LED products quality assurance has been discussed a lot in the last several years. Many standards and system solutions were developed to control the lighting product quality. Lamp manufacturers and end-users has been usually focusing on better product performance, efficacy and product compliance.
This paper provides a detailed description of a novel solution which combines the CAD lighting design software with the on-site measurement supported by new technology illuminance spectroradiometers able to handle measurement procedure applied to verify and report lighting levels and other quality measures using CAD plans as a reference and reporting illuminance level back to the design software. It also discusses the use of Imaging Luminance Measurement Devices in the practical verification of luminance levels inside and outside buildings. Available design tools support designers with calculation and simulation of luminance levels but little is available when the onsite software and hardware tools are available for onsite verification. This is especially critical for verification of architectural and road lighting installation.
Citius, Altius, Fortius, which is Latin for “Faster, Higher, Stronger”. An Olympic slogan could easily be adopted to the modern LED lighting industry race that we are all observing nowadays. This market is moving very fast and almost every member of this industry is trying to deliver faster, better and more reliable products. At the same time with the increasing number of technical possibilities and expanding knowledge about the influence of light we struggle to tell the good from bad and make right decision regarding the selection of components and choosing the right products.
LED products quality assurance has been discussed a lot in the last several years. Many standards and system solutions were developed to control the lighting product quality. Lamp manufacturers and end-users have been usually focusing on better product performance, efficacy and product compliance.
However nowadays companies working to WELL Building Standard or those in need for California Title 24 standard compliance struggle with onsite lighting quality evaluation. This is primarily caused by the extended scope of new metrics and measurements needed for the verification of the quality of a complete light installation. There is also a huge gap between the Computer Aided Design software world and a real life lighting installation. Therefore, a growing need for dependable and practical on-site LED lighting verification has recently been observed.
Old Standards, New Metrics
Traditional approach for on-site lighting audits was focused on photometric quantities like illuminance or luminance. These are the two most important values for onsite lighting installation evaluation. They are included in a majority of lighting standards around the world as well. Therefore, lighting designers and lighting companies use these quantities at the design stage and also as a verification metrics when they deliver lighting solutions.
It should be noted that the above mentioned values are only based on a photometric approach providing information about the amount of light in a quantitative manner. Due to the fact that our knowledge about the influence of light onto the human body and performance is increasing and the technology is able to support the effective use of lighting to increase the well-being of people. There is a need for software solutions and measurement instrumentation tools for which could help in the verification of lighting quality.
Generally available CAD based lighting design software uses photometric data and is not able to handle other qualities beside the colour temperature that one can specify for the project as general information. Despite this, the good practices used by some designers and manufacturers aiming at delivering better lighting systems, superior to competitor products, are ready to verify additional quality measures to check the quality and prove the advantages of the solution they deliver. Approved new metrics which are valid for lighting product verification and which are commonly used for lighting audits purposes are available. They have not yet been agreed by standardization organizations as part of lighting audits requirements. These are newly approved rendering indices according to IES TM-30, Spectral Analysis as a basis for effective lighting verification, Equivalent melanopic lux (EML) suggested by LRC and used by WELL Building Standard to verify circadian lighting and all different flicker parameters which can be measured too at present as additional quantifiers.
Table 1: Traditional Light Quality Metrics vs. Modern
The traditional approach was focused around photometric values and traditional photometers were suitable to perform the lighting audit. Usually lux meters were, and still are, used for illumination i.e., lux level verification. And for luminance measurement simple spot luminance meters with a fixed field of view were used. The new augmented approach to light quality verification though requires the use of more advanced instruments, light spectral illuminance meters and imaging luminance meters which can be connected to a software interface and which are able to provide more information about the light quality beside information about the quantity.
There are two major obstacles slowing down common use of the new approach in lighting audits and these are: the accessibility of the instruments able to perform such evaluation and the fact that these new metrics are not yet the part of international standards. The industry and organizations such as Lighting Research Centre (LRC) or WELL are moving towards the more advanced lighting verification and put additional, newly proposed metrics as criteria to verify the conformance of lighting installations. At the same time official mandatory regulations based on standards do not cover these new factors at all.
The accessibility issue can be explained by the instrument pricing because in order to perform an onsite measurement that covers additional metrics there is a need for a reliable spectroradiometer which is obviously more expensive than a standard lux meters commonly used on a building site. This is why these instruments are not yet accessible for everyone in the industry. Another issue is that despite the fact that there are a number of portable spectrometers available on the market, not many are properly calibrated or accurate enough for these more advanced applications and not all of them are able to calculate new values.
New Concept for the Illuminance Lighting Audits
Every lighting installation should be verified before it is commissioned. This requirement, depending on the lighting installation and its location, is either mandatory by law or required by the contract. The new concept for modern lighting audits was particularly triggered by the requirements developed by large lighting companies. They are in need for practical instruments which can use the digital data they have available from the CAD design software which is generating the measurement grid layer with designed illumination levels. The concept was to use this data as reference values and provide the software and hardware solution to be able to verify the existing lighting levels onsite. The process should support the auditing person in the measurements and also allow for the preparation of a comprehensive report.
The R and D team of GL Optic assigned software and hardware engineers to develop a solution to close the gap between the available sophisticated CAD design tools and a real life installations. The objectives of the project were as follows:
- Single instrument to measure lux and other parameters
- Interface to communicate with CAD design software to obtain reference data
- Firmware supporting the onsite measurements process
- Reporting tools to compare the measured values with the designed lighting levels
Figure 1: Spectroradiometer with flicker measuring module
For the purpose of this development only the lux values verification was considered. Nevertheless, each measurement data file incorporates additional spectral characterization which can be used for colour and active radiation verification for specific application like human centric lighting metrics or light flicker evaluation. This concept has been designed and evaluated with the use the DIALux software where the lighting design was made.
