Opinion: The usefulness of light sources in human centric lighting

In the emerging concept of human centric lighting, every user of a lighting system is considered individually (according to their age, health, culture, profession and current activity as well as external parameters like the presence of daylight, season, weather, time of the day). The spectral, spatial and temporal radiance distribution of the light source has to be optimised accordingly. For a successful optimisation, an appropriate numeric target quantity is needed. For this purpose, luminous efficacy (in lumens/watt) is not appropriate (this has been pointed out several times in the literature). The new target quantity (let us call it usefulness here) shall not only depend on the spectral, spatial and temporal radiance distribution of the light source and its input electric power but also on the individual needs of the given user in the given application. To compute usefulness, the descriptor quantities of the numerous different benefits of lighting, in the sense used by Rea and Bierman in their paper "A new rationale for setting light efficacy requirements", shall be weighted with positive, negative or zero weights according to the individual characteristics of the user and the lighting application and then, they should be added together.Descriptor quantities might include a colour rendition index and measures of scene brightness, visual clarity, detection performance or the circadian stimulus. If the aim is to create a relaxing atmosphere then the usefulness of a cool white light source at a high illuminance is low. The reason is that although we use much electric energy, the users (e.g. friends sitting in a restaurant in the evening) will not be satisfied. They rather need a warm white light source at a moderate illuminance (at which usefulness will have its maximum). But the same cool white light source will obtain high usefulness if it is used for a task that requires a high level of alertness or high visual performance. If the aim is to illuminate an office, a multi-channel LED light source programmed to have an intense red component to oversaturate red and orange objects achieves only a small usefulness value because this oversaturation is disturbing for office work. However, the same light source might obtain a high usefulness value when it illuminates a theatre scene in which a strong emotional effect is required. To derive a formula for usefulness, Rea and Bierman’s universal luminous efficiency function,U(λ), is a good starting point because it does indeed "provide multiple benefits to users". But in the next step, the resulting universal luminous efficacy shall be multiplied by the above mentioned weighted sum of the descriptor quantities of the different benefits, which cannot always be expressed as single integrals because they might be spectrally non-additive. This provides an appropriate target quantity for the spectral optimisation of a multi-channel LED engine for the given lighting application and the given user