Exposure units Info
Exposure units facts.
Light sources for exposure units basically fall into three categories: fluorescent (multi-point), metal halide (single point) and halogen-fluorescent (combination of multi-point and single point). There are other types, but these are the most common.
Fluorescent. Fluorescent exposure units employ multiple fluorescent tubes as opposed to a single bulb or "point light source." This type of unit is best for lower resolution images such as line artwork and coarse halftones. While most can properly expose diazo-sensitized emulsions, photopolymer-sensitized emulsions
Halogen-Fluorescent
Halogen-fluorescent combination units are the next step up in price. The combination of a multi-point fluorescent and single-point halogen light captures slightly finer detail than fluorescent lights alone. This type of light source is best used with photopolymer or dual-cure emulsions. Combination units tend to work well for small- to medium-size shops that need a little better resolution, a bit more speed
Metal Halide
Metal halide units employ a single-point light source and offer the finest resolution and the fastest exposure. They are available with spectral output that is diazo-compatible at about 360 nanometers or photopolymer-compatible at about 320 nanometers
The higher-wattage halide units allow for faster exposures and make it possible to expose thicker stencils, such as those for high-density printing. Metal halide systems are usually the best choice for shops that want to capture the finest detail, need fast exposure, and/or desire the ability to create thicker stencils.
It takes about 20 to 30 seconds for all of the gases in a metal halide lamp to reach full temperature and spectral output, and therefore maximum exposure speed output.
Long exposure time does not fix weak light or a light with limited spectral
output. It all comes down to how the emulsion cross
links and how complete and organized the cross
linking is among the components in the emulsion. So
to get perfect screens you need both: a very good
exposure system, and a very good emulsion like
Murakami, simply the best there is, I’ve used them all,
So what’s the secret? Strong, multispectral light. The
only portion of the light that matters is the light that
a ects the sensitizer and initiates the cross linking.
Not the white and yellow light but the bright bluish
purple ultraviolet light.
400-420 nanometer wavelengths
as well as the 350 nanometer wavelengths
which is optimal for emulsion exposure.
Emulsion
Needs strong 350-420 wavelengths, strong enough to
penetrate the emulsionI
If the light strength reaching the squeegee side is weak,
or doesn’t have the right spectral mix, the screen
tends to be below optimal exposure no matter how
long you expose for. You can only make screens as
good as the exposure unit light allows, its all in the
physics of light.
500 Watt Halogen - Quartz Bulbs - Their spectral output is
very limited. It is mostly white light and requires long
exposures. While you can make screens with them the
quality of exposure is adequate for anything but
hand printing. On automatic presses they will exhibit
pinholes and stencil breakdown just like weaker
fluorescent systems.
Metal Halide Lamps are constructed quite diff erently
than any of the previous lamps. They contain rare
earth additives, mercury and Xenon gas to assist
electrical flow
When emulsion is exposed to a strong high
quality light the cross linking process is more
complete forming stronger molecule chains that
cannot be broken by squeegee abrasion, screen ex,
or chemical interaction with harsh inks like discharge
or for graphics strong solvent inks.
How halogen lamps work
Halogen lamps similar to normal incandescent light bulbs only hotter and brighter. When electricity flows through the filament, the lamp gives off light and gets hot. In a normal incandescent lamp, the filament is made of tungsten metal and surrounded by a non reactive ("inert") gas called argon. The argon is there to make the filament last longer: if the filament were surrounded by air, the oxygen in the air would make the filament burn up very quickly. But even with argon, the tungsten filament still gradually disintegrates over time, causin the bulb to blacken and grow dimmer before it eventually "blows".
In a halogen lamp, the bulb contains a tiny amount of a halogen gas such as iodine or bromine, which is at a higher temperature and pressure than in a normal bulb. The halogen constantly regenerates the tungsten filament, effectively "bouncing back" tungsten atoms to rebuild the filament when it starts to disintegrate. This means the filament lasts much longer than it does in a normal lamp. The temperature of a halogen bulb (typically 250-600°C or 480-1100°F) will give off enough uv light. To expose all diazo emulsions. On a nano meter the uv light will measure 350nm.
