Two assemblies cured under different UV LED settings can look identical when they come off the line and produce completely different bond strengths in pull testing. The explanation almost always comes back to a misunderstanding of two parameters that seem interchangeable but are not: UV intensity and UV dose. Getting these right is not an academic exercise — it is the practical foundation of any repeatable UV curing process.

Defining UV Intensity

UV intensity — more precisely called irradiance in radiometric terminology — is the instantaneous power of UV light arriving at the cure surface, expressed in milliwatts per square centimeter (mW/cm²). It is a rate: how fast UV energy is being delivered to the adhesive at any given moment. If a UV LED spot lamp is on and aimed at a bond joint, the irradiance at that joint is the intensity value.

Irradiance is determined by the lamp’s output power, the optical coupling efficiency of the delivery system, and the working distance between the cure head and the substrate. It does not depend on how long the lamp is on. A lamp operating at 3,000 mW/cm² delivers that irradiance whether it is on for 0.1 seconds or 10 seconds.

Defining UV Dose

UV dose — also called UV energy or fluence — is the total UV energy received by the adhesive surface over the full exposure period, expressed in millijoules per square centimeter (mJ/cm²). It is calculated as:

UV Dose (mJ/cm²) = Irradiance (mW/cm²) × Time (seconds)

Dose is cumulative. It accumulates as long as the lamp is on and UV light is reaching the adhesive. A lamp delivering 3,000 mW/cm² for 1 second provides a dose of 3,000 mJ/cm². A lamp delivering 1,500 mW/cm² for 2 seconds also provides 3,000 mJ/cm².

Why They Behave Differently in the Curing Process

The equation above might suggest that intensity and time are fully interchangeable — that any combination producing the required dose will give the same cure result. For many adhesives, that is approximately true within a moderate range. For others, it is not, and confusing the two parameters leads to real process failures.

Intensity-sensitive reactions. Free-radical polymerization depends on generating reactive radical species faster than oxygen scavenging can consume them. If irradiance falls below the adhesive’s threshold intensity, oxygen inhibition dominates regardless of exposure time. The adhesive surface remains tacky even after extended exposure, because the initiation rate is never high enough to overcome the inhibition mechanism. Increasing exposure time does not help. The solution is higher irradiance, not longer exposure.

Dose-limited through-cure. In thick bondlines or with pigmented adhesives that limit UV penetration, the total energy reaching the interior of the adhesive layer determines how completely the deeper material polymerizes. Once the surface has cured and attenuates incoming light, additional dose delivered over a longer exposure helps drive deeper polymerization, while additional intensity primarily affects the already-saturated surface.

These two mechanisms mean that a process optimized only for dose — without verifying that irradiance exceeds the material’s threshold — may produce under-cured surface layers. A process optimized only for irradiance — without delivering adequate total dose — may produce hard surfaces over incompletely cured interiors.

How Adhesive Manufacturers Specify Requirements

Adhesive product data sheets typically specify both a minimum irradiance (mW/cm²) and a recommended dose range (mJ/cm²). These are independent requirements that must both be satisfied simultaneously, not alternative specifications to choose between.

A datasheet stating “Minimum 100 mW/cm²; recommended dose 2,000–4,000 mJ/cm²” means the process must deliver at least 100 mW/cm² at all times during exposure and accumulate between 2,000 and 4,000 mJ/cm² total. A system delivering 50 mW/cm² for 80 seconds reaches 4,000 mJ/cm² total dose but violates the minimum intensity requirement and will likely produce a defective cure. A system delivering 5,000 mW/cm² for 0.2 seconds meets the intensity requirement and accumulates 1,000 mJ/cm² of dose — also potentially inadequate for full cure.

Practical Measurement

A calibrated UV radiometer measures irradiance at the cure surface. Dose is derived by multiplying that irradiance by the exposure time, or by using a data-logging radiometer that integrates the measurement over the exposure period, which is more accurate when irradiance varies during the exposure (as it does in conveyor applications where the part passes under a lamp).

Radiometer selection must match the spectral output of the lamp. Because UV LEDs emit at discrete narrow wavelengths, a radiometer calibrated for broadband mercury lamp output may significantly misread the actual intensity from an LED source.

If you need guidance on selecting a radiometer and establishing measurement protocols for your UV LED curing process, Email Us and an Incure engineer will provide recommendations specific to your lamp system and adhesive.

Setting Up a Repeatable Process

Defining a UV curing process correctly requires establishing minimum irradiance, target dose, and exposure time as separate specifications — not just a single “cure time” setting. A complete cure specification includes:

• Measured irradiance at the production working distance (in mW/cm²)
• Required exposure time to achieve target dose at that irradiance (in seconds)
• Minimum acceptable irradiance threshold before the process requires intervention
• Re-measurement interval for irradiance verification as the lamp ages

This structure prevents the common failure mode where a lamp that has degraded over time still runs the same cure time as when it was new, delivering progressively less dose with each production cycle.

The Interaction with Wavelength

Irradiance and dose are both wavelength-dependent measurements in multi-wavelength environments. A radiometer that reports total UV power without wavelength discrimination cannot distinguish between 365 nm and 405 nm photons, even though those two wavelengths may have entirely different effects on a given photoinitiator. When specifying and measuring UV curing parameters for LED systems, using wavelength-matched measurement instruments eliminates one of the most common sources of process characterization error.

Contact Our Team to review your UV curing process specifications and ensure your intensity and dose parameters are correctly established.

Visit www.incurelab.com for more information.