Classroom Lighting 101

Below is an application story I wrote for the May issue of tED Magazine. Reprinted with permission.

As of November 2016, put-in-place construction spending in the United States reached $82 billion, a 6.3-percent increase over 2015. After power and highway and street construction, educational facilities are the largest construction market in the country.

Today, nearly 55 million students attend school in some 130,000 K-12 buildings in the United States. Arguably, the most important room in these buildings is the classroom, where the majority of instruction occurs.

General-purpose classrooms typically serve 20 to 75 students and are at least 350 sq.ft. In regards to education technology, many modern classrooms bear little resemblance to those used to teach previous generations. A large number of classrooms now use computers, mobile devices and interactive whiteboards as instructional tools.

This article discusses lighting and control for K-12 general-purpose classrooms based on three sources: ANSI/IES RP-3-13, American National Standard Practice on Lighting for Educational Facilities, 2014 national Collaborative for High-Performance Schools (CHPS) criteria (a points-based design rating system), and the 2010/2013 ASHRAE/IES 90.1 energy standards.

General lighting

The primary lighting layers in a classroom are general and supplemental lighting.

For general lighting, RP-3 recommends uniform lighting on horizontal task surfaces, which provides task layout flexibility while promoting alertness and visual acuity. Light levels should satisfy Illuminating Engineering Society (IES) recommendations. Design light levels depend on factors such as the luminaires’ placement, output and distribution as well as the room dimensions and surface reflectances. Recommended surface finish reflectances are 90 percent ceiling, 80 percent window, 70 percent whiteboard, 5-20 percent chalkboard, 60 percent wall, 25-40 percent task surface, and as light as practical for the floor.

The general lighting typically may be segmented into two zones, one for the educator and one for the students. The educator zone focuses on light on vertical surfaces (teaching wall), and the student zone focuses on light on horizontal surfaces (desktops). The designer must take care to avoid glare—reflections on computer screens and whiteboards and direct glare for the educator—which can be challenging with lower ceiling heights.

Daylight is a valuable source of light for general lighting in classrooms. Ideally, students will be seated with sightlines parallel to windows. The daylight entering the space should be controlled with accessories such as windows or blinds. CHPS imposes significant daylight requirements.

Luminaires may emit direct or indirect light distribution or a combination of both, such as direct/indirect. With a direct/indirect luminaire, the direct emission places light on the task and produces some shadowing for modeling. The indirect emission, meanwhile, provides soft, diffused ambient lighting that may be more visually comfortable and produce less reflection on computer screens, which may be tilted back.

The designer may add supplemental lighting to the educator zone. This lighting may be part of the general lighting or dedicated lighting such as a whiteboard luminaire. Its purpose is to raise vertical light levels on the education surface such as a whiteboard or across the entire educator zone. In the latter case, it also draws attention to and effectively models the educator.

Available equipment is constrained by energy codes, which limit interior lighting with a power allowance expressed in maximum W/sq.ft. ASHRAE/IES 90.1-2010 imposes a maximum allowable lighting power density of 0.99W/sq.ft. for school and university buildings if using the Building Area Method and 1.24W/sq.ft. for classrooms if using the Space by Space Method.

For commercial building applications where color rendering is important but not critical, a color rendering index (CRI) rating of 80+ is typically recommended. CHPS requires either a minimum of 80 or 85 CRI, depending on the selected points package.

CHPS options further require luminaires be RoHS compliant, have an L70 of 50,000 or 100,000 hours if LED, operate with an initial efficacy of at least 50 lumens/W, and/or produce a Percent Flicker that is 10 percent or less across the dimming range. For specific requirements that relate to different points packages, consult the CHPS criteria applicable to your project.

Flexibility

The large-scale introduction of projected images in general-purpose classrooms demands flexibility from the lighting system to produce optimal viewing conditions. RP-3 recommends controls that reduce or turn OFF during audiovisual (AV) presentations, with dimming being desirable for presentations using video and computer projection systems.

The lighting should be capable of at least two scenes, General (normal) and AV (multimedia) instruction. In the General mode, the lighting places 20-40 footcandles on desktops. In the AV mode, 5 footcandles, while limiting vertical light levels to 3 footcandles on the whiteboard or projection screen and 7-15 footcandles on the surrounding teaching wall.

CHPS encourages flexible controls by offering up to four points. For two points, the designer must provide indirect/direct lighting for all general-purpose classrooms. Control enables a choice of General or AV (10-30 footcandles in the student zone, maximum 7 on the screen) modes. Separate control must be provided for whiteboard vertical lighting. Where daylight-responsive controls are present, the light sensor takes precedence over manual dimming for the upper light level limit.

For two additional CHPS points, the designer can specify enhanced teacher controls, which provide teacher control at the front of the classroom for General/AV mode, whiteboard control and a manual override of the occupancy sensor time delay during written exams. The occupancy sensor signal in turn must be linked to a school-wide management system.

Tunable-white lighting allows deployment of another emerging dimension of control, which is correlated color temperature (CCT) tuning by activity or time of day. CCT tuning may be combined with intensity control to enable additional lighting modes throughout the day, such as “focus” (high intensity and cool shade of white light) for test taking, and “calm” (standard intensity and warm shade) to help calm an excited class.

Automatic controls

ASHRAE/IES 90.1-2010 and -2013 require manual control, occupancy sensing and daylight-responsive controls. Many commercial building energy codes are based on these standards or the International Energy Conservation Code (IECC).

At a minimum, the occupancy sensor must automatically turn the lights OFF within 30 minutes of the space being vacated. If the sensor automatically turns the lights ON, it must activate the lights to 50 percent or less of lighting power (bilevel switching).

For manual control, one or more manual switches must be installed at the entrance to control all lighting in the room. Additional manual controls may be installed as needed to support visual needs through flexibility.

Daylight-responsive controls must be installed where daylight is present through either sidelighting or toplighting. The output may be bilevel switching, step dimming or continuous dimming.

Learning with light

Lighting practice for educational facilities is changing alongside the teaching environment and its needs. Manufacturers have experience and offerings optimized for this market, and are therefore an excellent resource. To learn more about recommended practice, consult RP-3 published by the IES. To learn more about CHPS, download the applicable CHPS criteria at CHPS.net. To learn more about energy code requirements, consult the energy code in effect in the project’s jurisdiction.

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