What Are the Signs Before a Lens Goes Bad?
2026-10-9
Optical components rarely die suddenly. The vast majority of failures are gradual. The problem is that many warning signs are ignored until the power drops and the beam profile deforms, and only then do people look back for the cause.
1. Signs Visible to the Naked Eye
A foggy appearance on the surface
Appearance: When held up to a light, there is a faint fog on the lens surface that cannot be wiped off.
Judgment: Use a magnifier to examine the distribution. If it is concentrated in the beam-irradiated area and the edges are clean, it is basically coating hydrolysis—moisture penetrates the pores of the coating and forms a low-density, loose layer at the interface, increasing scattering. If the distribution is uniform, it may be overall moisture exposure or contamination.
Handling: Wipe first. If it cannot be wiped off, it is inside the coating.
Transmittance drop <3%: Place in a drying cabinet at 60°C for 2–4 hours; it can partially recover.
Drop >5%: Replace. After hydrolysis, the damage threshold drops significantly.
Colored fringes at the edges
Appearance: Irregular colored rings appear at the lens edge or around the beam edge.
Judgment: First distinguish normal interference color from failure. Normal interference color moves with the viewing angle and has smooth color transitions; failure color is fixed in position with sharp boundaries. Then use a polarizer to distinguish: if the color changes when rotating the analyzer → stress birefringence; if unchanged → coating peeling.
Handling: Stress birefringence: loosen the mount and see whether the fringes decrease. Fused silica stress birefringence should be controlled below 5 nm/cm.
Coating peeling: irreparable. If it is not in the beam, it may be used temporarily, but it must be marked for observation; if it enters the beam, replace immediately.
A halo around the beam spot
Appearance: When a low-power laser hits it, there is a ring of scattered light around the spot.
Judgment: Place white paper in the beam path to see the distribution. Linear and concentrated → scratches; a uniform ring → coating degradation or increased roughness; speckled → dust or localized damage. Wipe first; if scattering remains after wiping, it is a coating or substrate problem.
Handling: Measure surface roughness. If RMS deteriorates from <0.5 nm to >1 nm, the coating has clearly degraded.
Measure total integrated scatter (TIS). TIS >0.5% suggests replacement. At high power, scattering increases local absorption and forms positive feedback.
2. Signs That Require Instruments to Detect
Slow decrease in transmittance
Appearance: Output power decreases month by month, not much each time, but the trend is downward.
Judgment: Use a spectrophotometer to measure the curve. An overall downward shift → increased absorption or scattering; a dip near the design wavelength → change in the optical thickness of the coating (usually hydrolysis). If you only have a power meter, use the dual-beam comparison method to rule out laser degradation.
Handling: Establish a baseline and measure once a month.
Drop <2%: Continue using, increase inspection frequency.
Drop 2–5%: Mark and analyze the cause.
Drop >5%: Replace.
Increased wavefront distortion
Appearance: The interferometer measures PV changing from λ/10 to λ/5, and RMS from λ/50 to λ/25.
Judgment: Turn off the light and cool before measuring. If it recovers → thermal deformation (thermal expansion caused by lens absorption); if it does not recover → stress release or coating stress change. Mostly astigmatism → stress release; mostly spherical aberration → coating stress change.
Handling: Thermal deformation: measure absorptance. If >50 ppm, consider switching to a low-absorption lens.
Stress release: Loosen the mount and see whether it improves. If not, it is irreversible and replacement is recommended.
Coating stress change: If wavefront deterioration is >50% and affects the experiment, replacement is the only option.
Decrease in laser damage threshold
Appearance: It used to withstand 10 J/cm², but now it damages at 8 J/cm².
Judgment: The root cause is increased local absorption. Use photothermal microscopy scanning: if absorption hot spots coincide with the beam → contaminant-induced; if randomly distributed → subsurface damage or coating defects. If no equipment is available, perform an alternative test: fire 1000 shots continuously at 80% of the nominal threshold and see whether there are damage spots.
Handling: Contaminant-induced: if the threshold recovers after cleaning, continue using.
Subsurface damage: irreparable; replacement is recommended.
Coating interface degradation: irreparable; replacement is recommended.
3. Correspondence Between Different Failure Modes and Signs
4. Daily Monitoring and Judgment Thresholds
Daily (before and after each use)
Visual inspection under a light: fog, colored fringes, scratches
Compare output power with last time
Weekly
Measure transmittance at low power and compare with baseline
Check mount looseness and changes in clamping force
Record laboratory temperature and humidity
Monthly
Use a polarizer to check stress birefringence and see changes in extinction ratio
Use white paper or a CCD to check scattered light distribution and compare with last month
Check drying cabinet humidity
When conditions permit (no mandatory period)
Wavefront PV/RMS (requires removing the lens, high cost, only when a problem is suspected)
Surface roughness
Absorptance or TIS
Judgment thresholds—handle immediately if the following occur
Transmittance drops by more than 5%
Coating peeling or detachment is visible to the naked eye
The scattering ring cannot be wiped off
Wavefront PV deteriorates by more than 50%
Polarization extinction ratio drops by more than 20%
Handling methods
Drop <2%: Continue using, increase monitoring frequency
Drop 2–5%: Mark, analyze the cause, try drying or cleaning
Drop >5%: Replace
1. Signs Visible to the Naked Eye
A foggy appearance on the surface
Appearance: When held up to a light, there is a faint fog on the lens surface that cannot be wiped off.
Judgment: Use a magnifier to examine the distribution. If it is concentrated in the beam-irradiated area and the edges are clean, it is basically coating hydrolysis—moisture penetrates the pores of the coating and forms a low-density, loose layer at the interface, increasing scattering. If the distribution is uniform, it may be overall moisture exposure or contamination.
