A workplane is an imaginary plane where work is performed and illumination is specified. The standard height for the workplane is shown in the image below, but there are different methods for choosing how to define workplanes.
LEED Definition: The USGBC defines the workplane at 30″ above the floor, which is a standard height for table tops. If you are seeking LEED credit you will need to simulate at workplane height, 30″ above the floor.
BREEAM Definition: The Building Research Establishment Environmental Assessment Methodology (BREEAM) uses CIBSE LG10 to define the workplane as the horizontal, vertical or inclined plane in which a visual task lies. The working plane is normally taken as 0.7m above the floor for offices and 0.85m for industry.
Flexible Definition: However, not all furniture has a height of 30″, .7m, or .85m. Before deciding on a workplane height, consider researching the height of the work surfaces (i.e. tables and desks) that will be used in the space for a more accurate analysis. For example, tables in science labs are often 32 to 36 inches tall.
The workplane is a type of Illuminance Grid since it is made up of a grid of points and has the function of collecting light. The spacing of the points on the grid can be changed and will affect the precision and speed of the computation of the results. In other words, densely spaced grids will be more accurate yet more computationally expensive than a sparser point spacing. In the image below, we see how different spacing between points affects the daylight measurement. 2′-0″ spacing is standard for LEED, but choose an appropriate density that makes sense for your project and purpose.
An Illuminance Grid will typically be defined as a horizontal plane but can be placed to measure light on any plane inside the building. For example, they can be defined to measure light on a vertical surface such as a wall. This function can be useful in assessing situations like daylight hitting artifacts in a museum. Some users may also want to collect light on the ceiling, in which case the Illuminance Grid would be horizontal, and located just below the ceiling, with the active side facing down.