Signs of the Times - January 2013 - (Page 30)

LIGHTING TECHNIQUES By Marcus Thielen Marcus Thielen is a physicist and lighting-industry consultant from Duisburg, Germany. Bombarding: How and why? Achieving dual results can be tricky. The determines the producta life vacuum processing of neon tube and quality. More than once I’ve heard the phrase: “Bad vacuum processing can ruin every genius glasswork in seconds – or good processing can make ugly glasswork light forever.” Often, vacuum processing is called “bombarding” a neon tube – sometimes this wording is used synonymously, but, when splitting hairs, bombarding is only a part of the vacuum processing (the part in which the glass tube and electrodes are heated by an overload current passing through). Well, due to space constraints, I’ll focus on the heating part. What happens, and what equipment is proper? Two goals need to be achieved in one process, which makes it difficult: The glass tube first must be cleaned from impurities, and secondly, the electrodes are made stable for operation. Cleaning the glass Glass tubing stored over time not only accumulates dust inside, but Photo 1: Typical example of an improperly bombarded neon tube after a few hours of operation: Water stains on the fluorescent powder layer; a brown/yellowish deposit mars the front of the electrode due to incomplete conversion of the activation chemicals. the material itself (and the phosphor coating layers) attract moisture. A clear, soda-lime glass tube’s surface contains several percent of water in its upper 15-20 micrometers. If this water isn’t removed during tube processing, it will be released during tube operation and spoil the gas discharge (see Photo 1). Even short exposure of air to a clean glass surface will restore the water layer in seconds. However, tubebending can introduce further impurities like cork, rubber grains or spit. In any case, a vacuum pump can remove only gases – no solids or liquids. Consequently, liquid impurities (mostly water) need to be vaporized, and solids need to be “burnt” to become gaseous. This is achieved by heating the glass tube (the water absorbed in the first 10-20 microns of the surface is also driven out by the heat, as the vapor pressure gets higher than the retaining force.). To burn solids, at least some oxygen is required. Electrode conditioning I call the second bombarding task Photo 2: A representative color of correct, electrode-shell temperature (see pyrometer reading in the back in Centigrade) during breakdown of electrode-activation chemicals. 30 SIGNS OF THE TIMES / JANUARY 2013 /

Table of Contents for the Digital Edition of Signs of the Times - January 2013

Signs of the Times - January 2013
ST Update
Technology Update
Vinyl Apps
Strictly Commercial
Lighting Techniques
The Moving Message
Technology Review
Technology Review
Design Matters
New Products
Scenes from the Global Village
Crane Truck Safety Advice
Supreme Wrap
The Value of Signs
Industry News
Advertising Index
Editorially Speaking

Signs of the Times - January 2013