This post will go over what is an IES file, and how designers use it to conduct lighting simulations. In the 1980’s the Illuminating Engineering Society of North America (IESNA), developed a standard file format that can be used to store and represent photometric data of a light fixture. Photometric data is the specific luminous intensity (candela) and luminous flux (lumens) for a given fixture. Using this photometic data contained within the IES file, designers can model a light fixture in a three dimensional space to simulate and calculate luminous emittance (measured in lux or foot-candles) commonly referred in to in the industry as light levels.
The IES file is usually provided the light fixture manufacturer, and is specific to each light fixture and the light source it contains. For example, a fixture that can house both an HPS and LED lamp, will comes with two different IES files. The file itself has an extension that ends in “.ies” and can usually be found on light fixture manufacturers websites for most specification grade fixtures. Bellow shows the Phillips website with links for IES files to download.
It is also important to note that the same lamp in two different fixtures will have two different IES files. For example, a 100W HPS lamp in fixture with a reflector will result in different photometric data than the same lamp inside a fixture without a reflector. Thus again resulting in two different IES files.
To open and make use of the photometric data contained in the IES file, a lighting simulation software is required. The most popular lighting simulation software used by lighting designers today is AGi32. A cheaper alternative is Visual 3D, which has less bells and whistles but still gets the job done. Once the IES file is loaded up in the simulation software, you can usually set various parameters, such as the mounting height and light loss factor (LLF).
Most software lets you visualize the photometric output of the light fixture in the 3D space. Below is a street lighting fixture which has downward throw as well as some forward throw. It shown from various angles: plan view (looking top down), side view, and a birds eye view.
The designer can then proceed to place the light fixtures in the 3D space at the proposed mounting locations. A calculation area can be set, to determine what the resulting light levels will be based on the placing of the light fixtures. In the below example, we have modeled two different fixtures with different light distribution types. One has a type 3 distribution, which throws the light out in front of the fixture and also to sides. The other has a type 4 distribution, which throws the light down directly below it in a symmetrical circle. The numbers in the calculation zone below the light fixture tells you the lux reading at each particular point. The higher the lux level is at a given point, the brighter the light is at that point.
Building on this, a designer can now draw out the layout of an entire space, and analyze the average lux levels for a desired calculation zone. In the example below, the roadway around a building structure is the calculation zone, and new pole light fixtures are simulated around the roadway. Using Visual 3D, a report is generated to determine the average light levels along the roadway. In this case it is approximately 35 to 40 lux.