Irradiance is the variable most engineers underspecify when setting up a UV adhesive curing process. The number on a lamp’s data sheet tells you what the lamp can deliver at a specific distance under laboratory conditions. The number that matters is the irradiance actually reaching your adhesive surface at your production working distance — and whether that value is sufficient to drive the polymerization reaction to a complete, structurally sound cure.
What Irradiance Means
Irradiance is UV radiant power per unit area: watts per square centimeter (W/cm²) or milliwatts per square centimeter (mW/cm²). It represents the intensity of UV energy hitting a surface at a given moment. Irradiance is not the same as dose — dose (J/cm²) is irradiance integrated over the exposure time and represents the total UV energy delivered to the adhesive.
Both irradiance and dose must meet minimum thresholds for complete cure. An adhesive can receive a sufficient total dose at low irradiance over a long exposure time, but cure kinetics are not always linear. Very low irradiance can result in incomplete photoinitiation even with extended exposure. Very high irradiance can drive rapid surface cure while the adhesive interior remains undercured, particularly in thick bond lines or pigmented adhesive systems.
Adhesive Supplier Specifications
The starting point for determining required irradiance is the adhesive technical data sheet. Reputable UV adhesive suppliers specify:
- Recommended irradiance range (mW/cm²)
- Required cure dose (J/cm²) at the specified wavelength
- Minimum tack-free surface cure dose
- Minimum structural cure dose for full mechanical properties
If the data sheet does not specify irradiance and dose, contact the adhesive manufacturer directly. A supplier who cannot provide cure parameter guidance is a supplier to evaluate carefully.
Use the adhesive’s specified wavelength for all measurements. Irradiance specifications apply at a specific emission wavelength — irradiance measured at 365 nm is not the same as total UV irradiance, and using the wrong measurement wavelength produces incorrect dose calculations.
Typical Irradiance Ranges by Application
Industrial UV adhesive applications span a wide range of irradiance requirements:
Low irradiance (10–50 mW/cm²): Ambient or low-power UV curing, suitable for slow-curing adhesives with high photoinitiator loading. Rarely used in production environments due to slow cycle times.
Medium irradiance (50–500 mW/cm²): Standard range for most UV spot lamp industrial applications. Covers the majority of UV-curable adhesive formulations used in assembly. Cycle times of 2–30 seconds are typical.
High irradiance (500 mW/cm² – 5 W/cm²): UV LED spot lamps and high-power flood lamps in the upper irradiance range. Used for fast-cure applications requiring sub-second or 1–3 second cycle times. Some adhesive formulations are specifically designed for high-irradiance cure.
Very high irradiance (>5 W/cm²): Concentrated UV sources for specialized applications. Requires careful evaluation of substrate thermal tolerance, as even low-infrared UV LED sources can heat substrates significantly at very high irradiance.
If you are uncertain whether your target irradiance is achievable with available UV LED lamps for your working distance and spot size requirements, Email Us and an Incure applications engineer will model the irradiance for your process geometry.
How Irradiance Varies with Distance
Irradiance falls off with distance from the UV source. For a collimated UV LED source with a light guide, irradiance follows approximately an inverse-square relationship with distance in the far field. In the near field (close to the light guide tip), the relationship is more complex.
Practical consequence: moving a spot lamp from 10 mm to 20 mm working distance does not halve the irradiance — it typically reduces it by 60–80%, depending on the optical system. Always measure irradiance at your actual production working distance with a calibrated radiometer, not at the manufacturer’s reference distance.
Lamps with collimating optics maintain irradiance over a longer working distance than diverging sources. Evaluate the lamp’s irradiance-versus-distance curve across the range of working distances your process may use.
Irradiance Uniformity Across the Cure Zone
For spot lamps, irradiance is highest at the center of the spot and decreases toward the edge. The usable cure zone is typically defined as the area within which irradiance exceeds a minimum threshold — often 80% of peak irradiance. Bond areas extending beyond this zone receive insufficient energy for complete cure.
For flood lamps, irradiance uniformity across the cure area is specified by the manufacturer. A uniformity of ±10–15% is typical for industrial flood lamp arrays. Tighter uniformity (±5%) is available on precision flood lamps and is required for applications where cure time windows are narrow.
Measuring Irradiance in Production
Irradiance measurement requires a calibrated UV radiometer with a sensor matched to the lamp emission wavelength. Do not use a general-purpose UV meter — sensors calibrated for 365 nm will report incorrect values when used with a 385 nm or 405 nm source.
Measure irradiance at the adhesive surface, not at the lamp head or controller display. Measure across the full cure zone, not only at the center. Record the working distance at the time of measurement and hold it constant in production.
Establish a periodic irradiance verification schedule — UV LED sources degrade gradually over their lifetime, and irradiance measurements taken at commissioning will not remain valid indefinitely. Monthly or quarterly irradiance checks, or checks triggered by cure quality complaints, maintain process control.
Irradiance and Heat Generation
Higher irradiance means more UV power delivered per unit area per unit time. Even though UV LED lamps emit less infrared than mercury arc sources, high irradiance UV LED curing still heats the adhesive and substrate through absorption of UV energy and heat conduction from the adhesive as it polymerizes.
For heat-sensitive substrates — thin plastics, optical coatings, delicate electronic components — measure substrate temperature during cure at the maximum irradiance you intend to use. Confirm the temperature remains within the substrate’s thermal tolerance. If substrate heating is excessive at the required irradiance, evaluate pulsed UV delivery (short UV on-times with cooling intervals) to maintain the required dose while reducing peak temperature.
Qualifying Irradiance for Your Process
Set up the lamp at production working distance and measure irradiance. Compare to the adhesive supplier’s recommended range. Cure test specimens at the measured irradiance and target exposure time. Measure cure completeness using methods specified by the adhesive supplier — shore hardness, DSC conversion, or destructive lap shear testing. Confirm that bond strength meets the required specification.
Document the qualified irradiance, working distance, exposure time, and adhesive lot number. Use these as the controlled process parameters for production.
Contact Our Team to discuss irradiance requirements and UV LED lamp selection for your specific adhesive bonding process.
Visit www.incurelab.com for more information.