SSLDesign News Staff
November 8, 2013…The US. Departement of Energy has published a CALiPER report about the subjective evaluation of PAR 38 lamps. The report is the first in a series of four special investigations designed to extend the findings of CALiPER Application Summary Report 20: LED PAR38 Lamps, which was published late last year. The new report, CALiPER Report 20.1: Subjective Evaluation of Beam Quality, Shadow Quality, and Color Quality for LED PAR38 Lamps, focuses on a subset of the lamps from Report 20, which were separately assessed by members of the Illuminating Engineering Society of North America. The report is available online at http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/caliper_20.1_par38.pdf.
Lighting quality is not easily quantified, and while measurements such as CRI are a start, as the DOE points out, CRI does not reveal smoothness or color consistency of a beam. The report states,“Unfortunately, many of these facets of lighting quality are not easily quantified using readily available metrics. Thus, it can be difficult for manufacturers to make improvements and for consumers to differentiate products. Subjective, comparative evaluations can help to identify product attributes that lead to better lighting quality, as well as what attributes specifiers consider desirable. “
Researchers at the Pacific Northwest National Laboratory constructed a demonstration mock-up at their facility in Fairfield, Oregon to learn more about lighting preferences. Members of the local IES chapter were invited to view the 26 differen lamps, and were asked to complete a questionnaire that focused on beam quality, shadow quality, and color quality.
According to the report, “A total of 26 lamps were installed and aimed at a wall covered with off-white paint typical of a basic commercial space. The lamps were aimed at pushpins placed in the wall, which were spaced approximately 10 inches on center to match the spacing of the lamps.”
The report describes the demonstration configurations for beam quality, shadow quality, and color quality. “Each performance attribute had a different demonstration configuration: beam quality was evaluated with lamps aimed at a blank wall; shadow quality was evaluated using a series of synthetic flowers positioned in a vase about 12 inches from the wall; and color quality was evaluated using an X-Rite Color Checker Classic Card, fabric samples, and a board with Rosco Scenic Paint.”
The results revealed that many of the LED-based lamps compared favorably to halogen benchmarks in all attributes considered. Some preferences emerged in the rankings. LED lamps using a single-emitter design were generally preferred for their beam quality and shadow quality. Conversely, poor color consistency within the beam, and stray light outside the main beam pattern, were the attributes most likely to be noted by the observers as negative features.
LED lamps with narrow-spot distributions were generally viewed as having less-acceptable beam quality than their narrow-flood or flood counterparts, although there was substantial variation in perceived quality within any of the groups. Shadow quality was the most universally agreed upon by evaluators with single emitter B6 lamps generally giving the best shadow quality followed by diffused B1 lamps. For color quality, the observers generally preferred 3000 K LED lamps over 2700 K LED lamps.
Noteably, the ranking of color quality did not always match the rank-order according to the product’s color rendering index (CRI). So CRI by itself was not a perfect measurement of subjective color quality. The report notes that the effect of CCT and Duv on color appearance should not be overlooked in favor of choosing a product based solely on CRI—especially when the CRI is above a minimum threshold (e.g., 80). In the future, the adoption of new color-quality metrics should help manufacturers design better products and specifiers choose better products.
The DOE reports that LED PAR38 lamps still have room for improvement in even the best performing lamps. Another conclusion that the report made was that because of substantial variability in both LED PAR 38 lamps and halogen lamps, buyers will have to make careful purchases. The report points out that some of the factors to consider before choosing a lamp include: electronic compatibility with control equipment, dimming performance, form factor, cost.
The report points out that lumen depreciation and color shift of 40 lamps is currently being monitored for an extended period of time using the Lumen Maintenance Test Apparatus (LMTA) at PNNL.