Axial segregation in a metal-halide lamp at 1–10G measured by emission spectroscopy

Summary form only given. High intensity discharge (HID) lamps are high pressure lamps based on arc emission. They are used for example for street or shop lighting. Metal-halide (MH) lamps, which are HID lamps contain a buffer gas (often Hg) and a rare earth additive (in our case Dy, dosed as salt) which radiates in the visible. When the lamp is burning vertically, the combination of diffusive and convective flows cause axial segregation of the additives; colour segregation appears. The non-uniform light output decreases the efficiency and the colour rendering of the lamp. The axial segregation is maximum when the diffusion and convection are in the same order of magnitude. Convection is induced by gravity, and to understand the flow phenomena we vary the gravity at the lamp by means of a centrifuge (1g-10g). The measurement techniques include imaging laser absorption spectroscopy and emission spectroscopy; we will focus on the latter. The setup consists of the lamp and a 0.25 m monochromator. The lamp is imaged on the slit, which is in vertical direction, this allows us to measure at the lamp axis (radial position 0). This technique is fast and easy compared to the ILAS measurements. By comparing these results with ILAS a qualitative measure for axial segregation is obtained. The setup also allows for investigating commercially available lamps. The COST lamp contains 5 or 10 mg Hg and 4 mg Dyl₃. The intensity of several Hg and Dy lines are plotted as function of axial position. We focus on the ground state atomic Dy 642.19 nm line, which was also used to obtain the Dy density in the ILAS measurements. When we go from the bottom to the top, we first observe an increase in Dy intensity at the bottom region caused by the arc that is not fully developed yet. Next the Dy intensity decreases again. The slope gives the amount of axial segregation in the lamp. The shape of these curves are in agreement with those of the ground state atomic Dy densities obtained b- - y ILAS. To measure cataphoresis effects in the lamp we decrease the shutter time (~1 ms) of the camera and trigger on the square wave of the ballast (f= 120 Hz).