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Talking Photometry: Understanding Photometric Data Formats

One of the most common tests that we're asked to perform at Photometric Testing is the generation of photometric data files. To many, the contents of an EULUIMDAT file are a complete mystery - this article will shed some light on photometric data files and why they are so useful. We will explain: 

  1. What information is in a photometric data file?
  2. What formats they come in?
  3. How to generate one>
  4. What to do with one once you have it?

The Photometric Data Itself

What goes into a photometric data file? There are two photometric units that we need to consider here, total luminous flux and luminous intensity. Total luminous flux is the total amount of light emitted from a light source, corrected for the spectral response of the human eye to light. It is measured inlumens. The luminous intensity defines the amount of lumens in a given direction, per solid angle. This is measured in lumens per steradian, or candela.The luminous intensity in a photometric file will be defined in many different directions. A typical photometric file that we create at Photometric Testing will contain luminous intensity values for 855 different angles. A photometric data file will also contain the input electrical power that the light source consumes (the Watts).

The File Formats

The photometric file itself is an ASCII (delimited text) file. There are three formats: IES; EULUMDAT; and TM-14.

In 1986 the Illuminating Engineering Society of North America (IESNA) created the standard LM-63-86, “IES Recommended Standard File Format for Electronic Transfer of Photometric Data.” This has been updated twice, in 1995 and 2002. The IES file is the most common format in North America but is also widely used in Europe. IES photometric data files have the .ies filename extension.

EULUMDAT (also known as LDT, after its file extension) is the de-facto industry standard photometric data file in Europe. Without a recognised standards organization to define and maintain it, EULUMDAT has remained largely unchanged since its introduction in 1990. Unfortunately, it also means that there is no publication available which officially documents the file format. The only documentation currently available is the on-line specification Despite these drawbacks, LDT files are still widely used and most customers request their data in both formats. LDT photometric data files have the .ldt filename extension.

In 1988 CIBSE introduced a photometric file format called TM14, but this need not detain us. The format has not caught on in the lighting industry and in our many years of experience, we have never known a customer ask for a TM14 file.

There are some differences but many similarities between the IES and LDT formats. Firstly, the LDT format contains lines that specify the correlated colour temperature (CCT) and colour rendering index (CRI). The IES file does not contain this data. However, both IES and LDT files can be formatted as either absolute or relative files. When absolute photometric measurements are performed, the luminous intensity values are recorded and the total luminous flux value is calculated from these values. When relative photometric measurements are taken, the candela values of the luminaire are measured but the total luminous flux of the lamp is measured separately from the luminaire itself. In a relative measurement, the luminous intensity values are reported in candelas per kilolumen. This allows for the measurement of the efficiency of the luminaire by comparing the bare lamp lumens to the lumens of the luminaire. The candela per kilolumen value from the fitting should therefore be independent of the luminous flux of the lamp, as it is entirely dependent on the design of the luminaire itself.

However, in the age of LED lighting there is one big disadvantage associated with the use of relative photometric data files. The thermal performance of an LED will be different depending on whether it is mounted in a luminaire or as a bare lamp, and the lumen output will therefore be very different. Relative photometry in this case can lead to misleading results as the calculated efficiency value will be incorrect.

How to Generate a Photometric Data File

The luminous intensity data needed for photometric files is generated by a goniophotometer. A goniophotometer is a mechanical device to support and optionally position the light source (a luminaire or lamp) and a photosensor which measures luminous intensity at each angle, at a set distance. The goniophotometer set-up can be as simple as a manual engineering turntable and a light meter. This has the advantage of being cheap, but the disadvantage of being incredibly time consuming (measuring the previously mentioned 855 angles would take the best part of a day). Most photometric files are generated by an automatic goniophotometer. In this case, the luminaire is mounted to an automatic turning frame, which rotates the device under test through two axes. A measurement of luminous intensity is taken at each angle. The files can be generated by hand but most goniophotometer post processing software generates the files automatically.

How to Use the Photometric Data Files

The luminous intensity data presented in photometric files is useful because it allows lighting designers to observe both the total light output and the angular spread of the light output. The angular information is typically presented as polar diagrams and cone diagrams. Neither of these diagrams is included in the file format itself, but can be generated from the files by using lighting analysis and design programs such as Photometrics Pro, Photometric Toolbox, Relux or Dialux.

Figure 1. Polar Intensity Diagram

The polar diagram above shows the angular spread of the luminous intensity. The cone diagram shows the beam angle (typically defined as 10% of peak intensity for architectural lighting and 50% of maximum intensity for theatrical and broadcast lighting) and the illuminance levels at beam centre for various distances.

Figure 2. Cone Illuminance Diagram

This data is useful to lighting designers because it allows calculations to be made on how many luminaires are needed to achieve the desired illumination in a specific installation.

Photometric Testing provides measurement services on both traditional and LED-based solid state lighting products using state-of-the-art goniophotometric equipment. Please use our Enquiry Form to request further information or a quotation.