Further aspects of the artificial illumination of plant growth chambers

Abstract

Tubular low-pressure mercury fluorescent lamps with and without an internal reflector are compared. When an efficient external white reflector is provided and the lamp environment is dust-free, plain lamps give 10% greater light intensity than reflectorized lamps. Using a double-tier lighting unit, individual switching of lamps is shown to offer a wide range of intensities between 3500 and 400 lm/ft 2. Dimming of only six lamps extends the range down to 10 lm/ft 2. Reasonable horizontal uniformity of illumination in a horizontal plane is retained in all cases. An electrical circuit is described for use with photo-periodic work in which selected lamps in a lighting unit can be used as a night-break or for an extended day. The range of night-break duration is from 5 s to several hours using standard commercial time-switches. A design of fitting for incandescent lamps is described which is physically interchangeable with a tubular fluorescent lamp. Recommended lamp spacing and the resultant intensity distribution in a horizontal plane is given. Using a lighting unit containing tungsten filament and fluorescent lamps, the relative contributions to intensities are given in both energy and photometric units. Values of total radiation are used in conjunction with spectral curves of the lamps to predict the “red” to “far-red” ratio. The significance of the artificial sky type of illumination is discussed. Using a 27-in cube, various light-reflecting materials are compared. Self-adhesive metallized Melinex is recommended for lining the walls of growth chambers when both the lamp bank and the plant plot extend right up to the walls; white paint, aluminium sheet and stainless steel sheet are shown to be unsuitable.