Spot size and working distance are inseparable variables in UV spot lamp specification. A spot size without a working distance is meaningless — the spot expands with distance, so the same lamp delivers a 5 mm spot at 10 mm and a 15 mm spot at 40 mm. A working distance without a spot size tells you nothing about whether the cure zone covers the bond area. Engineers who specify both together — at the same conditions — get quotes that are directly comparable and equipment that performs as expected in production.
Why Specifying Both Together Matters
UV LED spot lamp manufacturers optimize their lamps to perform at defined conditions. When an RFQ asks only for “spot size” or only for “working distance,” suppliers respond with specifications measured at their preferred conditions — which may not match your process.
Supplier A specifies a 5 mm spot at 10 mm working distance. Supplier B specifies an 8 mm spot at 30 mm working distance. Without knowing both values simultaneously for the same system, you cannot compare these — and you cannot predict what either lamp will deliver at your actual production conditions.
Specifying spot size and working distance together, at conditions representative of your production process, eliminates this ambiguity and produces comparable, actionable responses from suppliers.
How to Define Your Production Conditions First
Before writing the RFQ specification, determine two things:
1. The required working distance. This is set by your part geometry and fixture design. The working distance is the gap between the light guide tip and the adhesive surface in your actual production fixture. Factors that determine it:
- Physical clearance needed to load and unload parts without striking the lamp
- Height of components or features that the light guide must clear to reach the cure point
- Fixture arm length and adjustment range
- Operator access requirements
If you have not yet designed the fixture, establish a target range: “working distance 15–30 mm, with 20 mm preferred.” This gives the supplier a realistic range rather than a single point and allows them to provide irradiance-versus-distance data across the range.
2. The required cure zone. What is the diameter or maximum dimension of the adhesive bond area? This sets the minimum spot size at the specified working distance — the spot must cover the bond area with irradiance above the adhesive’s minimum threshold.
If the bond area is circular and 10 mm in diameter, the spot must deliver irradiance above the adhesive minimum across 10 mm diameter at the production working distance.
Writing the RFQ Specification for Spot Size and Working Distance
Once production conditions are established, write the specification as a combined requirement:
Option 1: Point specification (single working distance)
“Spot diameter: minimum 12 mm at 20 mm working distance from the light guide tip, measured at the irradiance contour corresponding to 80% of peak irradiance. Supplier to confirm irradiance at the 12 mm boundary at 20 mm working distance.”
This form is clear, unambiguous, and directly testable. The supplier must provide data at exactly the conditions specified.
Option 2: Range specification (working distance range)
“Supplier to provide irradiance versus working distance data for working distances of 10 mm, 20 mm, and 30 mm from the light guide tip, including: (a) spot diameter at each working distance, defined as the area within which irradiance ≥ 500 mW/cm²; and (b) peak irradiance at the spot center. Cure application requires minimum 500 mW/cm² irradiance across a 10 mm diameter zone at a working distance to be finalized between 15–30 mm.”
This form gives the supplier more flexibility in presenting their product and gives you the data to determine which working distance best satisfies your requirements.
Including Irradiance in the Combined Specification
Spot size without irradiance is incomplete — a large spot with insufficient irradiance does not cure the adhesive. Write all three parameters together:
“At 20 mm working distance from the light guide tip:
– Minimum irradiance: 1,000 mW/cm² at the center of the spot
– Effective spot diameter: minimum 12 mm, defined as the contiguous area in which irradiance ≥ 500 mW/cm² (the minimum irradiance required by the specified adhesive at the cure wavelength)
– All irradiance values to be measured with a calibrated radiometer at [365 nm / 385 nm], with calibration traceable to [NIST / national standard]. Measurement method and conditions to be documented in the quotation.”
This specification is complete — it defines the wavelength, the irradiance level required throughout the cure zone, the spot diameter required to cover the bond area, and the measurement conditions for verification.
If you would like assistance writing a combined spot size and working distance specification for your application, Email Us and an Incure applications engineer will help you define the requirements based on your adhesive system and bond geometry.
Requiring Beam Profile Data
Nominal spot size claims from suppliers may be based on theoretical calculations or measurements at idealized conditions. Request actual beam profile data — a map of irradiance as a function of position across the spot — at your specified working distance:
“Supplier to provide measured beam profile data at 20 mm working distance, showing irradiance (mW/cm²) as a function of radial position from the beam center, measured with a calibrated detector. Data to be presented graphically (irradiance vs. position) and in tabular form.”
A beam profile curve reveals whether the irradiance distribution is smooth (Gaussian or flat-top) or has hotspots and cold regions. Adhesive cured under a spot with a central hotspot and low-irradiance edges will cure unevenly — center overcured, edges undercured — even if the nominal spot covers the bond area.
Setting Acceptance Criteria
Include acceptance test criteria in the RFQ to define how the purchased equipment will be verified:
“Upon delivery, the purchased UV LED spot lamp will be tested for compliance with the following specifications at the buyer’s facility:
– Peak emission wavelength: [365 ± 5] nm (measured by calibrated spectrometer)
– Irradiance at [20] mm working distance: minimum [1,000] mW/cm² at spot center (measured with calibrated 365 nm radiometer)
– Irradiance across [12] mm diameter zone: minimum [500] mW/cm² (verified by radiometer measurement at center, ±4 mm, and ±6 mm from center)
Equipment failing any acceptance criterion will be returned to the supplier at supplier’s expense for replacement or correction.“
Clear acceptance criteria make the purchase order a contract on performance, not just equipment delivery.
What Suppliers Should Provide in Their Response
A complete supplier response to a spot size and working distance specification should include:
- Irradiance at the specified working distance (center, measured value)
- Spot size at the specified working distance (contour at the specified irradiance threshold)
- Beam profile data at the specified working distance
- Light guide diameter used for the specified configuration
- Working distance range over which the lamp meets the specification
- Measurement method, calibration standard, and measurement uncertainty
Suppliers who cannot provide this level of data are not equipped to confirm that their product meets your specification — a signal to evaluate them carefully or request a demonstration unit before committing to a purchase.
Documenting the Agreed Specification After Purchase
Once the lamp is purchased and delivered, document the confirmed spot size and working distance as controlled process parameters:
- Record the measured irradiance at the production working distance (not the supplier’s catalog value)
- Record the measured spot diameter at that irradiance threshold
- Document the working distance as a controlled parameter for fixture setup and maintenance
- Establish periodic re-verification of irradiance at working distance to confirm lamp performance has not degraded
This documentation becomes the basis for production process validation and for detecting lamp aging that would push irradiance below the required minimum.
Contact Our Team to discuss UV LED spot lamp specification for your production process or to request application testing at your specified conditions.
Visit www.incurelab.com for more information.