Radiation Levels in Growth Chambers Fitted with High Intensity Discharge Lamps, with or without Thermal Barriers1

Abstract

Levels of photosynthetically active radiation that approach full sunlight can be achieved in growth chambers fitted with high intensity discharge lamps, but the high levels of photosynthetic radiation may be accompanied by abnormally high levels of nonphotosynthetic radiation. Downward irradiances of photosynthetic radiation (400 to 700 nm) shortwave radiation (280 to 2800 nm), thermal radiation, and total radiation (shortwave plus thermal), were measured in growth chambers fitted with metal halide, high pressure sodium, and low pressure sodium lamps, some with glass or plastic barriers, and some without. Ratios of photosynthetic to shortwave radiation were generally at or above those found in natural sunlight (about 0.5), but ratios of photosynthetic to total radiation were less than half of this value, in a chamber fitted with metal halide lamps and no barrier. This was due to the large flux of thermal radiation from the bulbs and fixtures. Leaf temperatures were as much as 16°C above air temperature. A plate glass barrier removed 75% of the thermal radiation, while transmitting 81% of the photosynthetic radiation and 74% of the shortwave radiation. With the barrier, leaf temperatures were close to air temperature. It is concluded that barriers are essential in chambers fitted with metal halide or high pressure sodium lamps. The low pressure sodium lamps operated at a lower bulb temperature, causing thermal radiation fluxes to be lower and leaf temperatures to be closer to air temperature, without the use of a barrier.