Illuminance is the most important and most frequent measure in lighting installations. It is measured at the centre of the measuring grid which either spans the entire room, the working area or only the workstation using luxmeters. Luxmeters consist of a photoelement (photoelectric receiver) and most often a digital display device directly calibrated in lux. It is important for the measurement that neither people nor objects nor part of the standard equipment are blocking the light receiver. For the purpose of occupational health and safety, however, it can be useful to measure illuminance at normal posture of the person at the workstation. From the measured point illuminance values, average illuminance and its uniformity are calculated (see section Evaluation).