Electric lighting

At five o'clock on a December afternoon, Abernathy takes the students to two libraries a few blocks apart. The first, refitted ten years ago, is lit by rows of bright ceiling panels that give an even, shadowless glare from end to end; the floor, the shelves and the people all look the same, and the windows, now black, reflect the panels back like mirrors. The second, an old branch recently renovated, has warm light washing its shelves and walls, pools of light on the reading tables from low lamps, a soft glow on its vaulted ceiling and almost no visible light source anywhere. Its light meter readings on the tables are about the same as the first library's. 'Same lux,' says Abernathy. 'One feels like an airport and the other like a living room. Lighting design is not about putting enough light on the floor. It is about deciding what people should see, and how the room should feel when the sun has gone, which in this city is by half past four for two months a year.'

Light sources

The efficiency of a light source is described by its luminous efficacy, the light it produces for each watt of electricity, in lumens per watt. The incandescent lamp, which lit buildings for a century, produced about 15 lumens per watt and turned most of its energy into heat. Fluorescent lamps raised this to between about 60 and 100. Light-emitting diodes, or LEDs, now produce well over 100 lumens per watt in typical architectural luminaires and keep improving; they have replaced almost every other source in new buildings. LEDs are small, dimmable, long-lived and controllable, but their quality varies widely, and cheap products can flicker, shift in tint as they age or render colors poorly.

Two other properties describe the quality of light. Correlated color temperature, measured in kelvins, describes how warm or cool white light appears: about 2,700 K is the warm light of a traditional lamp, 3,000 to 3,500 K a neutral white often used in homes, libraries and restaurants, and 4,000 to 5,000 K a cool white common in offices and schools. The color rendering index, CRI, measures how faithfully a source reveals the colors of objects compared with a reference, on a scale up to 100; 80 is the usual minimum for interiors and 90 or more is preferred where faces, food, art or materials matter. A newer method, published as IES TM-30, gives a fuller picture of color quality, and designers increasingly ask for it.

Layers of light

Lighting designers think in layers. Ambient light provides general illumination so that people can move about safely and see the space. Task light provides higher levels where detailed work happens: on a reading table, a kitchen counter or a reception desk. Accent light picks out objects, such as books on a shelf, a painting, a stage or a sign. Light on vertical surfaces, walls and shelves, makes a room feel bright and open even when the light on the floor is modest, because people judge brightness mostly by what they see at eye level. Decorative luminaires add sparkle and character. And the light in a room must also connect to the light of the spaces around it, so that people's eyes are not forced to adapt suddenly as they move from a bright corridor into a dim hall.

The second library in the December visit used every layer. Its shelves were washed with light from concealed fixtures, its tables lit by lamps, its ceiling lit from above the shelves, and its general light kept low. The first used one layer for everything, at the same level everywhere, and lit the floor more than anything people actually looked at.

How much light

The Illuminating Engineering Society publishes recommended illuminances for different tasks and spaces, adjusted for the age of the occupants and the importance of the task. Typical recommendations are in the order of 30 to 50 footcandles for reading and desk work, 10 to 20 footcandles for corridors, lobbies and stacks, 50 footcandles or more for a gymnasium used for sport, and higher values for detailed or demanding tasks. Older people need considerably more light than young people for the same task, because the lens of the eye yellows and the pupil shrinks with age. Uniformity matters too: a ratio of the minimum to the average illuminance across a working area of about 0.6 or more avoids patches that are too dim.

The lumen method is a quick way to estimate how many luminaires a room needs for a target average illuminance. It divides the light needed on the working plane by the useful light each luminaire delivers there, after allowing for the light absorbed by the room's surfaces, captured by the utilization factor, and the loss of output as the luminaires age and gather dirt, captured by the maintenance factor.

# Lumen method and lighting power for the hub reading room
area = 3000                    # floor area (ft2)
target = 40                  # average illuminance on the tables (fc)
lumens = 4000                 # output of one LED luminaire
watts = 30                    # input power of one luminaire
uf, mf = 0.6, 0.85            # utilization and maintenance factors for a pale room
n = target * area / (lumens * uf * mf)
count = int(-(-n // 1))       # round up
lpd = count * watts / area
print(f"Luminaires needed: {n:.1f}, so {count}; efficacy {lumens / watts:.0f} lm/W")
print(f"Lighting power density: {lpd:.2f} W/ft2")
print(f"With fluorescent fittings of 60 lm/W for the same light: {count * lumens / 60 / area:.2f} W/ft2")
# Layered alternative: low general light plus table lamps and shelf lighting
general = target * 0.4 * area / (lumens * uf * mf) * watts
tables, lamp_w = 20, 10
shelves = 200                 # length of shelving to wash with light (ft)
shelf_w = 1.8                # watts per unit length of linear LED
layered = general + tables * lamp_w + shelves * shelf_w
print(f"Layered scheme: general {general:.0f} W + tables {tables * lamp_w} W + shelves {shelves * shelf_w:.0f} W = {layered:.0f} W")
print(f"Layered lighting power density: {layered / area:.2f} W/ft2")

Luminaires needed: 58.8, so 59; efficacy 133 lm/W
Lighting power density: 0.59 W/ft2
With fluorescent fittings of 60 lm/W for the same light: 1.31 W/ft2
Layered scheme: general 706 W + tables 200 W + shelves 360 W = 1266 W
Layered lighting power density: 0.42 W/ft2

The uniform scheme meets the target with a power density well within current code limits, thanks to the efficacy of LEDs; with fluorescent fittings, the same light would take about twice the power. The layered scheme uses less power and spends it differently: less on the floor, more on the tables and the shelves, where people look. It will feel brighter and more inviting, and it allows the general light to be dimmed or switched off in zones that are not in use.

