The RADIANCE Lighting Simulation and Rendering System

Gregory J. Ward /


1. Introduction

2. System Design Goals

2.1 Ensure Accurate Calculation of Luminance

Accuracy is one of the key challenges in physically-based rendering, and luminance (or the more general "spectral radiance") is probably the most versatile unit in lighting. Photometric units such as luminance are measured in terms of visible radiation, and radiometric units such as radiance are measured in terms of power (energy/time). Luminance represents the quantity of visible radiation passing through a point in a given direction, measured in lumens/steradian/meter^2 in SI units. Radiance is the radiometric equivalent of luminance, measured in watts/steradian/meter^2. Spectral radiance simply adds a dependence on wavelength to this. Luminance and spectral radiance are most closely related to a pixel, which is what the eye actually "sees." From this single unit, all other lighting metrics can be derived. Illuminance, for example, is the integral of luminance over a projected hemisphere (lumens/meter^2 or "lux" in SI units). Luminous intensity and luminous flux follow similar derivations. By computing the most basic lighting unit, our simulation will adapt more readily to new applications.

To assure that a simulation delivers on its promise, it is essential that the program undergo periodic validation. In our case, this means comparing luminance values predicted by Radiance to measurements of physical models. An initial validation was completed in 1989 by Grynberg [9], and subsequent validations by ourselves and others confirm that the values are getting better and not worse [14].

2.2 Model Both Electric Light and Daylight

2.3 Support a Variety of Reflectance Models

2.4 Support Complicated Geometry

2.5 Take Unmodified Input from CAD Systems

3. Approach

3.1 Hybrid Deterministic/Stochastic Ray Tracing

3.2 Cached Indirect Irradiances for Diffuse Interreflection

3.3 Adaptive Sampling of Light Sources

3.4 Automatic Preprocessing of "Virtual" Light Sources

3.5 User-directed Preprocessing of "Secondary" Sources

3.6 Hierarchical Octrees for Spatial Subdivision

3.7 Patterns and Textures

3.8 Parallel Processing

3.9 Animation

3.10 Implementation Issues

4. Applications and Results

4.1 Electric Lighting

4.2 Daylighting

5. Conclusion

6. Acknowledgements

7. Software Availability

8. Bibliography

9. Appendix