Grafica News - Aug 2015
Grafica participates in SPAI's Cross Country Table Top Exhibitions : Pune/Solapur
This is in continuation of the technical article titled “Understanding Exposure Lamp Variables Part 1” which was published in the May 2015 issue of Grafica News India
All types of exposure lamps lose UV intensity or level of actinic light as they age. While a lamp may seem bright, UV light output needed for exposure cannot be seen with the human eye. UV intensity will steadily decline and eventually lose the ability to adequately expose the stencil.
The best option to address these progressive changes is to use a light integrator for exposure instead of a timer. Light integrators measure the quantity of UV light reaching the screen using a photocell, and automatically adjust the exposure length to ensure the light dose is the same each time Figure 1. This effectively compensates for lamp decline and minor voltage fluctuations.
Integrators use 'units' of light instead of minutes and seconds (although most integrator controllers allow you to set actual time if you wish.) Initially when a new lamp is installed the integrator photocell is calibrated so 60 units equals approximately 60 seconds of time at a specified distance. This provides an easily understood unit equivalent for entering/determining exposure times.
To actually quantify the intensity and quantity of UV light arriving at the exposure surface, an integrating radiometer is needed. A radiometer is a portable device designed to provide independent readings of UV light emissions through the use of a photo sensor. There are two main configurations offered, self-contained designs and types including a remote photo sensor attached with a wire Figures 2 & 3 .
Radiometers come with different photo-sensors to read different sections of wavelengths, making it necessary to select an appropriate range for the intended application. There are several photo-sensor options available, each addressing a different band of the UV spectrum Figure 4 . The best UV sensor types for our emulsions sensitivities are the UVA and UVV with UVA being the most common in our industry. There are two UV output measurement functions available on radiometers, irradiance and radiant energy density.
Irradiance is the radiant power arriving at the working surface at a given moment, and is referred to as “intensity” expressed in watts or milliwatts per square centimeter (W/cm2 or mW/cm2). Radiometers often report the average peak irradiance. For screen exposures, the higher the value, the shorter and more efficient exposure. Higher lamp wattages give higher irradiance readings than lower wattages.
Radiant energy density is the timeintegral of irradiance or total quantity of light received at the surface during exposure time. It is expressed in joules or millijoules per square centimeter (J/cm2 or mJ/cm2) and more commonly known as “dose”. It is calculated by integrating the area under the irradiance curve, which is necessary since irradiance is not constant throughout the entire exposure.
You can use a radiometer to help ensure exposures remain the same over time if you don't have an integrator. The radiometer must be equipped with an external photo-sensor and must read radiant energy density mJ/cm2 (dose).
1. Determine the appropriate exposure in time using a stencil exposure calculator.
2. Tape the radiometer probe on exposure glass at a position it can remain or be placed consistently.
3. Make an exposure for the correct time with radiometer in place. Read the radiometer dose in mJ/cm2 and record as the proper exposure.
4. Check the radiometer reading each week (for that exposure and distance) to ensure it reaches the same reading. Increase exposure time as needed to reach the correct value.
Monitoring Lamp Life
It is important to monitor the lamp in some way so it can be replaced at the proper point. As the lamp ages it loses intensity and should be replaced when its UV spectral intensity falls below acceptable levels, not when it burns out. At some point the same exposure will not produce the same results, even if an integrator is used.
So tracking the lamp using your integrator response or a radiometer is important, especially for high tolerance or fine detail work. The best way of monitoring lamp change is to measure peak irradiance with a radiometer when the lamp is new using the procedure below.
1. Calibrate a new lamp to count one unit = one second at a specified distance.
2. Find the benchmark value by reading irradiance (W/cm2) with a radiometer in the center of exposure area using the selected exposure. Take a total of 5 readings, average and record.
3. Take a reading every two weeks and record to track lamp life. Sensor must be placed in the approximate same location on the glass and same lamp distance & exposure used as the original test.
4. Replace lamp when the reading drops to 25%-50% of original value (depending on print tolerance requirements).
Tracking the integrator response can also help monitor lamp life. Simply track the time it takes for the same units to count down every two weeks (use an exposure over 60 units long), and when it exceeds a predefined amount of seconds the bulb can be replaced.
In part 3 of this series we will explore uniformity of light distribution and vacuum table variables including glass, vacuum blanket and draw.