Pro Lite Technology

Lamps & Lighting Applications

Lighting

You would think that LEDs have taken over the world but traditional lamps and luminaires still need to be measured against application-specific requirements for light output, colour, and illuminance. Pro-Lite specialise in photometery and colorimetry and choosing equipment from us means you can tap into our expertise and have confidence in your results.

Luminous Flux Measurements

Luminous flux is the total photometric power emitted in all directions from a light source, measured in lumens (lm). Luminous flux can be measured using a Labsphere integrating sphere photometer, the diameter of sphere being dictated by the size of the device under test. For improved accuracy, a Labsphere integrating sphere spectroradiometer measures the spectral radiant flux and computes the luminous flux and colour parameters. Uniquely, spectroradiometers also provide colour rendering information and can apply both daylight (photopic) and night time (scotopic) observer functions.

Integrating spheres are the fast way of measuring flux. However, the Radiant Imaging PM-NFMS near-field goniophotometer can calculate the luminous flux of a luminaire by integrating directional luminous intensity over 2pi steradians. This type of measurement is often performed on light fittings in order to calculate their light output ratio (LOR), which is a measure of their efficiency, being the ratio of the luminous flux emitted by the luminaire to that of the lamps installed in the luminaire.

Illuminance Measurements

Illuminance is the luminous flux per unit area received at a surface, measured in units of lumens per square meter or lux (lx). For a point light source in the photometric far-field, illuminance decreases
with the square of the distance away from the lamp. This is the so-called “inverse squared law”. The illuminance at a surface tilted at an angle θ to the direction of illumination is reduced by the cosine of θ. It is for this reason that an illuminance photometer (or for improved accuracy a spectrometer configured for spectral irradiance measurements) is normally equipped with a cosine diffuser which scales the off-axis illuminance to take account of the reduced illuminance at higher angles of illumination. Luminous flux and intensity are intrinsic properties of a light source, whereas illuminance varies with the distance from the source. Consequently, measurements of the illuminance of a luminaire must be performed at a defined distance.

An alternative approach can be taken when measuring the spatial illuminance from sources such as projectors or flashlights. ProMetric imaging photometers are powerful, CCD-based spatial light and colour measurement instruments that provide for increased productivity compared with traditional photometers and colorimeters. The beam of light is shone onto a matte white screen and the ProMetric photometer measures the illuminance, luminous intensity and colour of literally millions of points simultaneously. The measurement area can be selected in software after the measurement has been made, and moreover, any number of analysis points can be defined - and recalled - as required. In addition, because the ProMetric camera views the whole illumination pattern at once, localised illuminance and colour differences can be easily detected – artefacts that spot measurements might miss.

Chromaticity, Colour Temperature & Dominant Wavelength

The colour of a lamp is expressed in a variety of ways. The perceived colour of a light source depends upon its spectral power distribution, the human eye's tristimulus response and the relative amounts of red, green and blue in the light. To simplify the reporting of colour, we normally quote the CIE (x,y) chromaticity coordinates (1931 2° observer).

However, a further simplification can be made and for white light sources, we can refer to the correlated colour temperature (CCT), reported in units of Kelvin (K). A lamp with a CCT of about 3,500 K is referred to as being "warm white" due its output being weighted more to the red end of the spectrum. Conversely, a lamp with a CCT of about 5,000 K is considered to be "cool white" due to the light being weighted more to the blue end of the spectrum. Ironically, the higher the (colour) temperature, the "cooler" the light.

Colour is measured either using a tristimulus filter colorimeter, a spectroradiometer or a spectrometer configured for spectral irradiance measurements. The accuracy of filter colorimeters is always reduced when measuring narrow spectrum light sources such as CFLS or LEDs. For improved accuracy, a spectrometer or spectroradiometer measures the spectral power distribution and computes the photometric and colorimetric parameters. Uniquely, spectrometers and spectroradiometers also provide colour rendering information.

Colour Rendering Measurements

Colour rendering in an important metric for lamps and luminaires. The colour rendering index (CRI) defines how well colours are rendered by different white light sources compared to a defined standard illuminant. Colour rendering can only be computed for a given light source if you know the full spectral power distribution, hence CRI cannot be measured using a tristimulus (filter) colorimeter. Instead, CRI must be measured using a spectroradiometer or a spectrometer configured for spectral irradiance measurements.

View Angle Measurements & Standard "Photometric" Data

The instrument used for measuring the angular variation from a lamp or luminaire is called a goniophotometer. The goniophotometer measures luminous intensity versus angle in the far-field.

Instantaneous hemispheric goniometric measurements for a small lamp can be made using the Radiant Imaging IS-LI Imaging SphereThe Imaging Sphere is an innovative, ultra-fast goniophotometer based upon a ProMetric imaging photometer for instananeous angular analysis of small light sources. With the Imaging Sphere, you measure the light distribution for all angles instantaneously.

For far-field measurements of larger lamps and luminaires, and for the generation of standard "photometric" data, Radiant Imaging PM-NFMS goniophotometer is available. The cost of a typical far-field goniophotometer system (combined with the associated, large dark room) can be prohibitive. However, the PM-NFMS near-field 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 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. 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.

Near-Field Luminance & Radiant Source Model Files

It is rare to want to measure the near-field luminance from an individual lamp, but for optical designers this data is invaluable for modelling how a particular lamp will perform in an optical system. Near-field data is exactly what you need to know if you are to analyse how the light from a lamp will interact with lenses or other optical elements placed close to the emitter.

Radiant Imaging's Source Imaging Goniometers (SIGs) are fully automated, computer-controlled goniometric systems that use a ProMetric CCD imaging photometer to capture a precise model of a light source’s near-field output. The image data and the Radiant Source Model (RSM) file generated from it provide a complete and precise characterisation of the light source output that can be used for design evaluation and imported into any major optical design software to allow accurate modelling of a lighting system.

The measurement data collected by a SIG is formatted as a .rs8 file, which contains information on luminance versus angle and image data. Ray sets containing an arbitrary number of rays can be generated from a .rs8 file by Radiant Imaging's ProSource software for export to other optical and illumination system design software packages such as ASAP, FRED, LightTools, LucidShape, Opticad, OSLO, SimuLux, SPEOS, TracePro and Zemax. A generic file format is also available for use with other optical design programmes. Ray sets generated by ProSource from RSMs are more efficient than random Monte Carlo generated ray sets as they contain equivalent information with only 20% of the number of rays - resulting in reduced optical design time and models with higher accuracy.

Related Products
Presentation

> Testing Solid State Lighting

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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