Problems with photometric data files.
A Guide by 42 Partners Ltd in partnership with the LIA.
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The importance of photometry for the lighting industry is undeniable. It’s the science of measuring light, which is fundamental for all aspects of our industry. A lot of photometric data in the form of LDT and IES files pass across our desks, and many of them can be problematic.
Understanding photometric testing is a challenge due to the technical complexity and lack of available training. There is a heavy reliance on manufacturer knowledge with a clear disconnect between Lighting Design and Photometric Testing.
Lighting design software like Relux and DIALux has removed the entry barrier into lighting design which has enabled more people to become lighting designers without the need for higher education or formal lighting training. Such software has also made it possible to import and use any data file and complete a full lighting scheme. There is often a blind trust in photometric data as users have a “usable file” from a lighting provider.
In this guide, we try to explain some of the common pitfalls in Photometric testing to raise awareness and provide further education to all users of photometric data.
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Included stray light
Goniophotometers measure the light distribution of luminaires. This data is usually collected in absolute intensities. Ideally, they only capture light directly emitted by the luminaire. However, some light reflects from the walls, ceiling, and goniometer itself and can be included in the measured data. This could easily be deemed an acceptable tolerance but unfortunately, this can have negative consequences for the end user when a design is compared to the installation.
Often this shows as uplight where there should be none. Uplight is often used within lighting to improve the ambience and enhance the aesthetic of a building. Erroneous uplight can be a consequence of incorrect testing and data processing. Any collected stray light artificially inflates the lumens per watt of the product making the luminaire seem more efficient than it is in reality. With the government’s commitment to achieving net zero by 2050, every Watt can count. Designs might be completed using metrics such as LENI or an end-user purchase could be based solely on the efficiency of the product used.
Stray light, particularly stray uplight, will affect glare calculations. Fittings might achieve a good glare rating because of the addition of the stray uplight but the reality could be that the installation is uncomfortable to work in as the lack of real uplight might produce discomfort glare. With an artificially inflated lumen output often a scheme could be designed to a higher light level than the installation will achieve. This can harm employee well-being or productivity.
A well-run photometric laboratory takes steps to minimise stray light and performs additional measurements to quantify its impact. This data allows for the correction of the raw goniophotometer data, removing the influence of stray light and providing more accurate results.
In conclusion, even a few percent of stray light can significantly alter the perceived performance of a luminaire. By minimising stray light and correcting for its influence, photometric laboratories ensure the accuracy and reliability of goniophotometer measurements. This leads to better informed decisions when choosing luminaires for building projects.
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Incorrect or missing dimensions
Lighting layouts rely on accurate physical dimensions to ensure luminaires don’t interfere with other building elements. We are placing luminaires in close relation to other services such as sprinklers, AC ducting and vents. It is imperative that the dimensions are accurate otherwise your luminaire layout plan is worthless. Significant investments have been made in the utilisation of BIM (Building Information Modelling) to overcome such issues but if the file is incorrect it won’t be apparent until the installation.
The second set of dimensions that is critical is the luminous portion of the luminaire. The luminous portion of a luminaire refers to the part of the fixture that actually emits light. This is typically the area where the light source is housed and where light-shaping elements like lenses, diffusers, or reflectors are positioned to control the light distribution pattern.
The main value dependent on these dimensions being correct is the RUG (UGR) table. If the luminous size is wrong the values in the standard glare table are incorrect. Often the luminous area is noted in the file as the dimensions of the whole luminaire. Usually the luminous area is significantly smaller; in extreme cases a fifth of the overall size. Having the lumen output spread over a smaller area increases RUG values often to the point where the luminaire will not meet the required glare limits.
Often overlooked by the industry, IES files only describe the luminous dimensions, there is no way for an IES file to provide physical dimensions. So either they will be incorrect on glare or make building integration very challenging.
An IES file for a luminaire with no luminous height will have a height/thickness of zero so will not be visible in your layout from certain points of view. Whole areas of a design could be calculated incorrectly, particularly if used for accent or supplementary lighting.
The key takeaway is that accurate dimensions in photometric data files are crucial for both avoiding physical clashes during installation and ensuring proper glare calculations in lighting design. LDT files offer a clear advantage in this regard by providing both physical and luminous dimensions.
