Growers have often assumed that a 1% increment in light results in a 1% yield increase. In this study, this rule of thumb has been evaluated for a number of greenhouse grown crops: fruit vegetables (cucumber, tomato, sweet pepper), soil grown vegetables (lettuce, radish), cut flowers (rose, chrysanthemum), bulb flowers (freesia, lily), flowering pot plants (poinsettia, Kalanchoe), and non-flowering pot plants (Ficus, Dracaena). A literature survey was first carried out on the effects of light on growth, dry matter production and partitioning, dry matter content and harvestable yield. Subsequently, yield data for cucumber, poinsettia and rose from commercial growers were analysed. Finally, growers were interviewed to assess their crop management in relation to the available light. For most crops a 1% light increment results in 0.5 to 1% increase in harvestable product. As a rule of the thumb the following values may be used: 0.8-1% for soil grown vegetables, 0.7-1% for fruit vegetables, 0.6-1% for cut flowers, 0.25-1.25 for bulb flowers, 0.5-1% for flowering pot plants and 0.65% for non-flowering pot plants. These are average values, which depend on several factors. For instance, the relative effect of light on growth is greater at lower light levels, at higher CO 2 concentrations and at higher temperatures. Consequently, the relative effect is larger in winter than in summertime. The effect of light on growth also depends on the duration and moment that the light level is changed. Besides a positive effect on yield quantity, light usually has a positive effect on quality as well. Light should not be considered as a separate growth factor in greenhouse horticulture, as it forms an integral part of the total farm management. Many growers, for instance, choose a higher temperature and adapt their plant density and cultivar choice when the light level is increased. INTRODUCTIONThe growth rate of a crop largely depends on the radiation it receives. Considering a given solar radiation, the grower has several options to increase the amount of incident light on a crop, such as building a greenhouse with high light transmissivity, or using assimilation light. On the other hand, the use of some measures such as screens may reduce the amount of incident light on the crop.In order to judge whether a measure affecting the light intensity is profitable, the grower needs to estimate its effect on production. For more than 20 years Dutch growers have usually taken a 1% additional light results in 1% additional growth and production as a rule of thumb. For cut flowers and pot plants there are indications that effects of light on production are less strong than in vegetable crops; for these crops 1% additional light it is often assumed to result in 0.5% additional growth and production.Light is the driving force for photosynthesis, while it may also affect plant development, morphology, dry matter partitioning and water content. Despite the assumed proportionality between light and production, it is well known that leaf p...