Why Are Goniophotometer Important?
One of the main characteristics of a light source would be its optical power. This criterion is typically done using an integrating sphere and the values are represented in luminous flux (lumen) or radiant power (watt). Integrating spheres measures the sum of light received by the detector and doesn’t account for its luminous intensity distribution. Hence, another approach to this quantification would be by using a goniophotometer.
While goniophotometers are known to be more time-consuming compared to using integrating spheres, it is much more precise. Also, it is the method of choice if lamps with different luminous intensity distributions must be measured. Another distinguishing feature of goniophotometer compared to integrating sphere is the capability to measure partial luminous flux and angle of half intensity.
Near-Field Vs Far-Field
For some applications where LEDs are integrated within an assembly the source can be treated as a point source. There are other applications whereby the source should be treated as an extended source. This would mean that the source that has physical extent and so has spatial variation in light output from point to point on the source. Knowing this characteristic of the source is vital as it would determine if a near-field or far-field goniometer is required.
A near-field goniometer is used to measure extended sources whereas the far-field is for point sources. The near-field model is expressed for optical design purposes as a ray set. The quality and usefulness of the ray set will be a function of both the number of rays in the set and how the rays are statistically sampled based on the near-field measurements of the source. Also, while near-field model can be extrapolated to a far-field model, but the reverse is not true. This is because the far-field model is a limiting case of the near-field model with a collapsed light source.
A rule of thumb for determining the measurement distance limit between the near- and far-field regions for optical devices is about 10X the largest dimension of the source. So, for an LED, the difference between capturing a near-field and a far-field measurement would be a few centimetres, but for a luminaire this might be tens of meters. Beyond this range, the far-field model and the near-field model will give essentially the same results.
Solutions Available for Near-Field & Far-Field Measurement
To characterise an LED device based on near-field measurement, a source imaging goniometer (SIG) used as shown in Fig 1.0. Radiant Vision System offers the SIG-400 which utilized an imaging colorimeter which placed on a motorized fixture and is moved around the light source and capture the output light distribution at the source from multiple. This would provide measurements of up to thousands of viewing angles. This information can be stored as raw data or converted to ray sets on the fly. This ray sets are then exported into light design software for analysis/modelling.
On the other hand, for a far-field luminous distribution, a photogoniometer would the choice. Instrument Systems offers the LGS-1000 goniometer (Fig 2.0) also uses motorized moving arms that would move the LED devices relative to the photometer or spectroradiometer sensor. This has the advantage of allowing multiple measurement devices to be employed to vary the information obtained from the scan. The disadvantage of the photogoniometer is the length of time required to make the measurement and the complexity of achieving the required mechanical accuracy. Contact us for quote