It is shown that when planning a lighting system for a large “ working plane” using the standardized industrial reflector (B.S.S. No. 232—1926), it is better to place the lamps over the centres of certain contiguous rhombuses than over the centres of contiguous squares of equal area. A new definition is proposed for the term “spacing ratio.” A convenient method of drawing isolux diagrams is described and such diagrams are given for the above-mentioned standardized reflector at a spacing ratio of 1.5. It is shown how uniformity of illumination varies with spacing ratio with the above reflector. A plea is made that manufacturers of other reflectors should publish similar information relating to their goods. Finally, some brief practical rules for designers of electrical lighting systems are given, from whose point of view the paper has been written.B.Sc.(Eng.)