Arc Lamp vs LED UV Spot Curing: Which Source Is Right?
Every spot-curing system has one job: put enough of the right wavelengths on the bond line to crosslink a photo-initiated adhesive in seconds. How the light gets made is the fork in the road. Arc systems drive a mercury short-arc lamp whose output spans a huge stretch of the spectrum, the 200 W SunSpot 2 emits from 275 to 650 nm. LED systems use solid-state emitters that pour all of their energy into one narrow band; the SkyBeam offers 365, 385, 395, or 405 nm heads.
Neither technology is simply better. Each wins a distinct set of applications, and the deciding factor is almost always the adhesive, not the hardware. This guide works through the trade-offs, spectrum, intensity, duty cycle, consumables, and heat at the part, using the two benchtop systems we stock as the worked example. The full three-way spec table lives on our system comparison page.
Spectrum: Broadband Coverage vs Narrowband Matching
Adhesives cure when their photoinitiators absorb light at the wavelengths they were designed for. Most modern formulations are built around initiators that respond in the long-UVA range, roughly 365–405 nm, exactly where curing LEDs emit. If your adhesive datasheet specifies 365 nm or 405 nm, an LED source puts every milliwatt into a band the chemistry can use and wastes nothing on wavelengths it can't.
Legacy and specialty chemistries are a different story. Formulations developed around broadband mercury lamps often rely on UVB or even UVC content, particularly for surface cure, where oxygen inhibition can leave a narrowband-cured joint tacky on top. Dual-cure and surface-inhibited adhesives are the classic cases. The SunSpot 2's 275–650 nm output covers UVB and UVC through visible light, which is why many labs keep an arc source on the bench next to their LED lines: it cures nearly everything, including materials a single-wavelength LED can't fully cure.
The rule of thumb is blunt. If the datasheet names one wavelength, LED is the cleaner match. If it calls for broadband output or UVB/UVC content, the arc isn't a preference, it's the requirement.
Intensity: Reading the Numbers Correctly
On paper the arc looks far stronger. The SunSpot 2 delivers more than 18,000 mW/cm² of UVA, measured across 320–390 nm, while the SkyBeam peaks at 11,000 mW/cm² at a 3 mm spot at 365 nm. But the two figures aren't measured the same way: the arc number integrates output across a 70 nm window, while the LED concentrates its output near a single peak, right where a matched photoinitiator absorbs most strongly. For an adhesive specified at 365 nm, the LED's usable dose is closer to the arc's than the headline numbers suggest.
Spot size matters as much as source. The SkyBeam's irradiance steps down with lens diameter, 11,000 mW/cm² at 3 mm, 5,600 at 6 mm, 1,500 at 12 mm, while the SunSpot 2's delivered intensity is set by light-guide depth and a 25–100% front-panel knob. Size the light guide to the bond geometry first, then confirm the irradiance at that spot meets the dose on the adhesive datasheet.
| Spec | Arc, SunSpot 2 | LED, SkyBeam |
|---|---|---|
| UV source | 200 W mercury short-arc, quick-change cartridge | Solid-state UV LED heads |
| Spectral output | 275–650 nm broad spectrum, UVB/UVC capable | 365 / 385 / 395 / 405 nm (guide-dependent) |
| Peak intensity (typ.) | >18,000 mW/cm² UVA (320–390 nm) | 11,000 mW/cm² @ 3 mm spot (365 nm) |
| Warm-up / restart | 1–2 min; 2–5 min cool-down to re-strike | None, instant on/off |
| Source life (typ.) | >2,000 hr lamp (UV1866 quick-change) | 20,000 hr LEDs |
| Output adjustment | 25–100% (knob & guide depth) | 10–100%, front panel |
| Regulation | Lamp power ±1.0% | LED current ±1% |
| Price | $4,220 | $2,900 |
Warm-up, Duty Cycle, and Consumables
Warm-up is the arc's daily tax. The SunSpot 2 needs 1–2 minutes to reach full output, and a hot lamp needs 2–5 minutes of cool-down before it will re-strike. In practice the integral shutter manages this: the lamp stays lit between cures and the shutter gates each exposure, so part-to-part timing stays fast. The cost is that the lamp accrues burn hours whenever the system is running, including idle time between parts. LEDs are instant on and off, emit only during the exposure, and deliver full irradiance the moment you trigger.
That difference drives the consumables math. The SunSpot 2's UV1866 lamp module is rated for more than 2,000 hours and swaps as a quick-change cartridge; the SkyBeam's LEDs are rated for 20,000 hours. That's ten times the service life before you account for duty cycle, and because the arc burns through idle time while the LED only accrues hours under exposure, the calendar gap is wider still. On most lines the LED source simply outlasts the deployment, with no lamp inventory to stock and no changeouts to schedule. An arc lamp, by contrast, is a recurring maintenance item: plan for replacement modules and the few minutes of swap and warm-up each one costs. If your chemistry requires the arc, that's a reasonable price for spectrum you can't get any other way, just budget it up front.
Heat at the Part
Both technologies cure heat-sensitive assemblies without oven cycles, that's the point of spot curing. But radiant heat at the part differs. An arc lamp's broadband output extends through the visible range, so the SunSpot 2 ships with the UV2554 heat filter (a 300–450 nm bandpass) factory-installed to strip unwanted energy before it reaches the light guide, and dual filtered DC fans cool the chassis. LEDs never generate that off-band energy in the first place: the SkyBeam's controller is convection-cooled and its guides are fan- or convection-cooled. For the most temperature-sensitive optics and electronics, narrowband LED delivery is the gentler default; for everything else, the arc's filtered output is a solved problem rather than a disqualifier.
When Each Source Wins
Choose an arc source when the chemistry demands broadband. If your adhesive datasheet calls for UVB/UVC content or a mercury-lamp dose, or your lab runs a mix of legacy and modern chemistries, the SunSpot 2 covers 275–650 nm at more than 18,000 mW/cm² UVA for $4,220, with the timer, shutter, foot pedal, and light guide included.
Choose LED when your adhesive is specified at 365–405 nm. For a defined chemistry on a production bench or line, the SkyBeam delivers matched narrowband output with instant on/off, 20,000-hour sources, four independent channels, and RS-232 plus PLC-style signal I/O for $2,900, a lower purchase price and a lower running cost.
And if the part can't come to the bench at all, the cordless SkyWand puts a 365 or 405 nm LED in your hand for $790, built for rework and field repair rather than fixtured production.
Common Questions
Can an LED system cure an adhesive that was qualified under a mercury lamp?
Often, but not always. If the photoinitiator absorbs strongly at 365–405 nm, a matched LED can reach full cure, but formulations that rely on UVB/UVC content for surface cure may stay tacky under narrowband light. Check the adhesive manufacturer's datasheet for an LED-cure rating before switching sources.
Why is the arc's intensity number so much higher than the LED's?
They're measured differently. The SunSpot 2's >18,000 mW/cm² integrates UVA output across 320–390 nm; the SkyBeam's 11,000 mW/cm² is concentrated at a single 365 nm peak at a 3 mm spot. Compare delivered dose at the wavelength your adhesive actually absorbs, not the headline figures.
What routine maintenance does each source need?
The SunSpot 2 uses a quick-change lamp cartridge rated for more than 2,000 hours, cooled by filtered dual fans. The SkyBeam's 20,000-hour LEDs have no scheduled replacement, keep the guide tips and lenses clean. Both systems carry a 2-year OEM warranty and ship in 24–48 hours.