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No Symmetry or incorrect symmetry
A photometric data file for use in lighting design is supposed to represent the average performance of an average luminaire of that type. Not a single unit with potential manufacturing variations. Photometric data released to clients/users should be the average data of the production run of that luminaire which requires symmetry to be applied
Commonly, poor photometric data files will have no symmetry applied. If a manufacturer is developing a luminaire, they would want the test data from the prototypes to be ‘warts and all’ so they could see if it has been manufactured correctly.
If the luminaire is meant to have some symmetry and the test data does not, either the test set-up was incorrect or the test sample was wrong. This is commonly seen in inaccurate data and can have a dramatic impact on a company’s reputation and perceived quality. If a product is supposed to have a smooth circular symmetrical distribution and it looks like two half circles badly patched together then their data and quality will likely be questioned.
Understandably this can have a massive impact on a design as the distribution of a luminaire can be completely inaccurate leading to shadows or over-illumination. It is even more critical that the lighting is correct for emergency lighting products where there are legal constraints and moral obligations.
In conclusion, symmetry in photometric data files ensures that lighting design software accurately predicts the lighting distribution pattern of the luminaire in the final installation. A good photometric testing lab should provide clients with symmetrised data files. If the file is not correctly symmetrised then it is likely the other information is inaccurate.
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Too much / not enough detail
Photometric tests should be performed using data steps small enough to record the full details of the luminaire distribution including any unexpected anomalies. Routine tests would be in 1° steps in elevation and 5° steps in horizontal with the ability to reduce this step size when required. This is test data, it is not suitable for release for use with lighting design software.
Lighting design programs calculate the average effect of a luminaire over a small area and then repeat the calculation for the next small area. Visual representations and maximum and minimum values are obtained by using the results from each of these areas, while the main calculations are based on the combined effect of all these small areas in the lighting scheme. Each area requires the calculation of an average intensity from the luminaire at various angles, the more detailed the file the longer it takes to calculate the average.
If we put this into practical application a dense opal downlight has an almost perfectly smooth Lambertian distribution. This can follow a simplified dataset with fewer entry points to reduce the calculation time. Whereas if we take a 10° narrow beam spotlight applying 5° elevation data steps would be wrong and very inaccurate. A luminaire with such a narrow distribution should have a more complicated dataset measuring the elevation in 0.5° steps.
A polar curve showing straight lines is a pretty good indication that the data steps are too wide to correctly describe the luminaire. If a file shows a very jagged polar curve, this can be a good indication of a lot of data. Providing extra detail is not harmful but it is wasteful and unnecessary and will slow the lighting designer down.
In conclusion, the ideal photometric data file for lighting design should strike a balance. It should provide sufficient detail to capture the luminaire’s essential characteristics without being excessively detailed and slowing down software calculations. The level of detail needed depends on the specific luminaire type and its distribution pattern. A good testing house should be able to review a product and provide the correct dataset to ensure your file is accurate and useable.
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File name & catalogue number
This is a relatively simple matter to make the file name and catalogue number match in some way. This makes it easy for designers to find a particular product data file from a list. A surprising number of file names have no relationship to the catalogue number. In some cases the catalogue number and file name that are supposed to match are different. This makes it very difficult for a Lighting Designer to know which data to use. Manufacturers and laboratories should make it easy for designers to find and verify your data.
The LDT file format specifies a maximum of 78 characters for the luminaire name and luminaire number, the subsequent ELX format specifies a maximum of 24 characters. The default maximum expected length for lighting design programs is now 40 characters. If key product data is featured in the latter part of the code this will not be present in the design software and not visible in the design report.
In conclusion, a simple fix like aligning file names with catalogue numbers and keeping them concise can significantly improve the workflow for lighting designers. Standardisation in this area would eliminate wasted time searching for data and cleaning up overly long file names. By prioritising clear and user-friendly data presentation, manufacturers and laboratories can demonstrate their commitment to client efficiency and ultimately, better lighting design outcomes.
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Final thoughts
Accurate photometric data files are crucial for efficient lighting design. By following these recommendations, manufacturers and laboratories can ensure their photometric data files are accurate and user-friendly, and ultimately contribute to better lighting design outcomes.
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