The principle is as follows:
- Lighting design is made in DIALux and the measurement grid including the designed illumination levels is exported to the CAD software as a layer.
- This layer is then imported to the GL Spectrosoft where the selected points for the verification are marked to create the list of points for the onsite verification.
- The list is uploaded to the measurement device the GL SPECTIS 1.0Touch + Flicker
- Now the lighting audit can be made on-site and the subsequent measurements are made point by point with the device and the operator can observe on the screen the actual measured point number and can locate this on the general plan.
- Once the measurements are made the list including the measured values can be imported to CAD software as an additional layer.
- The measured values are displayed next to the design values, which simplifies verification and supports comprehensive report preparation.
New Concept for Luminance Lighting Audits
Luminance distribution is another challenge for engineers performing onsite lighting audits and it is the most important quantity in lighting design and the most important measure in road design and architectural illumination. LEDs are very bright light sources, which are characterized by high light efficacy. Unfortunately, the annoying effect of glare is a specific side-effect of LED construction. The assessment of glare is dependent on the luminance value in relation to the background luminance level. Designers and lighting companies often use design software and the available visualisation functions for lighting installations, where luminance plays a key role. However, there is a big discrepancy between the assumptions of the designer and the design and the actual implementation of the lighting system. As the technological possibilities of LED luminaire construction and lighting control increase, the need for more reliable verification of lighting quality at the stage of completion of construction works increases. The client, the customer or the contracting authority more and more often demand a report and a declaration of conformity of the installation with the design and the order.
Traditional luminance meters using a single lens optical system and appropriately adjusted photodiode, the so-called spot luminance meters, allowed for precise targeting and precise focusing at a small point. They are fast and reliable, but have two main disadvantages due to their technical characteristics. Firstly, they usually have a very small angle of view, i.e., from a given distance we can measure an area (point) with relatively small dimensions, which in the case of luminance distribution measurements forces the user to make multiple measurements and calculate the luminance distribution of a given area manually. Secondly, the matching class of a photodiode with a V-filter (lambda) may cause more or fewer measurement errors depending on the type of light source or the colour temperature. These meters were perfect for measurements of light sources with a wide range of radiation. On the other hand, in the case of some white LED sources, especially RGB systems, the errors can be up to 20 percent for a good class meter.
New solutions available on the market, using CMOS or CCD sensor technology, commonly used in digital cameras, offer far greater possibilities to use this technology both for measuring and testing the luminance of illuminated surfaces, as well as illuminated elements and the luminaires and light sources. With the use of a camera luminance measurement system, the so-called Imaging Luminance Measuring Device (ILMD), it is possible to perform measurements and compare luminance values on the basis of the image analysis.
These meters are equipped with a high-resolution sensor and optical system consisting of a lens and a V (lambda) filter adjusting the sensitivity of the sensor to the sensitivity of the human eye. In this way, the image recorded by the sensor is subject to computer analysis, and the recorded brightness level corresponds to the impression received by the human eye, i.e., it is the level of absolute luminance value. In contrast to an ordinary camera, where the level of brightness in different places of the image is a relative value, in the case of a luminance matrix meter we have an image showing the distribution of luminance, and on its basis we can analyse, compare and measure the level, uniformity, changes in values, etc., for each point individually or for a given area of the image.
Thanks to this technology, with the use of a matrix with several million pixels, we can record the entire image of a given surface, a lighting element or the entire interior of a building. The recorded images can be analysed in detail with the supplied software. With image analysis tools it is possible to quickly mark the fields or points of interest and easily assess, both visually and metrologically, the connections between different measurement areas. Dedicated software for such matrix systems allows for additional presentation of levels in pseudo-colours, isocandels, 3D charts, in the form of histograms, to create reports and contains many other useful functions.
When it is necessary to determine luminance coefficients throughout the scene, a traditional, point-based meter seems to be an uncomfortable solution. Point by point measurements would have to be made, which would be very time-consuming and would not be applied in practice. Similarly, the measurement of small parts cannot be carried out with such a luminance meter because the measuring angle is constant and usually not small enough. The method with an ILMD meter compared to a spot meter allows for a wider view of the scene. All the luminance information in the frame can be stored in a single image, so this method takes much less time.
Repeatability is also an important advantage compared to a point by point measurement of luminance, as the measured image can be recorded and re-assessed at a later date. Problems with filter mismatch and possible correction of indications for different types of light sources or colour temperature can be solved by combining a matrix system with a spectroradiometric system. This increases the accuracy of the luminance measurement and additionally allows for colorimetric analysis, e.g., of colour temperature, colour rendering index. In advanced systems, this is part of a complex measuring instrument. It is also possible to use a two-stage measurement for this purpose, where we first measure the luminance with an ILMD meter, and then we measure the spectral distribution of the lighting with a spectroradiometer. So with this software it is possible to combine measurement data and present consistent reports.
As the new metrics are introduced, more and more widely available and also cheaper measuring instruments and new software give hope that reliable measurements of lighting installations will improve the quality of lighting while reducing energy consumption. The future of lighting quality control system should provide a practical assistance to lighting industry as the comprehensive lighting evaluation is becoming a necessity for every day application and is no longer limited to a laboratory environment. In qualitative terms, we should be providing light where it is necessary, as much as is necessary and whenever it is needed.
Author: Miko Przybyla, COO, GL OPTIC, 70 Poznanska, 62-040 Puszczykowo, Poland
Key Image: Lycs Lycs
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(c) Luger Research e.U. – 2018