Common terminology, definitions, and some misconceptions
Following is a list of the most common terms typically used to describe a UV Bulb’s power or intensity
Watts
This is the total amount of energy the bulb will use at 100% power. Unfortunately, it is not a useful indication for UV because UV lamps are extremely inefficient.
Total watts: White Light, Infra-Red (IR) Heat, and a substantially low proportion of Ultra-Violet (UV) energy. ~92% waste energy and ~8% UV
WPI “Watts per Inch”; this is the same as Watts, but it is broken down into each linear inch of the UV Arc Lamp. An example would be as follows: You have a 10-inch arc lamp that operates at 300WPI. 10 inches of arc X 300 per inch = 3,000 Watts. (In some parts of the world,
W/cm = Watts per centimeter) W/cm2
This is a value for energy intensity and if the correct UV Radiometer was used; it will accurately describe the peak output power of a UV Lamp. Intensity can be also expressed in lower levels as mW/cm2 (milliwatts) or even lower as µW/cm2 (micro-watts). The portion at the end (/cm2) stands for per centimeter squared, or per square centimeter. It represents the detector size used within the radiometer. All radiometers calculate based on a square centimeter detector so that there is some uniformity within the industry.
J/cm2
This is a value for energy dose and accurately describes the total accumulative exposure of UV received. It is W/cm2 multiplied by time. Similar to W/cm2, in lower doses, it can be represented as mJ/cm2 (millijoules) or even lower as µJ/cm2 (micro-joules). Note: 1 Joule = 1 W/cm2/second Typically, the best place to get the lamp UV number should be the manufacturer of the Ink, Coating, or Adhesive. However, we find many times the answers our clients receive from them are vague at best. A good example is when the specification requires “exposure using a typical 300WPI UV lamp to cure”. Just from the glossary above you already know that this is not helpful. Here are several additional bullet points as to exactly why.
What is a typical 300WPI UV Lamp? Is it Mercury, or a metal halide type like Iron or Gallium? These are spectrally three vastly different lamps but are all available in 300WPI.
What is the UV Lamp in? Lamp housings have reflector chambers surrounding the bulb to redirect the lamp's energy. These reflector chambers vary significantly because they are designed for a specific purpose. The shape of the reflector, or ellipse, greatly affects the amount of UV ultimately received at the target and the intensity of that UV. You can put the exact same 300WPI mercury lamp in 6 different UV reflector housing styles and achieve 6 significantly different results.
How clean is the reflector and how new is the UV lamp? A typical electrode type UV lamp in Mercury has a useful life of 1,000 operating hours, Metal Halide additive type are only about 500 hours. Reflector aluminum becomes foggy or hazy due to exposure to UV and ozone. It discolors from intense heat and can become pitted from dirt being baked on while it is operating. The reflector is responsible for redirecting at least 50% or more of the UV energy emitted from the bulb back toward the target. So, a dirty reflector instantly affects UV intensity, even with a new bulb installed.
When was the last time you checked the lamp housings alignment and distance to the substrate, or the UV lamps focus within the reflector chamber? Most lamp housings have a very shallow sweet spot for focus, and UV intensity also decreases exponentially over distance. Minor damage to the fixture’s alignment or changes to the lamp holder's positioning can have significant negative impacts on intensity.
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