Accurate Determination of the Near Field Illuminance from Narrow Beam LED Arrays using Ray Files

Luminaires are conventionally modeled using a far-field representation. To calculate this representation, a photometer revolves around a light source at fixed distance and illuminance is measured in a set of angular directions. The far-field intensity of a light source, also termed luminous intensity distribution (LID) can be calculated, using the inverse-square-law, which states that illuminance decreases with the square of the distance to the source. For Lambertian light sources, a far-field representation yields accurate results from a distance of five times the maximal dimension of the source. This distance is often called the limiting photometric distance. Before this distance, an apparent intensity is measured. As opposed to the far-field intensity, the apparent intensity does depend on the measurement distance. Moreover, it depends on the dimensions of the source and its beam width. When increasing the measurement distance, the difference between the apparent intensity and the far-field intensity becomes smaller.