Near-field and far-field goniophotometry of focused LED arrays

Luminaires are conventionally modeled using a far-field representation. To calculate this representation, a photometer revolves a light source at fixed distance and illuminances are measured in a set of angular directions. Using the inversesquare- law, the far-field intensity, also termed luminous intensity distribution is then calculated. For Lambertian sources, the far-field starts from a distance of five times the maximal dimension of a light source; which is called the limiting photometric distance. The advent of luminaires composed of LED arrays with narrow beams have shown that this limit is no longer valid and far larger distances (up to 15 times the maximal diameter) are suggested by the lighting community. This problem is even more outspoken when the individual LEDs are focused at close distance, as in e.g. surgical luminaires. To overcome these problems, we exploit the use of a near-field representation to describe an array of two narrow-beam LEDs focused at close distance. For such a test source, this paper shows how a near-field luminance goniometer is able to construct ray-data. Ray files can be used to calculate a near-field representation and far-field representation of a light source. These measurements are validated by a theoretical derivation of the intensity of an array, using a simple analytical model to describe the emission of the individual LEDs. This near-field approach makes discussions to determine the far-field photometric distance superfluous.