Metal Reflectivity at Fiber Laser Wavelengths: Copper, aluminum and gold
All three metals reflect most of the light at common near-infrared processing wavelengths. A cold-surface reflectance value helps explain difficult process starts, but it cannot predict the energy absorbed by a changing melt pool, a textured surface or a thin plated layer.
Use the 1064 nm comparison below to understand the optical balance, then qualify the actual material, surface and laser system before choosing a process setting.
Representative materials; photographs do not establish reflectance. Image credits and licenses.
What does reflectivity tell you about absorbed laser energy?
Reflectance is the fraction of incident optical power that leaves the sample as reflected light. Absorptance is the fraction absorbed, and transmittance is the fraction passing through. For an opaque piece of metal, transmission through the complete sample is usually negligible, so the absorbed fraction is approximately one minus the reflected fraction.
Here, “fiber laser wavelengths” means the common near-1 µm infrared wavelengths used for metal processing, including 1064 and 1070 nm. Fiber delivery alone does not specify a wavelength. A reflectance value must always name the wavelength, surface condition and measurement geometry.
Use total reflectance for the energy balance. A detector that collects only the mirror-like, specular reflection can miss light scattered in other directions. Counting that missing light as absorbed would overestimate heating.
Copper, aluminum and gold reflectance at 1064 nm
These values provide a consistent model baseline. They are calculated for normal incidence from air onto an ideal, smooth, opaque elemental-metal interface, without an added oxide or coating layer.
On a narrow screen, scroll the table horizontally.
| Metal | Modeled reflectance, R | Implied absorptance, A | How to use the value |
|---|---|---|---|
| Copper | 97.8% | 2.2% | A small initial absorbed fraction; surface changes can have a large relative effect. |
| Aluminum | 94.9% | 5.1% | A pure-metal baseline, not a measurement of an alloy or anodized surface. |
| Gold | 97.4% | 2.6% | A bulk-metal baseline; it does not isolate absorption within a thin gold coating. |
Calculation method: Oceanplayer Laser calculated these values from the Lorentz–Drude parameters in Tables 1–2 of Rakić et al. (1998). The model gives the complex refractive index n + ik; normal-incidence reflectance is R = [(n − 1)² + k²] / [(n + 1)² + k²]. Results are rounded to one decimal place. They are model outputs, not guaranteed reflectance ranges for purchased material.
Do not turn this table into a process ranking. A larger absorbed fraction does not by itself make a material easier to weld, mark or cut. Heat flow, joint geometry, oxide, alloy chemistry and the required result still matter.
Estimate the optical balance
Enter total reflectance and the optical power incident on the workpiece. The presets use the rounded 1064 nm model values above. Choose a custom value when you have suitable measurement data for your sample.
Use a value from 0 to 100. Editing this field selects Custom.
Power at the sample, after delivery losses. Enter 0 or a positive value.
Absorbed optical power
- Absorbed fraction
- 2.2%
- Reflected optical power
- 978 W
At 97.8% reflectance and 1,000 W incident power, an opaque sample absorbs 22 W and reflects 978 W.
This estimate holds the reflectance constant. For a pulsed beam, using average power gives an average balance only if that reflectance remains applicable; it does not describe the pulse peak.
For a transmitting sample, use A = 1 − R − T instead. An opaque coated stack may have negligible overall transmission, yet this calculator still cannot tell you how much energy is absorbed in the coating versus the substrate.
Why actual absorptance changes during processing
The first interaction with a cold surface is only one stage. Oxide, residues, texture and incidence angle can change the starting optical response. As heating continues, the surface condition, temperature and geometry can change again. There is no universal rule that absorption rises smoothly with temperature.
Measured example: copper with different surface histories
Mattern and colleagues measured Cu-ETP at 1064 nm using an integrating sphere and a 12° incidence angle, from room temperature through melting. Their sample groups included untreated rolled, mechanically polished, oxide-reduced and resolidified surfaces.
The curves differed between surface groups and between samples. Some showed nonmonotonic changes before melting. The practical lesson is to record cleaning, finish and storage history alongside the grade: a polished-metal value cannot represent every incoming sheet or busbar.
The measurements used nitrogen and relatively slow heating. The authors caution that surface reactions may behave differently during the much faster heating of laser welding, so the measured curves are not a direct production recipe.
A melt pool or keyhole changes the optical geometry
Once the surface deforms, the beam no longer meets a flat interface. Light entering a keyhole can encounter the metal repeatedly, increasing effective coupling. NIST measurements of 10 ms, 1070 nm spot welds in 316L stainless steel illustrate this dynamic behavior. They explain the mechanism; they are not a copper-specific absorption curve or a power threshold for your part.
A cold-surface absorptance is therefore a poor basis for prescribing weld power on its own. Absorbed optical energy also spreads by conduction or leaves through other loss mechanisms, so it is not equivalent to useful melting energy.
What changes for copper, aluminum and gold?
Use the metal name to start the investigation. The purchase specification and actual surface tell you what the laser will encounter.
Copper: control the start and the heat path
Low initial near-infrared absorption and rapid heat spreading can make process initiation sensitive to the surface and delivered intensity. Increasing nominal power changes only one part of that interaction.
For a busbar joint, record grade, thickness, finish, contact gap and clamping. Compare the start of the weld with the established seam; a stable middle section can conceal an inconsistent start. Evaluate the joint against its electrical and mechanical requirements.
The copper busbar welding guide covers the joint design and quality checks that a reflectance value cannot supply.
Aluminum: specify the alloy and surface layer
A pure-aluminum optical model does not describe every aluminum product. Bare, brushed, oxidized, anodized and coated surfaces are different optical systems. An anodized marking sample should not inherit the bare-metal preset as a measured value.
