This paper discusses the relationship between the physical characteristics of the direction of light to be used for lighting design and luminous comfort in living rooms that can be provided by less electric power usage, namely lower exergy supply, than present lighting systems. The purpose of investigating such a relationship is to understand the perception of moderate brightness to be provided by an optimal combination of task and ambient lighting with the minimal electric power usage in living rooms. The ratio of mean (non-directional) vertical illuminance of horizontal illuminance, RVH, is proposed as an extension of the previous indices such as vector-scalar ratio and mean cylindrical illuminance, and its applicability to be an index for expressing the quality of ambient lighting in living rooms is examined by a subjective experiment. We first examined the characteristics of the RVH index for a variety of lighting conditions including outdoor clear sky and overcast sky. The RVH values range from 0.2 to 1.1. The larger the RVH, the more light tends to come in parallel to horizontal surface. On the other hand, the smaller the RVH, the more light comes from overhead. The RVH index represents the variation of luminous conditions more consistently than the previous indices do. The subjective experiment was conducted using an experimental living room with five different types of lighting to investigate subjective brightness sensation, luminous comfort and their relations to the RVH index. The values of RVH range from 0.1 to 1.4 in this set of experimental conditions. There seems to be a range of RVH values together with the associated mean vertical illuminance indicating the luminous condition, with which the subjects perceive sufficient brightness.