The Problem with Luminous Efficacy

A rate is a ratio of unlike quantities, such as miles/hour, beats/minute, and points/game. Efficacy is a special type of ratio that represents a rate of consumption, such as miles/gallon and gallons/flush, where one side of the fraction represents a benefit and the other a cost. With miles/gallon, the benefit is miles driven and the cost is gallons of fuel consumed. With gallons/flush, the benefit is the removal of waste by the flush and the cost is gallons of water consumed. Efficacy is intended to be a measure of the ability of something to do what it is supposed to do.

Luminous efficacy is defined as a ratio of lumens per watt and is applied to light-emitting devices such as LED emitters, lamps, and luminaires. Implicit is that lumens are a suitable proxy for lighting’s benefit and watts are a suitable proxy for lighting’s cost. Uncomfortably, this is often not true when light-emitting devices are employed in lighting applications.

Lumens may be good or bad. A “good lumen” is one that does what it is supposed to do. Good lumens enable a person to read a book, reinforce architectural form, create visual delight through desirable patterns of shade, shadow, and highlight, or evoke an alerting effect when alertness is desired. A “bad lumen” is one that does a poor job for its purpose or produces deleterious effects on vision or health. Most lumens generated never enter a person’s eyes; those lumens do not support any beneficial outcome and are waste. Sometimes unwelcomed lumens enter the eyes. This includes lumens that cause visual discomfort as with excessively bright luminaires, lumens that reduce task visibility through veiling reflections, lumens that distort colors due to poor spectral quality, and lumens that disrupt circadian rhythms due to inappropriate timing, intensity, spectrum, or duration of exposure.

The benefits of light to people are as a stimulus for vision that enables visibility, perceptions of color, and psychological reinforcement, and as a stimulus for health-related effects that include alertness, entrainment of circadian rhythms, and sleep quality. We cannot expect a simple ratio to encapsulate all of these potential benefits, but we can question the veracity of lumens as a suitable proxy for any of them.

Watts are an imperfect proxy for the cost of light for two primary reasons. First, watts are a measure of power, not energy. The cost of lighting in end-use applications, both in dollars and in carbon emissions, is time-dependent. Light-emitting devices that are frequently switched off consume little energy even if they have low luminous efficacy. Second, electrical power can be generated with different technologies. A watt from a coal-fired power plant is environmentally costly, but a watt generated by a photovoltaic cell via conversion of sunlight is benign. As renewable energy production expands, the energy source is increasingly relevant to the calculus of lighting’s costs.

Some things that reduce the luminous efficacy of light-emitting equipment may produce desirable benefits in application. Luminaire optical elements reduce luminous efficacy by absorbing or redirecting lumens that may otherwise cause visual discomfort. Higher quality color rendering is associated with lower efficacy. LEDs with lower CCT tend to have lower efficacy than those with higher CCT, but may better support user needs in some settings.

In summary, luminous efficacy is a narrow performance measure for light-emitting devices, whereas a holistic evaluation of applied lighting requires a contextual assessment within the built environment. Total system performance depends not just on lamps and luminaires, but also upon how those products are installed and controlled, occupant outcomes, and the application environment—aspects that cannot be reduced to lumens or watts.