Lettuce growth data are presented that show the importance of the daily light integral for predictablevegetative growth. Dry mass accumulation is shown to be proportional to the light integral, and a consistent daily lightintegral is proposed to be central to consistent production. Supplemental lighting control rules are defined and describedand a computer implementation is used in conjunction with ten years of hourly weather data to test (by simulation)adequacy of the rules to control supplemental lights and movable shades in greenhouses to achieve a consistent dailyintegral of Photosynthetically Active Radiation (PAR), mol-m 2 -day 1 , on days of either insufficient or excess solarirradiation, which are most days. The rules require neither historical data bases of weather characteristics nor dailyweather forecasts. Control decisions are suggested to be made hourly, based on the current days accumulating solar PARintegral inside the greenhouse. The model is sensitive to time-of-day electricity rates, changing seasons, weather,greenhouse and component characteristics, and greenhouse location (latitude and longitude). The rules containparameters with values suggested for northeastern United States solar conditions but which may be adjusted for localsolar climates that are significantly different.
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Albright et al. (2000) studied this question.