Handling: Wipe first. If it cannot be wiped off, it is inside the coating.
Transmittance drop <3%: Place in a drying cabinet at 60°C for 2–4 hours; it can partially recover.
Drop >5%: Replace. After hydrolysis, the damage threshold drops significantly.
Colored fringes at the edges
Appearance: Irregular colored rings appear at the lens edge or around the beam edge.
Judgment: First distinguish normal interference color from failure. Normal interference color moves with the viewing angle and has smooth color transitions; failure color is fixed in position with sharp boundaries. Then use a polarizer to distinguish: if the color changes when rotating the analyzer → stress birefringence; if unchanged → coating peeling.
Handling: Stress birefringence: loosen the mount and see whether the fringes decrease. Fused silica stress birefringence should be controlled below 5 nm/cm.
Coating peeling: irreparable. If it is not in the beam, it may be used temporarily, but it must be marked for observation; if it enters the beam, replace immediately.
A halo around the beam spot
Appearance: When a low-power laser hits it, there is a ring of scattered light around the spot.
Judgment: Place white paper in the beam path to see the distribution. Linear and concentrated → scratches; a uniform ring → coating degradation or increased roughness; speckled → dust or localized damage. Wipe first; if scattering remains after wiping, it is a coating or substrate problem.
Handling: Measure surface roughness. If RMS deteriorates from <0.5 nm to >1 nm, the coating has clearly degraded.
Measure total integrated scatter (TIS). TIS >0.5% suggests replacement. At high power, scattering increases local absorption and forms positive feedback.
2. Signs That Require Instruments to Detect
Slow decrease in transmittance
Appearance: Output power decreases month by month, not much each time, but the trend is downward.
Judgment: Use a spectrophotometer to measure the curve. An overall downward shift → increased absorption or scattering; a dip near the design wavelength → change in the optical thickness of the coating (usually hydrolysis). If you only have a power meter, use the dual-beam comparison method to rule out laser degradation.
Handling: Establish a baseline and measure once a month.
Drop <2%: Continue using, increase inspection frequency.
Drop 2–5%: Mark and analyze the cause.
Drop >5%: Replace.
Increased wavefront distortion
Appearance: The interferometer measures PV changing from λ/10 to λ/5, and RMS from λ/50 to λ/25.
Judgment: Turn off the light and cool before measuring. If it recovers → thermal deformation (thermal expansion caused by lens absorption); if it does not recover → stress release or coating stress change. Mostly astigmatism → stress release; mostly spherical aberration → coating stress change.
Handling: Thermal deformation: measure absorptance. If >50 ppm, consider switching to a low-absorption lens.
Stress release: Loosen the mount and see whether it improves. If not, it is irreversible and replacement is recommended.
Coating stress change: If wavefront deterioration is >50% and affects the experiment, replacement is the only option.
Decrease in laser damage threshold
Appearance: It used to withstand 10 J/cm², but now it damages at 8 J/cm².
Judgment: The root cause is increased local absorption. Use photothermal microscopy scanning: if absorption hot spots coincide with the beam → contaminant-induced; if randomly distributed → subsurface damage or coating defects. If no equipment is available, perform an alternative test: fire 1000 shots continuously at 80% of the nominal threshold and see whether there are damage spots.
Handling: Contaminant-induced: if the threshold recovers after cleaning, continue using.
Subsurface damage: irreparable; replacement is recommended.
Coating interface degradation: irreparable; replacement is recommended.
3. Correspondence Between Different Failure Modes and Signs
|
Failure mode |
Typical signs |
Judgment method |
Failure speed |
|
Coating hydrolysis |
Foggy appearance, appears first in the center |
Magnifier to see distribution; dip in spectral curve |
Slow, months to a year |
|
Coating detachment |
Colored fringes at the edges |
Polarizer to distinguish stress birefringence |
Medium, weeks to months |
|
Stress birefringence |
Wavefront distortion, polarization characteristics change |
Loosen the mount and see whether it recovers |
Slow, may release suddenly |
|
Subsurface damage |
Increased scattering, decreased threshold |
Photothermal microscopy to locate hot spots |
Slow, then sudden breakdown |
|
Contaminant carbonization |
Black deposits at the beam edge |
Photothermal microscopy; compare after cleaning |
Medium, depends on power density |
|
Thermal deformation |
Wavefront changes with power |
Turn off light, cool, then measure |
Fast, changes in real time with power |
4. Daily Monitoring and Judgment Thresholds
Daily (before and after each use)
Visual inspection under a light: fog, colored fringes, scratches
Compare output power with last time
Weekly
Measure transmittance at low power and compare with baseline
Check mount looseness and changes in clamping force
Record laboratory temperature and humidity
Monthly
Use a polarizer to check stress birefringence and see changes in extinction ratio
Use white paper or a CCD to check scattered light distribution and compare with last month
Check drying cabinet humidity
When conditions permit (no mandatory period)
Wavefront PV/RMS (requires removing the lens, high cost, only when a problem is suspected)
Surface roughness
Absorptance or TIS
Judgment thresholds—handle immediately if the following occur
Transmittance drops by more than 5%
Coating peeling or detachment is visible to the naked eye
The scattering ring cannot be wiped off
Wavefront PV deteriorates by more than 50%
Polarization extinction ratio drops by more than 20%
Handling methods
Drop <2%: Continue using, increase monitoring frequency
Drop 2–5%: Mark, analyze the cause, try drying or cleaning
Drop >5%: Replace
