products and guaranteed to provide color-consistent replacement modules in the future.
FINDING: Many LED lamps currently available do not have the same light output and color rendering properties as incandescent lamps. SSL products with improved light output that are color consistent from product to product will be needed for the public to readily accept these as replacements for incumbent lighting technologies.
SSL lighting applications are currently intended to duplicate or be similar to incandescent and fluorescent lighting. SSL has many unique qualities that have not been present before in lamp technology, and these may result in new applications in the future. The attributes of SSL make possible new ways of lighting such as uniform surface washing and grazing, close-to-task effectiveness, and surface integrated light. Tunable spectral distribution adds flexibility for color adjustments.
The current status of SSL applications is summarized below. This is, of course, a snapshot of a moving target, as the field is changing rapidly.
Best. Because the current LED products have relatively low lumen output, the best current applications for SSL are those in which the light source is close to the task and the required lighting levels are low. Examples of these are task
FIGURE 5.4 Current and projected efficacies. NOTE: pc-LED = phosphor-converted LED; lm = lumen; W = watt; Qual = qualified data point having satisfied the criteria for cool white (color rendering index (CRI) of 70-80, correlated color temperature (CCT) of 4,746-7,040 K) or warm white (CRI of 80-90, CCT of 2,580-3,710 K). Results are for 25°C package temperature and are normalized to current densities of 35 A/cm2. SOURCE: DOE (2012b).