Why am I here?

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 architectrual lighting industry for predicting product performance without testing a physical prototype.

How do you simulate a photometric file?

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 then traces rays from the lamps, to the materials, and out of the luminaire. If the geometry, lamps, and materials are modeled accurately, there is no reason that simulated photometry can not be as accurate as physical testing.

Why a simulation instead of physical measurements?

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, there are so many configurations that 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. Typically manufacturers will have a physical test done on the final part to ensure it is built correctly, but some trust Photopia enough to send simulated photometry to their customers to do layouts with.

Why can I trust this file? Is the simulation accurate?

Typically, yes. There are many factors which affect the accuracy of both simulated and tested photometry, so saying one is more or less accurate than the other would be too general. 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. We also have many comparisions of Photopia photometry versus lab tested photometry which always show very strong correlation.

What can cause errors in physical photometry?

• 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 Miro Silver reflector 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. There are many cases where a reflector may only be tested with the 35W CMH lamp, and then simply scaled to create an IES file for the 70W and 150W CMH lamps. This may not be very accurate, but is common practice.

What can cause errors in simulated photometry?

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. Compact fluorescent lamps, induction lamps, T5 fluorescent lamps, and 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?

How accurate are lighting simulations?

This page covers some of the issues in measured and simulated photometry, but photometry isn't the only area of uncertainty in prediciting light levels in a space. Laying out a fixture in a space model to predict light levels has its own issues. The predicted light levels could vary from installed values for any of the following reasons:

• Surface reflectances and gloss
• Power conditions (is the running voltage the same as the lab test?)
• Running temperature (the same fixture will perform very different in a refrigerated room and an uncontioned space in Texas)
• Lamp Lumens (the lamps on the job may not have the standard lumen output)