Notes on the Assignment of Target/Ambient Illuminance Ratio on the Last Surface in Lighting Design Objectives Procedure

In applying the concept of lighting design objectives (LiDOs) procedure, a lighting designer shall specify the mean room surface exitance (MRSE), which serves as the indicator of spatial brightness of the room. To guarantee that the specified MRSE can be achieved in the actual result, recent literature suggested that the designer may choose the desired target/ambient illuminance (TAIR) for each surface and object, except for the last (n-th) surface, which must be computed individually. However, the choice of which surface to be assigned as the last surface may yield different design outcomes and may, at times, render the design impractical. This brief report aims to observe the impact of choosing different surfaces as the last surface in a hypothetical room and to compute the probability of having an impractical design outcome. Monte Carlo simulation is applied to compute the output of LiDOs procedure in 1,000 random combinations of specified MRSE, room sizes, and surface reflectance values. It is recommended to assign the ceiling, whose reflectance is typically the highest in the room, as the last surface in LiDOs procedure, since it gives the lowest probability of having an impractical design outcome.