||Building on the original SL*18 compact fluorescent lamp from Philips, the concept went on to be offered in an attractive spherical format from 1987 - the Decor SL*18D. Essentially the only change was the use of a spherical opal bulb 120mm diameter, instead of the usual prismatic body.
Shortly afterwards this SL*18R Reflector version was launched. By coating only the upper portion of the bulb but leaving the lower area clear, a greater proportion of light is directed out of the front of the lamp. From the outset it was directed predominantly into the horticultural lighting market, for photoperiodic illumination.
Photoperiodic lighting is widely used in greenhouses to control the out-of-season flowering of certain plants, which require specific ratios of light to dark periods. Of course HID lamps are still used to provide high illumination levels for photosynthesis, but flowering can be triggered with much lower illumination levels that interrupt the dark period, and with suitable lamps this can be timed to occur when there is a particular market advantage.
Incandescent 150W lamps are traditionally employed in this application, but they are power-hungry devices. Additionally their strong IR and far red output triggers growth mechanisms leading to undesirable elongation of stems and weak, spindly plants. During the 1980s and 1990s the SL*R Agro enjoyed popularity as an energy saving alternative, with longer lifetime. However today, the advances in HID lighting systems have favoured a trend back to the incandescent lamps for photoperiodic lighting, their IR output being used to control other stages of plant growth.