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The importance of UV light measurement for process monitoring

UV processes usually rely on a minimum UV light irradiance level or dose to achieve the desired outcome. Each application and process will vary, but it’s essential that the minimum levels are met to ensure the process runs smoothly.

Tips for UV measurement

  • Use a UV light meter with a similar spectral response to the UV light source output
  • Check units of measurement and apply conversions where necessary
  • Take measurements at the same distance or in the same position for accuracy and consistency
  • Ensure meters are calibrated in line with manufacturer recommendations

Examples of process monitoring

UV fluorescent inspection – UV irradiance

UV fluorescent inspection processes usually involve a 365nm UV source, to produce bright fluorescent effects over a particular area. Some processes which form part of critical quality control procedures state a minimum UV light irradiance level, usually measured at a specific distance from the source.

It’s essential that the minimum UV irradiance levels are maintained to ensure parts, components are inspected to the required standards and meet the process specification.

UV irradiance (uW/cm2 or mW/cm2)

A UV light irradiance meter can be used to monitor the UV output over time.

Where the minimum UV irradiance levels are not known, this can be determined by performing a simple test:

  1. Use a UV light meter to take a measurement at the typical distance from the UV lamp to the inspection surface.
  2. Increase the distance between the UV lamp and inspection surface until the fluorescent effects are too dull to spot.
  3. You can then use the UV irradiance value in step 2 as the minimum UV irradiance level when taking UV irradiance measurements at the typical distance in step 1.
  4. Regular measurements should then be made at the typical distance to ensure the UV irradiance level meet or exceed the required level.

UV disinfection – UV dose

UV disinfection processes often require a minimum UV dose to achieve a particular log reduction of microorganisms. The dose is calculated by multiplying the UV irradiance levels by time.

UV dose (J/m2) = UV irradiance (W/m2) x time (seconds)

For timed processes, it’s essential to monitor the output regularly to ensure the UV dose achieved meets the requirements of your process. It’s not always possible to measure or monitor the UV irradiance levels for disinfection applications, so a UV meter is used to monitor the dose.

With most UV light sources, as time goes on the UV irradiance values will decrease and there is potential for the UV dose to drop below the required levels. When this happens, you can either replace the UV source to increase the UV irradiance levels, or you would need to extend the exposure time to counter the reduction in UV irradiance.

For example, if your minimum UV dose is 450 J/m2 and exposure time is set to 10 seconds, the minimum UV irradiance value must be 45 W/m2. This is demonstrated in the tables below.

UV irradiance Exposure time Dose
50 W/m2 10 seconds 500 J/m2 (exceeds minimum UV dose)
47.5 W/m2 (5% loss) 10 seconds 475 J/m2 (exceeds minimum UV dose)
45 W/m2 (10% loss) 10 seconds 450 J/m2 (minimum UV dose)
42.5 W/m2 (15% loss) 10 seconds 425 J/m2 (below minimum UV dose)
40 W/m2 (20% loss) 10 seconds 400 J/m2 (below minimum UV dose)

Option 1: Replace the UV source(s) to increase UV irradiance levels:

UV irradiance Exposure time Dose
50 W/m2 10 seconds 500 J/m2 (exceeds minimum UV dose)

Option 2: Increase the exposure time to compensate for the drop in UV irradiance:

UV irradiance Exposure time Dose
40 W/m2 12 seconds 480 J/m2 (exceeds minimum UV dose)

In most cases, option 1 is preferred as it does not affect the process. For some smaller applications then option 2 is viable.

UV light curing – UV irradiance and/or UV dose

UV curing, bonding and drying processes often require a minimum UV dose to achieve a full cure of the material. The dose is calculated by multiplying the UV irradiance levels by time. UV light meters should be used to monitor and verify UV levels.

UV dose (mJ/cm2) = UV irradiance (mW/cm2) x time (seconds)

Similar to the example above, as the UV source ages, the UV irradiance level drops which can compromise the process, which can affect the strength and quality of the cure. This can often lead to product defects, recalls or safety problems.