Abstract: A high pressure sodium vapour lamp having a fill consisting of sodium, mercury and xenon in an arc tube with a sodium weight portion within the sodium-mercury-amalgam of approx. 12% to approx. 20%, having a xenon filling pressure in the cold state between approx. 180 Torr and approx. 350 Torr, having a D-line reversal width (distance of the tops of both wings of the sodium-D-line of the radiation spectrum) of approx. 110 Å to approx. 200 Å, and having approx. 14% to approx. 18% radiation portion in the red wave length range 635 nm to 750 nm and having approx. 7% to approx. 10% radiation portion in the blue wave length range 380 nm to 500 nm, in each case of the radiation power in the wave length range 380 nm to 780 nm for promotion of plant growth is provided.

Abstract: A high pressure sodium lamp having a fill within an elongated arc tube comprising an inert starting gas, mercury and sodium wherein said mercury and sodium being are in an amount less than two milligrams per cubic centimeter of said volume of the interior of the arc tube wherein the weight ratio of sodium to mercury is less than 1 to 20 whereby the lamp is saturated with sodium and unsaturated with mercury at said predetermined nominal output voltage and does not extinguish at an input voltage exceeding about 90 percent of said rated voltage.