Light

The units used when measuring the intensity of all wavelengths are watts per square metre (W/sq m) whereas lux (lumens/sq m) are used when only light in the photosynthetic range is being measured. More usually in horticulture, the light integral is used. The light sensors used for this measure the light received over a period of time and expressed as gram calories per square centimetre (gcals/sq m) or megajoules per square metre (MJ/sq m). These are used to calculate the irrigation need of plants in protected culture.

The usual method of measuring sunlight at a meteorological station is the Campbell-Stokes Sunshine Recorder (Figure 2.15), a glass sphere that focuses the sun’s rays on to a sensitive card; the burnt trail indicates the periods of bright sunshine. Another approach is to use a solarimeter, which converts the incoming solar radiation to heat and then to electrical energy that can be displayed on a dial.

Automation
Increasingly instrumentation is automated and, in protected culture, linked to computers programmed to maintain the desired environment by adjusting the ventilation and boiler settings. To achieve this, the computer is informed by external instruments measuring wind speed, air temperature and humidity, and internally by those measuring CO₂ levels, ventilation settings, heating pipe temperature, air temperature and humidity.