LED Array Light Laboratory
Strojkov Engineering's measurement and testing lab is equipped with the absolute latest laboratory equipment SPECIFICALLY produced for measuring LED and OLED lights, displays and arrays. There is no finer equipment anywhere in the world. With our engineering staff capabilities in optics we can also offer design improvement/optimization assistance once the laboratory tests have been completed.
LEDs are changing the landscape of lighting worldwide and across all industries. 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. This makes measuring LED output a critical activity, but their small size, narrow spectral emissions and directionality mean that traditional photometric and colorimetric measurement methods are not applicable. If you wish to have confidence in your results then only highly advanced and specific laboratory testing and measuring equipment for the LED task itself is appropriate.
Near-Field Goniophotometer in Strojkov Engineering
Strojkov Engineering's New PM-NFMS Near-Field Goniophotometer for Luminaires
A luminaire goniophotometer which generates standard IESNA & EULUMDAT photometric files
The PM-NFMS from Radiant 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).
The Near-Field Difference
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).
Comprehensive Co view angle performance characterization system for large light sources
The PM-NFMS™ system performs brightness and color measurements as a function of viewing angle for large light sources. It is designed to provide accurate near-field luminance distribution data for developers of large light sources, for automotive, transportation, architectural, and other applications. Because the PM-NFMS system captures a complete near-field model of the light source, it provides much more comprehensive information — in a much smaller measurement space — than can be obtained from traditional spotmeter-based measurements.
Extensive display analysis functions provide complete view angle performance analysis as well as extrapolation as an illumination distribution
The PM-NFMS system software supports light source performance analysis through an extensive variety of quantitative analyses, including isoplots, CIE color analyses and radar plots. In addition, the PM-NFMS software can be used to extrapolate a far-field illumination distribution from the measured near-field RSM. Because the illumination distribution is derived from the near-field data, this results in a substantial reduction in the space required to perform these measurements when compared to spotmeter-based methods where the measurement instrument must be place in the far field. The spotmeter cannot provide near-field data either.
• Near-field luminance distribution models for large light sources
• Generation of illumination distributions for large light sources in a compact space
• Illumination modeling of automotive and other headlamps
• Illumination modeling of room and architectural lighting systems
Some of the most critical measurements include:
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.
The Radiant Imaging PM-NFMS near-field goniophotometer in Strojkov's laboratory 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.
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 ? to the direction of illumination is reduced by the cosine of ?. 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 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 must use 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.
These software applications have been developed for the ProMetric imaging photometers for those working with LED arrays, OLEDs and LED displays and backlights:
PM-LED Measurement software accurately determines the colour and brightness of each individual LED emitter in an array or panel. Typical applications include LED panels, display signs, traffic signals, luminaires and automotive instrumentation. A simplified user interface automates many aspects of testing, including panel alignment.
PM-OLED software simplifies the measurement of OLED displays at the substrate, display and pixel level in both R&D and in on-line testing. Advanced analysis capabilities automatically check the luminance, chromaticity and uniformity of the OLED device and identify pixel and sub-pixel defects, line defects, mura, pixel shrinkage and track changes over time. OLED pattern generation is also provided.
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.
For far-field measurements of a luminaire and the generation of standard "photometric" data, Strojkov is equipped with the world-class Radiant Imaging PM-NFMS goniophotometer. 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 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.
PM 1200 Series™ Imaging Colorimeter /Photometers is a highly accurate, CCD-based camera system designed for making precise, spatial measurements of luminance and chromaticity. compromise.
Critical parameters to consider are:
The ability to accurately match measurements to a color coordinate system, such as the CIE or L*a*b* Color Space; proper selection of color filters and precise calibrations are required to achieve this.
The absolute accuracy of luminance measurements; calibration to compensate for system noise and photopic filter accuracy are the major factors that improve luminance accuracy.
The number of shades of gray into which the system divides luminance measurements; larger dynamic range together with low measurement noise yields greater measurement precision and enables measurement of higher contrast ratios within a single image.
Determines the ability to distinguish fine detail within an image; for a given field of view, higher CCD pixel resolution means greater spatial detail.
