Why Predicting Light Matters
An introduction to the importance of photometry and IES files
Light is essential to human vision – it determines what we see, how clearly we see it, and how we feel in spaces we occupy. Before any lights are installed, lighting professionals must predict how they’ll perform and compare different lighting options. Will there be enough light to work or navigate safely? Will it support visual comfort and meet the design intent? These questions are central to photometry, the science of measuring light.
Being able to anticipate how light will behave in a space – much like sound – is essential. It allows lighting professionals to:
- Compare and select appropriate lighting solutions
- Ensure the space meets safety code Make sure the space meets safety codes and design goals
- Plan for energy efficiency and lighting controls
- Create visual renderings to show what a space will look like
To do this, designers rely on a toolkit that includes:
- Standardized data files from manufacturers about how their lights perform (commonly known as IES files)
- Knowledge of how surfaces (walls, floors, ceilings) reflect or absorb light
- Software that uses scientific models to simulate lighting effects (AGi32, LightStanza, and others)
- Computers to run that software
Even with sophisticated visualization tools, designers thoughtfully interpret the results – often in combination with supporting materials from manufacturers. Photometric data helps them understand how luminaires (also known as light fixtures) interact in a space to:
- Highlight architectural details and conceal others
- Deliver recommended lighting levels for specific tasks
- Shape the overall experience and atmosphere of the environment
About IES Files
Definition
An .ies file is a digital representation of light distribution characteristics of a luminaire or source in a standardized file format. It’s created by a lab using the data from a test method called IES LM-79, which measures things like how bright the light is and where the light goes.
Details
IES files contain data about the luminous intensity distribution patterns of a luminaire, often measured in candela (cd) as a function of angle. Lighting simulation and calculation software interprets the IES file, and utilizes the characteristics of light within that file to create “realistic” lighting scenarios for design projects, and, to predict light levels within the built environment.
Software examples: DIALux, AGI32, Revit, AutoCAD, or 3ds Max. IES files are plain text files and can be opened with a text editor to view raw photometric data. The contents therein include:
- Fixture Information: Includes manufacturer, model, and description of the light source.
- Photometric Data: Details the intensity distribution across various angles.
- Units and Measurements: Specifies the details about the angular increments, fundamental units (meters or feet), spherical coordinate system used, and other information about the testing details that might be pertinent to the data).
Information Ownership
The data contained in an .IES file, SPD file, or lumen maintenance report belongs to the manufacturer of the light source or luminaire. The standards detailing the industry consensus testing method and reporting format belong to IES. Manufacturers request IES files, SPD files, and lumen maintenance information from testing labs (accredited and otherwise) when a new luminaire is designed, new LEDs or lamps are being considered for integration into a luminaire, and when otherwise relevant to design, marketing, etc.
Testing
- Setup: A goniophotometer or an integrating sphere is used to measure a lamp or luminaire (test subject). The test subject is generally mounted and oriented as it would be in real-world usage under controlled electrical and environmental conditions. Testing can be performed with equipment traceable to National Metrological Institute (NMI) standards.
- Data Collection:
- Goniophotometry: The testing equipment measures luminous intensity at various angles around the light source. The raw measurements are used to calculate multiple photometric quantities, graphs, and tables that can help the reader to evaluate the test subject and predict its performance within a particular application. The output includes:
- Light source description (manufacturer, model, etc.)
- Total luminous flux (lumens)
- Luminaire classification (by distribution type, for example direct or indirect)
- Multipliers for scaling data collected to other lengths/configurations
- Photometric data points (raw data)
- Goniophotometry: The testing equipment measures luminous intensity at various angles around the light source. The raw measurements are used to calculate multiple photometric quantities, graphs, and tables that can help the reader to evaluate the test subject and predict its performance within a particular application. The output includes: