Evidence from published trials is reviewed to investigate whether there is
any need to change the recommended optimal light intensity for laying fowls
from 5 lux. Although some recent trials have recorded excellent performanc
es from birds exposed to light intensities of less than 5 lux, it is conclu
ded that this has more to do with the ability of modern pullets to lay well
even without photostimulation than with any change in the threshold of lig
ht sensitivity. There is good recent evidence that the stimulation of early
sexual maturity by a large increment in the photoperiod during the growing
stage requires a minimal intensity of 2 lux, and that 4 lux is probably re
quired for maximal response. By combining the results from all available tr
ials, it is possible to demonstrate small but significant effects of light
intensity on egg size, food intake and mortality. An economic analysis, whi
ch takes into account all these responses, indicates that the optimal profi
t occurs at about 5 lux. Although the prices used in that analysis are both
local and ephemeral, the optimum will vary very little within other cost s
tructures, because it is determined almost entirely by the lower outputs pr
edicted at intensities below 5 lux and the greater electricity cost in prov
iding very much higher intensities. It is recognised that, because maximisi
ng profit is not always the sole objective in enterprises involving animal
production, considerations for the welfare of both the livestock and worker
s may make it prudent to recommend a slightly higher value such as 10 lux.