Evaluating a Trial Installation
“Seeing is believing”, as the expression goes. And no matter what can be confidently stated on paper about the light levels and energy use of a proposed system, lighting is ultimately a visual experience. With regard to many qualitative aspects there is simply no substitute for experiencing lighting equipment in application. However, there are numerous potential pitfalls in evaluating a trial installation. These must be understood and avoided to ensure a meaningful assessment.
We all have a natural tendency to evaluate lighting equipment by looking directly at it. This is certainly a valid way to assess various visual aspects of a luminaire, particularly in comparison to existing equipment or alternative product options. However, looking directly at a lighting product, particularly from directly beneath, can be misleading. This is not the way that occupants typically experience and interact with overhead lighting. For most visual tasks, including moving throughout a space, the direction of our gaze is generally horizontal or downward such that overhead lighting is typically in the periphery, or out of view. Luminaires that are in view are usually not those directly overhead. This is important to keep in mind when evaluating the overall lighting effect created by a particular type of luminaire. Some repetitive tasks do represent an exception however. For example, a forklift operator identifies or manipulates product at the top of a storage rack that may have luminaires very close to the direction of view.
Since trial installations are typically limited to small areas and relatively few sample luminaires, it is good to experience the lighting effect both within the evaluation area and when viewed from a distance. Distant observations are useful for gauging perceptions of overall brightness, object modeling and visual clarity (employing imagination to envision the appearance of a complete installation). Within the trial installation area, the luminaires being considered are largely responsible for the illumination of surfaces and objects. It is important to experience the actual visual tasks that are typically performed in the area in order to assess visibility, glare, color rendition, shadows, and the modeling of three-dimension objects. These evaluations should naturally be weighted based on their relevance to the tasks and objectives at hand. Glare potential (direct and reflected), should be evaluated both near and far (keeping in mind the issue of typical viewing angles discussed above).
Lighting impacts us in a variety of ways (see Why Lighting is Important below) and it is important to understand that light level measurements and predictions do not necessarily correlate well with our overall impressions of a lighting system. Glare, for instance, has a powerful impact on our perception of environmental brightness and yet illuminance readings on a horizontal task plane are in no way indicative of this effect. Lighting "says" things to employees, customers, visitors and management. It speaks of the company and its values. Therefore, it is entirely appropriate to evaluate how these less tangible, but very real considerations, align with one's assessment of a trail installation.
The lumen output of all electric light sources diminishes with time. For numerical analysis, this fact should be captured through the incorporation of a Light Loss Factor (see Interpreting Point-by-point Illuminance Predictions below). It is important to understand the light loss characteristics of the equipment being considered, but also that of the existing lighting system and any alternatives being considered. Since the existing lighting typically serves as a frame of reference for a trial installation, it is critically important that the role of lamp lumen and luminaire dirt depreciation be understood. Presumably, the luminaires used for the trial installation will be new while the existing system suffers from the considerable impact of these factors. With time, the new system will also be subject to their degrative effects and this is not typically evident in a trial installation. A most realistic and fair comparison can be achieved by simply establishing an equivalent reference area where an equal number of the existing luminaires have been carefully cleaned and re-lamped. However, it is important that the reference area be geometrically identical (same luminaire spacing and mounting height) to the trial installation area in order to provide meaningful comparisons.
With the above caveats in mind, illuminance readings can be taken to verify predicted numerical performance and/or compare with industry recommended levels. It is important that careful attention be paid to matching both the location and orientation (typically either horizontal or vertical) of the illuminance meter with the associated predicted or recommended illuminance values. Care must also be taken to avoid shadowing the meter from nearby luminaires with one’s body. To insure meaningful averages, illuminance readings should be taken with a minimum density of ¼ luminaire spacing between adjacent measurement points. These readings should be minimally taken in a representative quadrant of four luminaires (see plan view graphic at right). Note that taking fewer readings, or readings in different configurations can lead to errors in excess of 15% when averaging to arrive at a single representative illuminance value.
The more luminaires that are included in a trial installation, the more directly illuminance measurements can be attributed solely to the luminaires being evaluated (no significant contribution from adjacent luminaires in the existing system). However, installing a large number of luminaires is generally impractical for trial purposes. Thus, it is recommended that a minimum of four adjacent luminaires be used to establish a trial area. For the most accurate results, readings can be taken in fixed locations, with and without the trial luminaires energized. The "off" readings can then be subtracted from the corresponding "on" readings to unambiguously isolate the contribution of the subject luminaires alone. Similarly, light from nearby windows and skylights can also confound interpretations. It is recommended that illuminance readings be taken at night when daylighting is a significant contributing factor
Lamps are always labeled in terms of power consumption (wattage). Ballasts are always labeled with respect to lamp power, but not always in terms of the total power consumed (including ballast losses). Total
power can be determined, but it is often more convenient to perform such measurements on a bench-top rather than in application. Generally speaking, ballast manufacturers publish detailed specifications for their products. Total power consumption can usually be established in lieu of direct measurements so long as the ballast manufacturer and model can be determined.
Related Resources
Why Lighting is Important
Interpreting Point by Point Illuminance Predictions
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