Lighting designers are challenged with providing innovative solutions that are increasingly technical while staying sensitive to architectural aesthetics. Many of these solutions go far beyond a simple hand calculation-from calculating illuminance on a workplane to analyzing daylight factors for LEED compliance. The last year has seen significant development in the tools that are available to lighting designers to help them achieve these technical solutions. Lighting software differs from other modeling and 3D rendering programs in that it is physically based, and uses one or both of two methods for calculation: radiosity or raytracing.
Radiosity versus Raytracing
The radiosity method of calculation works by dividing all surfaces in a model into patches, where each patch has its own equation for the amount of energy leaving and landing on that surface. The calculation solves the system of equations that results from all of the patches in the model to determine the quantity of light on each patch. A benefit of the radiosity method is that it is view independent: There is one solution regardless of viewpoint, and it is progressive, which means that in most implementations, it can be stopped and re-started to check the progress of the calculation. A disadvantage of the radiosity method is that it only works with diffuse surfaces, so translucent, specular (shiny), and transparent materials cannot be included in the calculation. It can also be memory intensive.
The second method, raytracing, operates by sending calculation rays outward from the viewpoint; the program then follows the ray as it bounces off surfaces and creates additional rays. It is able to handle all material types, including translucent, specular, and transparent surfaces, but it is not view independent; renderings from different viewpoints must be calculated separately.
Lighting software uses one or both of the above approaches, with facilities for illuminance and luminance calculations, and methods to output lighting calculation data. Other program features important in any lighting software package include: glare rating calculations, lighting power density, and photo-realistic rendering. With the growing number of projects working towards LEED ratings, daylighting calculations, which include solar exposure studies and daylight factor calculations are also becoming a required component.
The two leading commercial lighting software developers are Lighting Analysts and Lighting Technologies. Lighting Analysts produces AGI32 (www.agi32.com); version 1.7 was updated with a revised daylighting calculation engine. Lighting Technologies (www.lighting-technologies.com) has released version 1.1 of Lumen Designer 2004, which takes the widely used Lumen Micro calculation engine to a more user-friendly CAD-oriented interface. Both of these programs use radiosity engines for their lighting calculations and have raytracing engines for photo-realistic rendering. They can also perform daylighting calculations, although they take different approaches to this functionality, and both are able to import and export a variety of 2D and 3D CAD formats.
Other recent developments in lighting software include Autodesk's VIZ-4 (www.autodesk.com). The program incorporates the radiosity engine of Lightscape, which is no longer being developed past version 3.2. Although VIZ-4 has a very complex modeling interface and easy animation capabilities, some of Lightscape's functionality has been lost. It is difficult to extract calculation data, the false color illuminance and luminance plots do not have scales, and there is no way to run calculation grids.
DIAL (www.dial.de), a European lighting services organization supported by manufacturers, produces the calculation program DIALux, which can be downloaded from DIAL's website, and has been updated to version 3.1. DIALux is not as flexible as Lumen Designer and AGI32, but is useful for simple calculations.
The last several years have also seen significant development in Radiance (www.radiance-online.org), a raytracing suite of lighting calculation tools developed at the Lawrence Berkeley National Laboratory to run on the Unix/Linux operating systems; it now also runs on both Windows and the Macintosh OS. While Radiance has a steep learning curve, because it uses raytracing for calculations and not just image rendering, it is able to handle a wider range of materials than the programs that use the radiosity calculations method. In addition, because of the Unix interface, which is text-based rather than point-and-click, it is easier to automate certain tasks such as creating multiple renderings for animations. Within the last two years, Radiance has been released as open-source software, meaning it can be downloaded for free, distributed and modified by designers as needed per project requirements.
There are also many alternatives for the designer who does not need the complex functionality of the above-mentioned programs. AGI-Light is a simplified version of AGI32, and the Simply Lighting series from Lighting Technologies consists of a set of easy-to-use tools for lighting calculations such as interior room calculations and economic analysis. Most of the programs discussed have trial versions or evaluation licenses available. Lighting software has progressed past point-by-point illuminance calculations and now offers more complete lighting calculation packages that include tasks such as glare rating, daylighting analysis and renderings. This adds new functionality and the ability to consolidate capabilities into one package, reducing the number of software programs a designer must incorporate into his or her practice.
Matt Franks is a lighting designer in Arup's New York City office. He has gained experience in the computer simulation of electric lighting and daylighting through a variety of project types, including museums, educational projects and residential developments.