Goniophotometers.

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The word gonio (γονιο) derives from the Greek meaning angle. A goniometer is an instrument that measures angles most accurately. Photometer derives from the Greek photon (φοτον) = light and is an instrument that measures light. A goniophotometer therefore performs the measurement of the spatial distribution of a radiation source and displays the photometric properties of the light visible to the human eye in relation to a defined angular position. The automotive and general lighting industries use goniophotometers for lighting research and as a control measure in their manufacturing workflow. (Ref: optronik’s website)

Near-Field Goniophotometer Basic Principles of Near-Field Photometry Far-field photometry is based on the five-times rule, but this rule is clearly violated by linear indirect fluorescent luminaires used in many offices. As an example, consider a four-foot indirect fluorescent luminaire mounted 16 inches below the ceiling plane. Far-field photometry models this luminaire as a point source in the center of the luminaire. However, the inverse square law predicts a ceiling illuminance directly above the luminaire that is over four times what is actually measured. This was not a problem when most lighting design calculations were performed by hand using the lumen method. The average workplane illuminance was still reasonably accurate. However, the introduction of IES RP-24-89, VDT Lighting (now incorporated in RP-1, Office Lighting) with its recommended ceiling luminance ratios suddenly brought point-by-point luminance calculations to the forefront. Lighting design software such as Lighting Analysts’ AGI32 and Lighting Technologies’ Lumen Micro alleviate this problem by dividing each luminaire into an array of point sources and evenly dividing the luminous intensity distribution between them. This however assumes that the distribution is homogeneous along the length of the luminaire, which is not always true.

A more accurate approach – called near-field photometry – is to physically measure the distribution of light at distance close to the luminaire. There are two very different techniques. One is practical, while the other is (at least for architectural lighting purposes) mostly theoretical. Ref: IESNA Lighting Handbook, Ninth Edition, Chapter 2. IES LM-70-00, Guide to Near-Field Photometry

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The PM-NFMS from Radiant Zemax Imaging combines a motorised goniometer stage with a ProMetric CCD imaging photometer to perform a near-field angular analysis of the luminance and colour from LEDs, luminaires and solid state lighting. ProSource software then scales this to far-field luminous intensity data from which standard photometric files are generated (IESNA, EULUMDAT). So-called photometric data defines how much light a luminaire emits and into what directions; lighting designers then use this standardised data format with a variety of commercially-available lighting design programmes to determine the number and positioning of luminaires in order to create the desired illuminance (lux level).

Difference between The Near-Field and The far-Field The measurement of luminaires has traditionally been performed with an illuminance meter placed in the photometric far-field which views the light source at one angle of azimuth and elevation at a time. The device under test is mounted on a motorised goniometer stage (the "goniometer") and the photometer views the lamp from a distance of between 10 and 25m, the actual working distance depending on the size of the light fitting. The cost of a typical far-field goniophotometer system (combined with the associated, large dark room) was often prohibitive, which meant that most manufacturers have used the services of independent test laboratories. The PM-NFMS goniophotometer 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 the 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 cd/m2) 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 can then be generated. The photometric data reported also includes the light output ratio (LOR, a measure of the efficiency of the luminaire) as well as the integrated luminous flux (units of lumens). (Ref: Pro-lite’s Website)

Far-field goniophotometer Basic Principles of Far-Filed photomerty Goniophotometry is the measurement of the luminous intensity of a light source from different directions. A goniophotometer consists of a mechanical device to support and optionally position the light source (a luminaire or lamp) and a photosensor, together with associated electrical and electronic equipment to acquire and process the photometric data. Far-field photometry assumes that the light source is an infinitesimally small point source, for which the inverse square law applies (illuminance = intensity / distance-squared, or E = I d 2 ). This assumption generally holds true (to within ±2 percent) for most architectural luminaires when the distance from the luminaire to the measurement point is at least five times the maximum width of the luminaire (FIG. 4). This is the oft-quoted fivetimes rule for photometric measurements and calculations. (Ref: IESNA Lighting Handbook, Ninth Edition, Chapter 2. CIE 121-1996, The Photometry and Goniophotometry of Luminaires.)

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Goniophotometer SMS 10 C Website link: http://www.optronik.de/en/ Measurement Equipment for Automotive Lighting and Signal Lights and other Applications.