IES File from Photopia

You're likely here because an IES, TM-14 or EULUMDAT file you received had a web link to this page.

The photometric file you have was likely generated by Photopia, an optical design and analysis raytracing software package. Photopia is widely used in the architectural lighting industry for designing and simulating the photometric performance of luminaires.

Photopia (and other raytracing software) can provide a photometric file as part of their output. These programs require a complete 3D CAD model of the luminaire, as well as detailed information on the lamps and materials that are used. The simulation emits rays from the lamps that interact with the luminaire geometry and eventually exit the luminaire. The energy distribution of the rays emitting from the luminaire determine the intensity distribution and other types of results. If the geometry, lamps, and material properties closely match the physical luminaire, then the simulated photometry can be as accurate as physical testing.

Time and cost. Simulating is much faster than building the product and sending it to a lab to be measured. With custom products, the manufacturer may not be prepared or have time to build and test a prototype for each design. For some standard product lines with a large number of configurations, building and testing each variation is cost and time prohibitive. Simulations are quick and give the manufacturer an easy way to test different materials, lamps and geometries to design the best product. Manufacturers will typically have a physical test done on the final production sample to ensure it is built correctly, but some are confident enough with simulated photometry to distribute it to customers for use in lighting design layouts.

There are many factors that affect the accuracy of both simulated and measured photometry. Since the purpose of simulated photometry is to predict real photometry, every effort has been made in Photopia to ensure the highest degree of accuracy. Photopia takes the following into account:

• Material Properties - Photopia has a measured library of scattering and reflectance data for real materials, so what is simulated can be what is built.
• Lamp Properties - Photopia has a library of lamps that have been modeled to provide the most accurate simulation.
• Physical Geometry - A Photopia simulation also requires an accurate CAD model of the reflector and lens components of a luminaire.

Many manufacturers rely on Photopia to predict product performance and use it in their design process to save prototype costs and time. Just as you rely on a lighting software program to predict light levels, they rely on optical simulation to predict product performance. If they did not trust Photopia, they wouldn't use it.

You can also see this paper for more details about the factors that affect the accuracy of simulated and measured photometry.

No. The IES LM-79-08 standard defines protocols for physically testing LED luminaires. Following the LM-79 standard doesn't guarantee any specific level of accuracy, it only ensures that a consistent process is used among labs when collecting integrating sphere and goniophotometric data for LED luminaires. If a customer requires a photometric file tested according to the LM-79 standard or a file from a lab with accreditation from NVLAP, DLC or others, then a physical test must be pursued from an appropriate lab.

• Thermal Environment - Although the lab is supposed to remain at 25C during an entire test, this is often very difficult to ensure. Additionally, the installed location may not be at the same temperature as the lab, which will change lumen output and light levels.
• Manufacturing tolerance - Often only one sample of a product is physically tested. Lamp variations, material variations, and geometry variations can cause a wide range of performance that is not captured with a single measurement. The product tested may be one of the outliers instead of representing the vast majority of the product.
• Tested as Shipped? - Is the product that is shipped to a job the same as what was sent for testing? There is nothing (beyond reputation) to prevent a manufacturer from sending a premium reflector or lens for testing to achieve a high efficiency and then shipping product with a lower grade material to save cost. With LEDs a manufacturer may send the highest lumen bins for testing, but then ship product with other bins of lower lumen output.
• Adjusted IES File? - Does the IES file represent the product that it is supposed to. Was the product tested with an 11mm LES COB and then scaled up to a higher output 20mm LES COB using the same optical system? The distrubtion may be different between these two different LES optics.

There are also aspects of simulated photometry which may not be accurate:

• Thermal Effects - The lumen output of some light sources is very dependent on temperature, which isn't accounted for in a simulation since heat transfer is not analyzed. LEDs are known to be particularly temperature sensitive.
• Manufacturing tolerance - Simulations are typically done for the design condition, and often not for the range of tolerance on each component, including geometry, material properties and lamp output.
• Simulted as Built? - Is the product that is shipped out the same as what was simulated? Materials, lamps, lumen output, and geometry could all vary from the shipped product.

Interestingly, many of these issues are very similar to issues with physical testing. When asking if you can trust any photometry, it may be less about trusting the particular method of photometry, and more about trusting the company providing the photometry. Do you think they know what they are doing and have taken care to address the issues mentioned above? Do you think they're trying to make a quick sale, or develop a relationship? Are they willing to assure the predicted light levels or retest sample fixtures if they don't seem to perform as expected?

Lighting design software that uses luminaire photometric files (IES & LDT Files) has its own limits in accuracy. First, not all lighting design software uses the same computational methods so their individual performance will vary. In addition to this, other factors that affect the accuracy of application simulations include:

• Broad assumptions on room surface reflectances, gloss & objects in the room. Actual room surface reflectances and objects in the room are often far more complex than simulation models. Furthermore, all lighting simulation software assumes all room surfaces are perfectly diffuse.
• Simplified assumptions about luminaire shapes. In almost all situations, luminaires are considered to be simple rectangular boxes. This can lead to large inaccuracies in room simulations as outlined in this paper.
• Power conditions that vary from lab standards will affect the luminaire lumen outputs.
• Running temperatures that vary from standard lab conditions will also affect the luminaire lumen output.
• Broad assumptions about luminaire light loss factors may or may not be appropriate for any given application, affecting the lighting system's ability to meet the design requirements over time. Are luminaires cleaned on a regular basis to ensure their design performance is maintained?
• What is the tolerance of luminaire performance for any given product? Do they all emit precisely the same beam shapes and lumen outputs? Manufacturing tolerances & LED binning guarantee that there will be some variation from product to product. Good manufacturers will ensure that their products meet or exceed published lumen output levels, but none provide any type of assurance of beam accuracy.
• How accurately are luminaires mounted in the physical application? This can cause significant variances with narrow beam and aimed luminaires.

Because of limitations in the IES and LDT file formats, representing large complex luminaires in a single file can be a challenge. We have published a paper and put together a webpage that discusses this in detail. View more details about luminous shapes.