# Implementing Many-Lights Rendering with IES-Based Lights
## 3 Photometric Description of Light Sources  
Light sources in computer graphics are usually limited to extremely simplified approximations of real light sources. Point lights, directional lights, and spotlights are expressed just using a position in the world space, with no dimension, and with simple types of light emission. The only type of virtual light source that has an actual dimension is the area light. However, area lights are computationally expensive to calculate, thus limiting their actual use only to offline rendering. In real-time applications, illumination data from area lights are usually prebaked in _light maps_ and used during the rendering process through texture sampling operations. Another texture-based approach commonly used in real-time rendering to introduce complex illumination features is Image-Based Lighting [28]. Real light sources may have complex characteristics of emission. Indeed, light emitters may have different dimensions, and they may emit light with different intensities at different directions of emission. The IES file format [29], proposed by the Illumina Engineering Society [30], stores photometric data measured from real light sources (luminaires). IES files are largely used in the lighting design field and in PBR rendering engines [2], where they are used as input in the _importance sampling_ process of Monte Carlo methods. In recent years, they have also been used for real-time applications. Examples are the integrations in the Frostbite [31; 32] and Unreal [33] game engines. However, IES data are usually applied in game engines only as a sort of \"mask\" assigned to point lights in order to weight the intensity of light emission around the point light source in a non-uniform way. This process leads to a more realistic rendering of the surfaces close to the light source emission, which are primarily affected by the direct illumination component. However, the role of IES photometric data is usually not considered actively in the other processes related to the computation of GI effects. In practice, an IES file is an ASCII file, which contains information about a specific luminaire, and measured photometric data. Listing 1 shows the content of an IES file we have used in our test scenes. The content can be divided into three parts:  
* _Information about the luminaire product (line 1 to line 10)_: a list of metadata, providing information about the specific luminaire (e.g., model number, manufacturer, destination of use, model of the lamp(s) and ballast, type of mounting, etc.). The list of keywords is not fixed in type, order, or number of elements. Thus, IES files may differ in this section; however, it is not used during the rendering process. * _Information about the measurement test (lines 11 and 12)_: a list of values related to the measurement test on the actual luminaire. The values in line _11_ represent the _number of lamps_ inside the luminaire, the _luminous flux_ of each lamp, a _multiplication factor_ of light intensity, the _number of vertical and horizontal angles_ considered in the measurements, the _Photometric Type_ of the luminaire, the _unit of measurement_ (1 for feet, 2 for meters), and, finally, _length_, _width_, and _height_ of the emitting surface of the luminaire. The values in line _12_ provide information on the electrical ballast and the input power of the luminaire. * _Measured photometric data (line 13 to line 19)_: the actual measured data, which consist of the core part of an IES file and the data actually used in the rendering process. The luminaire's photometric data are captured by locating them at the center of an imaginary sphere and measuring light intensity values at some points on the surface of this sphere. These points are expressed using polar coordinates relative to a grid called _photometric web_. The measured samples define a volume that is called _photometric solid_. Lines 13 and 14 show the lists of the vertical and horizontal angles on the photometric web representing the measured samples.