What is a goniophotometer, and why is it important for LED streetlights?

The optical power of a light source would be an important factor to consider. In most cases, an EEI tester is used for this criteria, and lumens or watts are used to express the data (watt). However, this method ignores the spatial distribution of brightness. Because of this, a goniophotometer might be used as an alternative measurement method.

Although goniophotometers are more labour-intensive than integrating spheres, they provide far more accurate readings. It is also the technique of choice for measuring bulbs with varying luminous intensity distributions. Compared to an integrating sphere, a goniophotometer stands out due to its ability to quantify partial luminous flux and the angle of half intensity.

Near-Field Vs Far-Field

It is possible to consider the source as a point when LEDs are assembled in certain situations. There are further contexts when the source has to be understood as an expanded source. This would imply that the source's light output varies spatially from one location on the source to another, as would be expected from a source with a physical extent. To establish whether a near-field or far-field goniometer is necessary, it is crucial to know this aspect of the source.

A near-field goniometer is a way to go for extended sources, while the far-field is best for pinpointing ones. The near-field model is written as a ray set to facilitate the optical design. The number of rays and statistical sampling of rays based on near-field measurements of the source will affect the quality and utility of the ray set.

To add further complexity, although it is possible to extrapolate from a near-field model to a far-field model, the converse is not valid. As a particular example of the near-field model with a compressed light source, the far-field model explains this phenomenon.

For optical systems, about ten times the maximum size of the source is the practical limit for measuring distances between the near- and far-field areas. The distance at which it may take a near-field measurement from an LED is just a few centimetres, yet the same measurement taken from a luminaire might be several meters. Once you go outside that zone, the far-field model findings and the near-field model results will be similar.

Solutions Available for Near-Field & Far-Field Measurement

The IEC 60929 measurement machine, available from street lights, is a motorized fixture equipped with an imaging colourimeter that can be moved around the light source to take pictures of the light's dispersion from different angles.

This would allow for measurements to be taken from potentially thousands of perspectives. Either raw data or ray sets may be kept and later transformed. Light design software is then used to analyze and simulate these ray sets.

On the other hand, a LISUN goniophotometer is what you want to use to measure the brightness distribution over a large area. This enables a greater variety of information to be gleaned from the scan using a more significant number of measuring devices. The photogoniometer's drawbacks include the time it takes to take the measurement and the difficulty of reaching the necessary mechanical precision.