The Lumen Method - a.k.a. Zonal Cavity

The The LuMethod form and the additional calculators can be used to do Lumen Method calculations for spaces that range from simple to very complex. More information about the LuMethod calculator is available here.

Lumen Method is a technique for estimating the ambient illumination produced by a lighting system in an interior space. The technique combines the photometric data for a luminaire with the room geometry, reflectances and assumptions with simplification techniques to generate an average illuminance value over the horizontal plane designated as the task a.k.a. workplane.

The essential aspect of this method is that any space is simplified until it can be modelled as having just three planar surfaces. The bottom one of these surfaces is the horizontal rectilinear workplane for which the average illuminance is evaluated. The top plane has the same shape and is exactly 'above' the workplane at a distance that corresponds to the light sources' position relative to the workplane. The third surface is the sides, a 'rectilinear collar' composed of the four vertical planes between the bottom and the top a.k.a. walls. Note that the height of the sides is the distance between the top and bottom (sourceplane and workplane). Whenever the workplane is above the floor or the luminaires are installed below the ceiling, the height of the sides will be less than the height of the walls in the room being modelled.

For example, in a corridor with recessed luminaires, the height of the sides would be equal to the height of the room, since the sourceplane is the ceiling and the workplane is the floor. If the luminaires were wall sconces, the source plane would drop to the corresponding height above the floor (and further simplifications would be required too).

The MDI *LuMethod* offers both of the conventional Lumen Method calculations: one using a specified number of luminaires to get an illuminance value; the other using a specified illuminance to get the corresponding number of luminaires. Note that the number of luminaires is reported as a real number - but typically is changed to an integer to get the design to work. The form has a radio button to select between these two calculation variants.

These dimensions refer to the room cavity, which may be smaller than the room itself due to simplifications made as part of the Lumen Method calculation. Length and width relate to the bottom plane (workplane) dimensions and height is the distance from bottom to top (workplane to sourceplane). The corresponding value for the room cavity ratio (RCR) is shown. The units for all dimensions are selected at the top of the form, next to the LuMethod file name.

rhoBottom refers to the reflectance of the workplane surface (from 0.0 to 1.0), rhoTop refers to the reflectance of the sourceplane surface and rhoSides refers to the average reflectance of the sides of the room cavity. Any or all of these may be either the reflectance of a virtual surface or an averaged reflectance or a combination of both, depending on the simplification techniques required for the space.