29 LOHMANN TIERZUCHT › MANAGEMENT GUIDE 29 LIGHTING The principle for windowless houses “Do not increase the hours of light during rearing period and do not reduce hours of light during production period” also applies to “open” houses. The effect of the natural daylight must be considered when designing lighting pro- grammes, if natural light enters the building throughout the day or if the hens have free access to open-air runs. For example in Central Europe the natural day length increases in the course of the cal- endar year to about 17 hours until late June and then shortens to about 8 hours until late December. If flocks are moved to an open production house with windows that cannot be dark- ened, the lighting programme must be ad- justed to the natural day length at the time of rehousing. We distinguish between two variants: 1. Production starts as the natural day length decreases. 2. Production starts as the natural day length increases. In both variations the lighting programme at 17 weeks of age should be set to a lighting period of at least 10 hours, taking the natural day length into account, and to be increased by 1 hour every week to 14 hours until 21 weeks of age. Never switch on the artificial light before 04.00 o’clock in the morning (CE time). During the spring months the lighting pro- gramme is affected by the increase of natural day length and gradually extends to about 17 hours. When the natural day length be- gins to decrease in Central Europe from July, the 17-hour light period should be kept constantly until the end of the production period. This example can be accomplished in Central Europe very simply as follows: › 04.00* o’clock in the morning: lights on – dimmer switch off at ≥ 50–60 Lux. › Dimmer switch on at ≤  50–60 Lux– 21.00 * o’clock in the evening lights off. * Central European time These times should be varied depending on the condition of the flocks, the start of lay (production, egg size) and the facilities in the building. If for operational reasons a different diurnal rhythm from the one described above is applied, it should not differ too much from the dawn/dusk times stated above, having regard to the diurnal rhythm of the hens. Lighting Programme for Open Houses