For welding, separate the coupling question from oxide, contamination, porosity and solidification concerns. Improved absorption alone does not prove a sound joint.
Test material from the intended supply route, after the intended cleaning step. If results drift with finish or storage, include those conditions in the process specification rather than compensating with an undocumented power change.
Gold: distinguish bulk metal from plating
Bulk gold, a gold alloy and a thin gold layer over another metal require different assumptions. The appearance of a gold-colored surface does not identify its composition or thickness.
For plating, obtain the gold thickness, underlayers and substrate. Even if the complete stack is opaque, total absorbed energy may be distributed between layers. A bulk-gold reflectance value cannot determine that distribution or a safe engraving depth.
Define whether the task is surface marking, coating removal or substrate exposure. Inspect the finished layer and required function, not just mark visibility; stop if the permitted coating-loss or substrate-damage limit is exceeded.
Does a shorter wavelength solve the problem?
A shorter wavelength can improve initial coupling, but its value depends on the metal and process. In a specific example, Fraunhofer IWS reported 515 nm trials on copper, aluminum and gold with increased absorption compared with near-infrared processing. That supports testing another wavelength; it does not establish a universal machine choice.
Infrared processing remains a valid option. TRUMPF describes both green and infrared approaches to copper welding, including infrared beam strategies for controlling energy input. Compare accepted-part quality and repeatability under matched requirements.
Keep wavelength separate from pulse duration
Wavelength describes the light’s spectral position. Continuous-wave operation, nanosecond pulses and ultrashort pulses describe how energy is delivered over time. “UV” and “ultrashort pulse” therefore answer different questions. Average power, pulse energy, peak power, spot size and motion also affect the interaction; IPG’s fiber-laser overview explains these distinctions.
For a broader source comparison, use the metal-processing wavelength guide. For this article’s optical balance, always use reflectance data at the wavelength actually delivered to the sample.
Manage reflected light as a system issue
A surface reflecting 98% of incident power does not mean 98% returns into the laser source. The return path depends on reflection direction, surface shape, incidence angle and how the processing optics collect the light. The optical balance calculator estimates total reflected power, not power recoupled into the source.
Confirm suitability for the exact source, delivery fiber, processing head and operation. Protection is model-specific: for example, a TRUMPF system-builder report describes integrated back-reflection protection in TruFiber P. That feature should not be assumed for an unspecified fiber laser.
- Ask for the supported operating envelope: material, process mode, optical configuration and the manufacturer’s return-light limits or protective response.
- Include transient conditions: starts, stops, gaps and changing surface geometry may behave differently from an established track.
- Use an approved beam geometry: do not copy a generic tilt angle. A geometry change also changes the footprint, focus and reflected beam direction.
- Treat an alarm as a stop condition: investigate the cause within the equipment supplier’s procedure before repeating the run.
Source protection does not replace enclosure and reflected-beam controls for people around the machine. The highly reflective materials welding guide covers the wider process and system qualification work.
Qualify the real surface before choosing settings
Use reflectance to frame a trial, then accept the process on the required part result. A useful comparison keeps the material and inspection requirements visible instead of changing several unknowns at once.
On a narrow screen, scroll the table horizontally.
| Step | Record or verify | Decision it supports |
|---|---|---|
| Identify the sample | Grade, thickness, surface finish, cleaning, oxide or coating stack; representative incoming batches. | Whether the optical assumption and sample actually match production. |
| Define the laser input | Wavelength, power at the workpiece, spot size and focus, beam profile, motion and pulse conditions where applicable. | Whether a difference comes from the source setting, delivery or material. |
| Observe the transition | Process start and established track, visible instability, spatter, protective events and available monitoring signals. | Whether a usable operating window exists beyond one successful specimen. |
| Inspect the result | Weld section and specified joint tests; or mark durability, removal depth, coating retention and substrate condition. | Whether the part meets its actual functional limits. |
Release a defined operating window. Agree acceptance limits before trials, repeat the checks on representative surfaces, and keep a record of settings and results. Stop on a protective event, unacceptable coating loss or a failed part requirement; a calculated absorbed-power value is not evidence that the process is safe or capable.
Sources and image credits
Research behind the optical comparison
- Rakić et al., “Optical properties of metallic films for vertical-cavity optoelectronic devices”, Applied Optics 37, 5271–5283 (1998). Model parameters for the calculated 1064 nm baseline.
- Mattern et al., “Temperature-Dependent Reflectance of Copper with Different Surface Conditions Measured at 1064 nm”. Evidence for surface-dependent and temperature-dependent Cu-ETP reflectance.
- Simonds et al., “Optical Measurements of Dynamic Absorptance during High-power Laser Spot Welding”, NIST. The cited experiment concerns 316L stainless steel.
Equipment and wavelength examples are linked beside the relevant statements. Manufacturer examples describe particular technologies; they do not qualify an unspecified machine or workpiece.
Photographs
Copper busbars: Motimachine, CC BY-SA 4.0. Brushed aluminum: Andrezadnik, CC BY-SA 3.0. Both hero photographs are cropped to fit; their crops retain the respective licenses.
.jpg){kind=link}
Gold bullion: Stevebidmead, CC0 1.0; cropped to fit. Copper microscopy: Leiem, CC BY-SA 4.0; full frame.
The photographs illustrate materials and surface appearance. They are not Oceanplayer Laser process trials or images of the cited experiments.
Bring the material specification to the laser discussion.
Share the metal grade, surface condition, dimensions, coating stack and required result with Oceanplayer Laser so the discussion starts with the part you need to make.