The latest versions of ElumTools include the ability to perform calculations which consider only the direct illumination component. By ignoring all interreflected light, calculations complete significantly faster. And while this does come with an accuracy penalty, it can be a handy tool for design iterations or cases where direct lighting analysis is required by code (e.g. Emergency Calculations).
Direct Lighting vs. Global Illumination
The photometrically accurate lighting calculations and renderings produced by ElumTools and AGi32 are computed by considering 2 components, direct light and interreflected light. The global illumination solution is the sum of these components.
The first step in the calculation process is to determine the direct lighting component. This is a fast computation requiring only the inverse square cosine law of illuminance.
Once the direct results are calculated for each surface, the interreflected light between each surface pair can also be computed. This process takes significantly more time than the direct solution alone, but gives higher accuracy. For more information on how the global illumination calculation works, see Appendix A of the ElumTools help.
Accuracy Considerations
The accuracy sacrificed by skipping the interreflected calculation will vary substantially based on a number of factors. Application type, room geometry, and luminaire distribution type all have a significant effect on interreflected light. Data for some typical use cases are shown below.
Class room (27′ x 26′) lit with 2’x2′ troffers
| Global Illumination | Direct Only | Error | |
| Average Illuminance (fc) | 36 | 30 | -17% |
| Min. Illuminance (fc) | 17 | 13 | -24% |
| Max. Illuminance (fc) | 52 | 46 | -12% |
Hallway, lit with downlights and sconces
| Global Illumination | Direct Only | Error | |
| Average Illuminance (fc) | 19 | 16 | -16% |
| Min. Illuminance (fc) | 5.6 | 4.6 | -18% |
| Max. Illuminance (fc) | 33 | 29 | -12% |
Warehouse 150′ x 150′ x 50′, lit with HID high bays
| Global Illumination | Direct Only | Error | |
| Average Illuminance (fc) | 37 | 29 | -22% |
| Min. Illuminance (fc) | 20 | 12 | -40% |
| Max. Illuminance (fc) | 47 | 39 | -17% |
Exterior Site (parking lot).
| Global Illumination | Direct Only | Error | |
| Average Illuminance (fc) | 1.652 | 1.600 | -3% |
| Min. Illuminance (fc) | 0.101 | 0.095 | -6% |
| Max. Illuminance (fc) | 3.372 | 3.210 | -5% |
Analysis
The most important thing to note about the data in the preceding section, is the direct only calculations will always underestimate the total light levels. In other words, Direct Only calculations are a conservative estimate. And while this may result in over lighting, it is unlikely to lead to under lighting. For life safety applications like Emergency Lighting, this is often exactly what you want.
The other important takeaway is the margin of error for exterior and site lighting is often quite small with Direct Only calculations. For exterior environments with no ceilings or walls, light often escapes the environment after a single bounce. This reduces the interreflected component substantially. For this reason, the improved calculation time is often worth the associated accuracy reduction for exterior lighting applications.