A xenon bulb change takes 20–30 minutes; the alignment and calibration check that follows takes another 15. Shut down, let the housing cool for at least 30 minutes, wear lint-free gloves, and mark the reflector and bulb-seat positions before you loosen anything. After ignition, warm up for 15–20 minutes and confirm reactor-plane irradiance sits within ±5% of your recorded baseline.
Bulb replacement is mechanically simple and optically destructive. Opening the housing disturbs four adjustments at once:
The consequence is measurable. At a 200 mm working distance, a 1 mm arc offset typically moves the spot 3–5 mm and can cost 10–15% of the edge irradiance in a 50 mm window — enough to change a reported rate constant silently.
Run all six checks before the next experiment. If one fails, fix it rather than compensating in the analysis.
| Check | Method | Acceptance criterion |
|---|---|---|
| Irradiance at reactor plane | Calibrated power meter, same distance and filter as baseline | Within ±5% of the recorded baseline |
| Spot position | Target paper at the window plane, not at the wall | Centre within 2 mm of the mark |
| Uniformity | Centre reading vs 80%-radius reading | Deviation under 10% |
| Arc stability | Log output for 60 s after warm-up | Peak-to-peak fluctuation under 2% |
| Spectral output | Compare through the same band-pass filter | No new IR or UV leak; ratio within 5% |
| Cooling | Outlet air temperature at the housing | Within 5 °C of the previous reading |
Refocus only after the arc is stable. Low but uniform irradiance points to the bulb or reflector; a bright centre with a dim edge is a focus problem, not a bulb problem.
Fused silica at 600–900 °C is unforgiving about contamination. Salts from a fingerprint lower the local softening point, the envelope devitrifies, and the hot spot grows until the bulb fails — often within 50–100 hours instead of the expected 500–1000 hours. Uneven clamp torque tilts the base and pulls the arc off the reflector axis, so you lose intensity and uniformity while the bulb itself is healthy.
Plan for 45–60 minutes: 30 minutes of cooling before you open the housing, 10–15 minutes of mechanical work, and 15–20 minutes of warm-up plus verification. Rushing the cooling step is the most common cause of a cracked envelope.
No. A short-arc envelope at 600–900 °C will burn gloves and skin, and the internal pressure makes a hot bulb a handling hazard. Thermal expansion has also moved every mount, so alignment measured while hot is meaningless.
Not the meter itself, but you must re-measure the baseline at the reactor plane. The meter’s calibration is independent of the lamp; the irradiance it reads is not. Store centre and edge values with the bulb’s installation date and hour count.
Match electrical power, arc size and window material, not just wattage. A 300 W bulb with a different arc length focuses differently in the same reflector, changing both spot size and irradiance.
