Light is important for poultry, not only to find feed and water but also for reproduction. We therefor want an even distribution of light in the houses, to avoid dark and light spots. But how far should lamps be apart to create that even distribution?

The amount of light that a lamp give is expressed in lumen, a measure for the total amount of light that a lamp emits. The amount of lumen depends on the lamp type, the wattage etc. The light intensity is the amount of lumen per m² and is expressed in lux, where 1 lux is 1 lumen per m². This means that the light intensity on the floor depends on the amount of lumen that a lamp gives but also on the distance of the lamp to the floor. If a lamp is further away from the floor, it covers a bigger area of the floor, the lumen that the lamp gives will be spread out over a bigger floor area and the lumen per m² (the lux) will go down. If a reflector is used behind the lamp, more light will be directed towards the floor and the lux will go up.

A practical approach to determine distances between lamps in a poultry house is to see the light from the lamp towards the floor as a cone with sides of approximately 45 degrees. Directly under the lamp the light intensity will be the highest and on the edge of the cone the light intensity will go down, but is still acceptable as a uniform light distribution. According to the equation of Pythagoras, a triangle with a long side (hypotenuse) of 45 degrees will have two equal legs. That means that if we consider the edge of a cone of 45 degrees still acceptable for an equal light intensity, the edge of the cone will be on the same distance from the center of the cone as the height of the lamp over the floor. And to position the lamps, it means that the distance between two lamps should be two times the height of the lamps over the floor.

This set up works but has the disadvantage that there is still a difference in light intensity from the center of the cone (directly under the lamps) to the outside of the cones (in the middle between two lamps).

If we want to have a fully equal distribution, we have to set up the lamps in such a way that the cones overlap each other. If the outside of each cone reaches the center of the next cone (directly under the next lamp), the combined intensity at every place between the two lamps will be equal. The intensity of one lamp will decrease further away from the center, but the intensity of the next lamp will increase towards the next lamp. This is shown in the drawing below.

The practical consequence is that there is a double amount of lamps needed, but with a distance between the lamps equal to the height of the lamps above the litter we do get the most even light distribution.