by R.W. Stowe, Director of Applications Engineering, Heraeus Noblelight America LLC
Q. Is there a “quick and easy” way to estimate the UV exposure under a UV LED lamp?
All manufacturers of UV LEDs packaged for UV curing provide a specification of the irradiance or peak irradiance under the lamp, and typically at specific distances from the lamp face (watts/cm²). The model of the LED identifies the nominal wavelength.
But, what about exposure at various speeds? Perhaps an integrating radiometer (dosimeter) is not available. Many LED curing systems position the LEDs at 2 to 20mm from the work surface. What if the distance between the lamp and the work is too small for an integrating type radiometer or dosimeter? Or, what if one simply cant be sent through the system?
If you have a very thin, probe-type radiometer with a wavelength response that covers the particular LED, then you may be able to “map” the irradiance profile, as in Figure 1. This involves repositioning the probe carefully at many equally spaced points and making the following calculation:
Where E is the exposure, in J/cm²;
v is the desired velocity in inches/second (or mm/s);
i is the irradiance at each measurement point; and
?d is the increment of distance in inches (or mm).
But, I promised a quick and easy method.
The profile is unlike most medium-pressure mercury lamps that have a sharply-peaked profile and irradiance that “tails” on either side of the peak. The shape of these profiles and the “tails” make it difficult to calculate exposure (Figure 2). To find the area under the curve, which is proportional to exposure, we are usually forced to use an integrating radiometer.
LEDs are arrays of packed LED dies, so they usually have a characteristically “soft” irradiance profile. Profiles will be similar, even if the array has some concentrating optics applied. When examining the typical LED profiles, observe that the space above the curve inside and the space below the curve outside of the 50% points are similar – they have similar areas. This leads to a very rough approximation:
E = Ip x D/v
E is exposure in joules per cm² (J/cm²);
Ip is peak irradiance under the lamp head in watts per cm² (W/cm²) at a specific distance;
D is the distance (in the travel direction) between the 50% irradiance points, in inches (or mm) [This is approximately the width of the LED array and may be similar to, but smaller than, the window dimension]; and
v is the velocity of the surface under the lamp, in inches per second (or mm/s).
This method gives a good approximation of the exposure under an LED, or several LEDs, when a dosimeter or an integrating radiometer is not practical or available.
R.W. Stowe is director of applications engineering at Heraeus Noblelight America LLC. He can be reached at firstname.lastname@example.org.