Spot size is one of the most overlooked variables in UV LED system selection — until a production line is running and the cure results are inconsistent. Engineers who define the required spot size before specifying equipment build processes that cure reliably and predictably. Engineers who assume any UV spot lamp will cover their bond area discover edge-undercure problems after the equipment is installed. This is the guide for getting spot size right the first time.

What Spot Size Means

Spot size is the diameter (or area) of the UV light pattern delivered by the lamp at a specified working distance. For circular spots, it is typically expressed as a diameter in millimeters at the 1/e² intensity boundary (where irradiance falls to 13.5% of peak) or at the 80% or 50% irradiance level, depending on the manufacturer’s convention.

Different irradiance contours matter for different purposes. The area within which irradiance exceeds the adhesive’s minimum required irradiance is the effective cure zone. Adhesive that falls outside this zone — at the spot edges where irradiance is below threshold — will not receive sufficient UV energy to cure completely.

Always ask the lamp supplier which irradiance threshold their spot size specification refers to, and confirm that the threshold used is above your adhesive’s minimum required irradiance.

Defining Your Required Cure Area

Before selecting a UV LED system, define the geometry of your bond joint:

• What is the shape of the adhesive bond line? (Circular, rectangular, linear bead, irregular pattern?)
• What is the maximum dimension? (Diameter of a lens seat, length of an edge bond, width of a potted area?)
• What is the minimum dimension the lamp must cover?

The lamp’s effective cure spot must encompass the entire bond area within the usable irradiance zone. If the bond area is circular and 8 mm in diameter, the lamp’s spot must deliver irradiance above the adhesive’s minimum threshold across the full 8 mm diameter at your production working distance.

If the bond area is larger than available spot sizes from a single lamp, evaluate whether multiple light guides from a single controller can cover the full area simultaneously, or whether a UV flood lamp with a larger cure area is the appropriate tool for the application.

Working Distance and Spot Size Relationship

Spot size increases with working distance. As the light guide is moved farther from the substrate, the beam diverges and the illuminated area expands. Irradiance decreases as the spot expands, because the same total power is distributed over a larger area.

The tradeoff between spot size and irradiance at working distance is fundamental to UV spot lamp selection. A lamp that delivers a 5 mm spot at 10 mm working distance with 2,000 mW/cm² irradiance may deliver a 12 mm spot at 30 mm working distance but with irradiance reduced to 300 mW/cm². Whether the 300 mW/cm² at 30 mm is sufficient to cure your adhesive within your cycle time is determined by the adhesive requirements — not by the lamp alone.

Document the spot size versus irradiance data at multiple working distances from the lamp manufacturer. Use this data to identify the working distance that delivers both adequate spot coverage and sufficient irradiance for your application.

Spot Size in Automated vs. Manual Delivery

In automated UV curing systems — where the lamp or the part moves on a programmed path — spot size consistency is controlled by the robot or stage positioning the lamp. Working distance is held constant by the mechanical system, so spot size and irradiance remain consistent across every cure cycle.

In manual UV curing — where an operator holds the lamp and moves it over the bond area — working distance varies with operator technique. An operator who holds the lamp closer than intended produces a smaller, more intense spot. An operator who holds it farther away produces a larger spot with reduced irradiance. Fixturing the lamp at a fixed working distance eliminates this variability and is strongly recommended for any production process requiring consistent cure quality.

If you are designing a UV curing station and need guidance on spot size requirements for your bond geometry, Email Us and an Incure applications engineer will specify the lamp and working distance for your application.

Light Guide Diameter and Spot Size

For UV LED spot lamps with fiber optic or liquid light guides, the light guide exit diameter is directly related to the spot size. A 3 mm diameter light guide delivers a smaller spot than an 8 mm diameter guide at the same working distance. Larger guide diameters spread the energy over a larger area, reducing irradiance per unit area unless total lamp output is increased proportionally.

Light guide diameter selection involves a tradeoff: larger guide diameters provide more coverage but lower irradiance density. Smaller guide diameters provide higher irradiance density in a smaller zone. Match the light guide diameter to your bond area size and irradiance requirement.

For applications with multiple bond points of different sizes on the same assembly, interchangeable light guide tips allow a single controller and lamp head to serve multiple cure positions with different spot sizes.

Spot Shape for Non-Circular Bond Joints

Most UV LED spot lamps produce circular or near-circular spots. For rectangular bond joints — a linear adhesive bead along a straight edge, a rectangular gasket, or a square-frame bond — a circular spot may not provide efficient coverage. Options include:

• Circular spot with scanning: move the lamp along the bond line, providing sequential spot curing
• Rectangular beam shaping optics: specialty lamps with beam-shaping lenses that produce rectangular illumination patterns
• Multiple spot positions with a multi-output controller: multiple simultaneous cure zones

For linear bond beads in automated assembly, scanning the UV spot lamp along the bond line at a controlled speed provides uniform cure along the bead. Scan speed, irradiance, and spot size together determine the dose delivered per unit length of bond.

Measuring Spot Size and Uniformity

Spot size measurement requires a UV-sensitive detector array or a scanning radiometer that can map irradiance across the illuminated area. Beam profiling cameras calibrated for UV wavelengths provide two-dimensional irradiance maps that show spot shape, size, and uniformity.

For production setup, confirm spot coverage by placing UV-sensitive indicator paper or a UV-sensitive film at the working distance and making a brief exposure. The exposed pattern shows the cure zone boundary. Verify that the pattern covers the entire bond area before running production parts.

When to Choose a Flood Lamp Instead

If the cure area is large — more than 30–50 mm in any dimension — a UV flood lamp is typically more efficient than a spot lamp. Flood lamps deliver uniform irradiance across areas of 50 mm × 50 mm to 300 mm × 300 mm or larger, without the working-distance sensitivity of spot lamps. For tray curing, panel bonding, conformal coating, or other large-area applications, evaluate UV flood lamp systems rather than scaling up from spot lamp technology.

Contact Our Team to discuss spot size requirements and UV LED system configuration for your specific assembly process.

Visit www.incurelab.com for more information.