Imaging and Readout Speed:
This is the time required to capture and readout an image from the CCD. There is often a trade-off between speed, noise and the image detail that can be captured.
Interline vs. Full Frame CCD:
The type of CCD used determines parameters such as speed, accuracy and system cost. Interline CCDs allow fast imaging and are generally lower cost; full-frame CCDs capture more complete image data with typically higher dynamic range and lower noise if cooled and temperature stabilized.
Field of View and Aspect Ratio:
These parameters are determined by the CCD size and the lens selected. Field-of-view will determine the required working distance between the imaging colorimeter and the object being imaged. The aspect ratio of the CCD — square oe rectangular — can be chosen to suit the application.
Strojkov is equipped with the ProMetric PM-1200 Imaging Photometer/Colorometer
of View (half angle)
|+++||12 bits(4,096)||+++||±3%||1392 × 1040||Interline
Our ProMetric PM 1200 simplifies measurement set-up and control and provides easy access to a broad range of image analysis capabilities, including 2D and 3D plotting, histograms, defect detection, and Fourier analysis. Software functions can be externally controlled through PMEngine™ .Net controls (Framework 2.0) so users can build custom test and analysis sequence in Visual Studio 2005 or other .Net compatible programming languages. A limited set of ActiveX controls is also supported.
The PM-1200 uses a 12-bit (4,096 gray levels), 1,392 × 1,040 pixel, interline transfer CCD for high speed imaging along with CIE matched color filters for accurate color measurement. This delivers superior color accuracy compared to systems using color filters integrated on the CCD in a Bayer pattern. The optional integrated neutral density filters are on a filter wheel, so that all imaging configurations of color and ND filters are controlled usingProMetric software. Multiple lens options allow the PM-1200 to be configured with a field of view that matches the application requireme.
REMOTE FIELD MEASUREMENT CAPABILITIES of Strojkov's Lab:
Jaz Modular Optical Sensing Suite
Jaz is like nothing you’ve ever seen before -- a community of stackable, modular and autonomous components that combine to create a family of smart sensing instruments.
Jaz is unfettered by the limits of traditional optical sensing instrumentation. Its unique features and expandable platform makes it uniquely suited for field applications, remote sensing, process flow and quality assurance.
NEW! Jaz is now available with an LED module that allows you to switch out LED bulbs much more easily. Now, instead of having to replace the entire module, all that’s necessary is to replace the LED assembly – a small fixture with just three screws to manage. We offer modules and bulbs for white LEDs and for 450 nm, 590 nm and 640 nm wavelength LEDs. Consult an Applications Scientist for details.
• Rechargeable Lithium-Ion battery
• Up to 8 spectrometer modules
• Powerful microprocessor and onboard display eliminate the need for a PC
• Stackable, autonomous instrument modules allow you to customize the system to your changing application needs
• Ethernet connectivity plus an SD card for data storage make remote operation a snap!
Jaz Spectrometer Module
• Replaceable slits take spectrometer design to a new level
• Though the crossed Czerny-Turner optical bench design may be familiar, the rest of the bench is anything but conventional. For maximum flexibility, external slits are designed to be easily switched out by qualified users.
Jaz OLED Display Module
• Powerful onboard microprocessors eliminate the need for a PC while the OLED display offers clear, vivid viewing.
• The Jaz OLED module is the user interface. Its powerful microprocessor is the “director” harmonizing the interaction among the modules. The Jaz OLED module handles data processing and logging, distributed computing and user interface functions in the display.
Jaz EB Ethernet and Memory Module
• Ethernet supplies system power, makes remote access possible and provides memory and other functions.
• Our 100 Mb/S Ethernet connection is a “single-cable solution” that powers the system and enables remote access by any computer on the network. Communication can take place among modules that are plugged in anywhere on the World Wide Web. The module also includes an SD card slot for data storage.
Jaz MB Battery and External Memory Module
• Rechargeable battery and data storage functions take the hassle of external power supplies and PC handling out of field work.
• Lithium-Ion battery is rechargeable from the main board via Ethernet, USB or external power supply and allows autonomous data collection with power-conserving sleep mode for long-term measurements. This module also has two SD card slots for memory and other functions.