LED & OLED Applications
LEDs are being used in ever increasing numbers in lighting and display applications, ranging from solid state lighting luminaires, indicator lights, display backlights, large area video displays and signage. The human eye is very sensitive to brightness and colour differences and the problem with LEDs is they cannot be manufactured with consistent optical properties, even within the same batch. This makes measuring LED output an important activity, but their small size, narrow spectral emissions and directionality mean that traditional photometric and colorimetric measurement methods are not always the most appropriate. Here at Pro-Lite, we have been working with LEDs for over 20 years and we understand the science of optical radiation measurements. 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). With the proliferation of high brightness LEDs used for illumination, measuring the total flux of LEDs has become widespread. With OLEDs, measuring luminous flux allows you to compute the photoluminescent quantum yield of the material. 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.
An integrating sphere can be used to measure LED flux in two ways. Larger integrating spheres hinge open to allow the sample to be placed in the centre of the sphere. In this configuration, you record the luminous flux from both single emitters as well as LED arrays integrated over 4pi steradians of solid angle. Alternatively, you can place the LED at the input port on a small sphere or the external source port on the wall of a large sphere and record the partial (or forward) flux integrated over 2pi steradians. This latter approach is well suited to LED downlighters and has the benefit that the LED mount, heat sink and drive electronics remain outside of the sphere, which minimises absorption errors.
A recent development is the Labsphere "Half Moon" integrating hemisphere. These are ideally suited to measuring the output of LED-based solid state lighting. By mounting the sample and its drive electronics outside of the (hemi)sphere, thermal management problems are much reduced and measurement speed and accuracy are improved.
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 LEDs installed in the luminaire.
Luminous Intensity Measurement
Luminous intensity is the luminous flux emitted per unit solid angle, measured in candelas (cd). Intensity is what you measure when the lamp is a point emitter, in other words when you are in the photometric “far-field”. Conversely, when you move up close to the light source (the “near-field”), you transition to measuring luminance in candelas per sq. meter (cd/m2). Consider an array of 100 by 100 individual LEDs used in a video display panel. You would measure the luminous intensity of each LED but when you view the display as a whole, you would measure the luminance (or brightness). However, if you were to move sufficiently far away from the LED panel that it becomes a point source, you would then measure the luminous intensity of the panel.
Intensity should be measured in a defined direction; unless the lamp is isotropic, intensity will vary with direction of view. Intensity measurements can be performed with an illuminance photometer (or for improved accuracy a spectrometer configured for spectral irradiance measurements) placed approximately 10 to 20 times the source size away. This records the illuminance (in lux), and the conversion from lux to candelas is performed by multiplying the illuminance by the square of the distance between the lamp and the detector.
The problem with LEDs is that they don’t conform to the ideal of a point source which meant in the past that measurements of far-field intensity were highly instrument-specific. CIE's publication number 127 addresses this by defining standard measurement geometries. The CIE geometry defines – in effect – a near-field, or average intensity. A photometer (or spectroradiometer) with a collection area of 100mm2 views the LED at a distance of either 100mm (condition B) or 316mm (condition A), equivalent to view angles of 0.01 or 0.001 steradians respectively. CIE 127 applies to the measurement of individual LED emitters only. The Labsphere I-1000 and I-2000 are sensor heads that provide for measurements of the spectral radiant intensity of individual LEDs in accordance with CIE 127, condition A & B.
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
An alternative approach can be taken when measuring the spatial illuminance from sources such as projectors or lensed LEDs. 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.
Luminance is the luminous flux per unit area per unit solid angle emitted from a light source, measured in units of lumens per square meter per steradian, or in candelas per sq. meter (cd/m2). Historically, luminance has also been measured in units of nits, where 1 nit equals 1 candela per sq. meter. Luminance is the photometric term for what we think of as "brightness". Luminance is what you measure when you are in the near-field viewing an extended light source; intensity is the related far-field measurement for a point source. Luminance photometers (or for higher accuracy spectroradiometers) employ a lens to image a defined area or "spot" on the light source. Unless the light source is Lambertian, luminance can vary with viewing angle, hence it is necessary to specify the direction of view when making luminance measurements. Luminance is the property you would measure with an OLED panel or an array of LEDs, for example an LED video display.
