The measurement of luminaires has traditionally been performed with a goniophotometer which measures the light level from the light fitting at one angle of azimuth and elevation at a time. The device under test is mounted on a motorised goniometer stage and a photometer views the light source from a distance.
Luminaire manufacturers as well as lighting designers refer to so-called "photometric data" for a light fitting. By this, they mean standardised data files that describe the variation of light from the luminaire as a function of angle. The two most common file formats are IESNA (.ies) and EULUMDAT (.ldt). Lighting designers use these photometric files in conjunction with lighting design software in order to calculate the number and positioning of light fittings necessary to achieve the desired level of illumination in a building or street.
Luminaires have traditionally been measured using an illuminance photometer that measures the light level in lux in the photometric far-field. The far-field is that distance where the light source behaves as if it were a point source of light, hence the need to work at a distance of between 10 and 20 times the size of the luminaire. Measurements are reported in units of candelas; the candela is the unit of luminous intensity and in the far-field, intensity is the product of illuminance and the square of the distance between the light source and photometer.
There are several, additional metrics used to describe the performance of a luminaire. Luminous flux is the total light output and is reported in units of lumens. If you have a sufficiently large integrating sphere, you can quickly measure the total light output of a light fitting, but a goniophotometer will normally calculate the luminous flux based upon a summation of the directional luminous intensity values integrated over a hemisphere. The light output ratio (LOR) is a measure of the efficiency of the luminaire and is calculated as the ratio of the luminous flux of the luminaire to that of the lamp or lamps installed in the light fitting.
The cost of a typical far-field goniophotometer system (combined with the associated, large dark room) can be prohibitive, which has meant that most luminaire manufacturers have used the services of independent test laboratories in the past. However, the recently developed PM-NFMS near-field goniophotometer from Radiant Imaging changes all this by exploiting the latest advances in imaging photometry to make luminaire measurements more accessible and affordable.
Rather than using an illuminance meter in the photometric far-field to record illuminance as a function of angle, the PM-NFMS employs a ProMetric CCD imaging photometer to record spatially-resolved images of the near-field luminance emitted from the light source. Luminance (units of candelas per sq. meter) is the optical property one measures when working up close to the light source. Spatially-resolved images of the source luminance are recorded in Radiant Imaging's proprietary ProSource (.rs8) format for one angle of azimuth and elevation at a time. The associated, motorised goniometer stage scans the device under test over ± 88° in all directions. Radiant Imaging's ProSource software then performs a ray-tracing operation to scale the near-field luminance readings to equivalent far-field illuminance values at the click of a mouse. Standard photometric files in the IESNA (.ies) and EULUMDAT (.ltd) format are then generated. The photometric data reported also includes the light output ratio (LOR) as well as the integrated luminous flux.
Not all light fittings require a goniophotometric measurement. For downlighters, or for any luminaire that has a narrow beam angle, there is a simpler solution. The same ProMetric imaging photometer that is employed in the PM-NFMS goniophotometer can be used on its own to record a beam pattern that illuminates a white screen. This is a spatial illuminance measurement but the ProMetric software converts these readings into luminous intensity versus angle from which IES files are generated.
Pro-Lite was invited to give a talk on the metrics and methods for testing solid state lighting at the Ultra Efficient Lighting seminar held during the Photonex exhibition in October 2009
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Pro-Lite offers measurement services for LEDs and solid state lighting luminaires.