ACCURATE DETERMINATION OF THE NEAR-FIELD ILLUMINANCE FROM NARROW-BEAM LED ARRAYS USING RAY FILES

Most lighting manufacturers represent the angular distribution of the flux of their light sources by a far-field representation, also known as luminous intensity distribution (LID.) For spatially extended sources however, the inverse square law is only valid beyond the limiting photometric distance. By consequence, this distribution does not allow for accurate illuminances to be determined in the near-field region of the source. This problem is even more pronounced for arrays of LEDs with focusing optics creating narrow beams. We gathered a ray-file of several LED arrays using a near-field luminance goniophotometer. This paper reports on measurements and simulations using ray-tracing. It shows that a far-field representation cannot be used to calculate illuminance values in the near-field. In contrast, a near-field representation allows the calculation of accurate illuminance values at any target point, irrespective of the distance to the source and its luminous intensity distribution.status: publishe