Looking Fit 10/2003: wolff words: Understanding Low-Pressure And High-Pressure Lamps

Light Subjects
Understanding Low-Pressure And High-Pressure Lamps

Editor’s Note: The following is the first part of a two-part article discussing the differences between low-pressure and high-pressure sunlamps.

Low-pressure sunlamps are most prevalent in our industry by a large margin. These usually are the 5- or 6-foot linear tubes using fluorescent technology. The term low-pressure evolved as a result of the pressure contained within the glass tube. This pressure amounts to somewhat less than one atmosphere—actually a slight vacuum.

When the lamps are broken, they collapse or implode as the external positive atmosphere rushes to fill the space within the lamp at negative atmosphere. When low-pressure lamps receive the proper voltage and current from the ballast, an arc is struck from one end of the lamp to the other. A small amount of mercury is vaporized by this arc and causes ultraviolet emissions. A thin coating of phosphor or a blend of phosphors on the inside of the lamp wall is excited by the mercury’s UV emissions.

The phosphor selection determines the output of the sunlamp; the phosphor converts mercury output (similar to early high pressure; this will be discussed in next month’s column) to the tanning spectrum output common to low-pressure sunlamps (see figure). Most of the emissions are not visible; this is the UV needed to stimulate the tanning process. A small amount of the emissions are visible. Only a small portion of the visible light leads to pigment darkening, but it is a handy way to tell that the lamp is working.

Output from low-pressure sunlamps is a combination of UVB (280-320 nanometers) and UVA (320-400nm), plus the small visible component described above. Differing effectiveness from one lamp type to another is modified through adjustment of the phosphors. Despite the conventional wisdom that increasing UVB makes for a hotter lamp, this is not always the case. In fact, using such simple rules of thumb can lead one down an entirely different path.

Next month’s article will focus on high-pressure lamps and their differences with low-pressure lamps.