Hill Technical Sales Corp

Metal halide lamp

Metal halide lamp

Metal halide lamps are mercury vapour lamps with the addition of metal halogens. The metal halogens are added to create specific wavelength lines of ultraviolet radiation to match the sensitivity of the photopolymer being exposed or dried. Metal halogens are compounds composed of metal and halogen elements combined within a curing lamp to form salts. The electro-negative halogens chemically react within an uv curing bulb to cause a reaction in which the metals take on a positive charge. As the internal temperature of the metal halide lamp increases to the vaporization point of the metals, the positive ions being produced allow the metals to release their outer electrons causing ultraviolet radiation output at specific wavelengths. Special lamp ballasts are required for metal halide lamps. These metal halide ballasts have a higher open circuit voltage and special output waveform (crest-factor). This feature also extends lamp life while keeping the mercury and heavy metals within the lamp structure in a plasmatic state during temperature and voltage variations. A standard mercury arc lamp does not have these requirements.

Iron Metal Halide Lamps and Operating Temperature

Iron metal halide lamps are extremely sensitive to over heating and need proper cooling. If the lamps are over heated for even a short period of time, iron is separated from the salt that is formed with the halogen and irreversibly fuses to the quartz. The iron part of the spectrum is now lost being unable to radiate.

There are three ways to tell if this has occurred:

• Check the spectrum to see if the 390nm peak is present using a spectrometer with the correct filter setting.
• Use a fluoroscope to check of stress fractures in the quartz.
• Visually check for white specks, this method is very difficult even for a experienced lamp technician.

Metal halide lamps are used in many types of applications whose photochemistry requires different UV wavelengths to initiate the photo-polymerization process. These come in either Gallium-Indium (GaI) or Iron-Cobalt (FeCo). Other additives are available upon request.

Note that the Iron-Cobalt lamp has a broader spectrum with more spectral output in the 380 to 390nm region. The Gallium-Indium has an enhanced output in the 403 to 420nm region.

Another mix shown to the right is a multi-spectrum lamp, which is a mixture of Iron-Gallium producing an increased output over a broader region.