The Konica Minolta LS-100 and LS-110 are simple, affordable, hand-held photometers that provide for spot luminance (brightness) measurements. The Konica Minolta CS-100A is the colorimeter version of the LS-100. The accuracy of filter photometers is always reduced when measuring narrow spectrum light sources such as LCDs and LEDs. The Konica Minolta CS-200 is a hybrid "spectral colorimeter" that combines the ease-of-use and relative affordability of the CS-100A with a colorimetric accuracy approaching that of the research-grade Konica Minolta CS-2000 spectroradiometer.
As the name suggests, "spot” photometers measure the brightness and colour of an LED or display one spot at a time. At most, you can select from up to five measurement spot sizes, either by fitting close-up lenses or by selecting alternate measurement apertures in the photometer. If a more flexible and productive (albeit more costly) solution is more your thing, you should consider an an imaging photometer, specifically a ProMetric CCD imaging photometer made by Radiant Imaging.
ProMetric imaging photometers are powerful, CCD-based spatial light and colour measurement instruments that provide for increased productivity compared with traditional spot photometers and colorimeters. Whereas a spot photometer can only measure the brightness and colour of one point on a display or light source at a time, a CCD-based ProMetric photometer can measure literally millions of points simultaneously. The measurement spot size 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 display at once, localised luminance and colour differences (defects) can be easily detected automatically – artefacts that spot meters might miss.
A number of application-specific, "bolt-on" software applications have been developed for the ProMetric imaging photometers for those working with LED arrays, OLEDs and LED displays and backlights:
Chromaticity, Colour Temperature & Dominant Wavelength
The colour of an LED 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 LEDs, we can refer to the correlated colour temperature (CCT), reported in units of Kelvin (K). An LED 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, an LED 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.
For coloured LEDs, CCT has no meaning and instead we use the simplified colour metric called dominant wavelength, measured in nanometers (nm). It is a measure of the hue (or colour sensation) produced by the LED and should not be confused with peak wavelength.
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 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 solid state lighting luminaires. The problem of poor colour rendering can be easily understood if you consider that you can mix the light from a blue and a red LED to create white light. However, if you then illuminate a green surface with this light, the green surface will not appear green. The colour rendering index (CRI) defines how well colours are rendered by different white light LEDs 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
LEDs are not isotropic light sources, meaning that their intensity varies with direction of view. View angle is the simplified metric that defines the angular extent of emission. Generally, the view angle of an LED is taken to mean the angular range over which the LED’s emission falls to 50% of that at its peak. The instrument used for measuring the angular variation from a light source or display is called a goniophotometer. For individual LEDs and solid state light luminaires, the measurement made is of luminous intensity versus angle in the far-field. For an LED video display, the measurement made is of luminance versus angle in the near-field.
Instantaneous hemispheric view angle measurements for individual LEDs or small clusters can be made using the Radiant Imaging IS-LI and IS-LI TE Imaging Spheres. The 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 a luminaire and 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.
The same goniometer hardware that is used in the PM-NFMS can be employed with slightly different software for measuring the view angle performance of an LED video display. This is the PM-FPMS near-field goniophotometer. PM-FPMS provides fully automated measurements of brightness and colour uniformity as a function of viewing angle for both LED displays and backlights. Because the PM-FPMS system captures a full image of the display or backlight at each measurement angle, it provides much more comprehensive information than can be obtained from a spotmeter-based measurement.
Near-Field Luminance & Radiant Source Model Files
It is rare to want to measure the near-field luminance from an individual LED, but for optical designers this data is invaluable for modelling how an LED 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 an LED 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.
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.