Control and regulation
When utilising daylight to reduce artificial light, there are two distinct principles:
daylight-dependent control, and
control (see footnote) means that the amount of available daylight is measured at the most appropriate location to determine the additional amount of artificial light that needs to be generated by the electrical lighting installation to provide the visual task area with sufficient light (see figure a);
regulation means that the sum of incident daylight and artificial light is measured in the visual task area. Whenever daylight is insufficient for providing the required illuminance for the visual task area, it is supplemented with artificial light. The sensor's detection range for this purpose is located in the visual task area. The artificial lighting is added by the control device in a fashion that keeps the measured illuminance constantly at the required level, the so-called nominal value (see figure b).
In indoor workplace lighting applications, regulation has proved to be the superior and more reliable principle in most cases. Control, however, is difficult. The difficulty consists of the fact that the dependence of the available amount of daylight in the visual task area on the one available at the measurement location outside of the visual task area is usually hard to determine. This is due to the following reasons:
Daylight measurement must usually be performed outside of the artificially illuminated room to avoid distortions of the measurement by artificial light components.
The incidence of direct daylight (sunshine) and diffuse daylight (cloudiness) has a different respective impact.
There are great variations in daylight incidence according to time of day and season.
The influence of shading devices is hard to calculate.
The influences which would need to be registered and calculated in addition for control are contained in the measurement value in the case of regulation.
On the other hand, the following factors are required to ensure reliable lighting regulation:
the measuring plane is located in the visual task area,
there is sufficient reflectance within the visual task area,
there is diffuse reflection in the visual task area,
the measuring plane underneath the sensor is sufficiently large and the average reflection is temporally uniform, and
there are no significant distortions by interfering light on the measuring plane.
This can generally be achieved with little effort by choosing a suitable position for the sensor.
In practice, interfering light can be generated by reflections of bright, daylight-illuminated surfaces in the sensor’s detection range (see figure), but also by e.g. indirect luminous flux components of suspended luminaires.
Per definition, the term control is the umbrella term, which also includes regulation. Regulation is thus a special case of control. In this context, the term control describes cases where regulation is not performed.