Glare from luminaires

Bright luminaires in the field of view cause discomfort glare, just as bright windows do. The unified glare rating, UGR, used in many standards, estimates it from the brightness of the luminaires, their size and position, and the brightness of the background; lower is better, and about 19 is a common limit for offices and classrooms. Practical measures are simple: choose luminaires with shielding or diffusers that hide the bright LEDs, avoid exposed points of light at eye level, light the ceiling and walls so that the contrast with the luminaires is lower, and keep reflections off screens and glossy tables. At night, windows become dark mirrors; luminaires placed so that their reflections do not appear in the glass from the main seating positions make a room far more pleasant.

Controls and daylight

Controls save as much energy as efficient lamps. Occupancy sensors switch lights on when people enter and off when they leave; vacancy sensors require people to switch lights on but switch them off automatically, which saves more. Daylight-responsive controls, sometimes called daylight harvesting, use photosensors to dim the lights in zones near windows and under clerestories when there is enough daylight. Scheduling switches lights off after hours, and task tuning sets the maximum output of each zone to what is actually needed. Energy codes in both countries require many of these controls in new buildings.

Daylight controls work only if the lighting zones follow the daylight: rows of luminaires parallel to the windows, each on its own circuit, so that the row nearest the glass can dim first. In the reading room, with its south windows and north clerestory, the zones are the south band, the clerestory band and the middle.

# Daylight dimming in the reading room: lighting energy over a year of daytime hours
target = 40                        # fc on the tables
hours = 3000                        # occupied daytime hours per year
outdoor = [(300, 600), (800, 900), (1500, 900), (3000, 600)]   # outdoor overcast-sky illuminance (fc) and hours
zones = {"south band": (5.0, 1500), "middle": (1.2, 1500), "clerestory band": (4.0, 1200)}   # daylight factor, installed watts
for name, (df, watts) in zones.items():
    used = 0.0
    for e_out, h in outdoor:
        daylight = e_out * df / 100
        need = max(0.0, target - daylight) / target          # share of electric light still needed
        used += watts * max(need, 0.1 if need > 0 else 0) * h / 1000   # dimmed fittings keep a small minimum
    full = watts * hours / 1000
    print(f"{name:16s} {full:6,.0f} kWh without dimming, {used:6,.0f} kWh with dimming ({100 * (1 - used / full):.0f} of 100 saved)")

south band        4,500 kWh without dimming,    562 kWh with dimming (88 of 100 saved)
middle            4,500 kWh without dimming,  2,678 kWh with dimming (40 of 100 saved)
clerestory band   3,600 kWh without dimming,    720 kWh with dimming (80 of 100 saved)

Near the windows and under the clerestory, daylight dimming saves most of the lighting energy over the year; in the middle of the room it saves well under half. The savings are only realized if the sensors are placed and calibrated well, which is why commissioning, the testing and adjustment of systems after they are installed, is essential. Badly set daylight controls that dim the lights while people still need them are soon disabled by unhappy occupants.

Light, health and the night

Light affects the body's daily rhythm as well as vision. Light rich in blue wavelengths, as in daylight and cool white LEDs, suppresses the hormone melatonin and promotes alertness, while warm, dim light in the evening allows it to rise. This has led to tunable lighting that shifts from cool and bright in the morning to warm and dim in the evening, used in some schools, hospitals and seniors' homes. The science is still developing, and the clearest advice is simple: plenty of daylight during the day, especially in the morning, and warmer, dimmer light in the evening.

Outdoor lighting needs as much care. Light that spills upward and sideways from badly shielded fixtures wastes energy, causes glare for drivers and pedestrians, disturbs neighbors and wildlife, and brightens the night sky so that stars disappear. Dark-sky guidance asks for fully shielded fixtures that send no light above the horizontal, warm sources of about 3,000 K or less, lighting only where and when it is needed, and controls that dim or switch off late at night. For the community hub, beside a greenway used by wildlife and overlooked by homes, these are part of the brief.

Exercise 7: a lighting design

This exercise takes about one week.

• Choose two rooms of the hub with different uses and draw reflected ceiling plans showing the layers of light.
• Calculate the number of luminaires by the lumen method and the lighting power density for each room.
• Define the lighting zones and controls, including daylight dimming where it applies.
• Draw a night section showing how the room will look when it is dark outside, including reflections in the windows.
• Specify the correlated color temperature, color rendering and glare limits for each room and justify them.