Transmission Testing for Laser Processes – Measuring the Laser Transparency of Thermoplastics

Transmission testing determines what fraction of the laser radiation passes through the laser-transparent joining partner and therefore actually becomes available at the joint plane. In laser transmission welding of thermoplastics, the melt does not form where the laser is set, it forms where the photons arrive. Between the two lie reflection at the interfaces, absorption within the volume, and – for semi-crystalline, filled or pigmented materials – scattering. What matters, therefore, is the spectral transmittance at the process wavelength, the optical path length through the part, and the angular distribution of the emerging radiation. Only these quantities determine intensity, line energy and the temperature field at the joint surface, and with them the reproducibility of the weld.

There is no standalone, cleanly delimited market figure for transmission testing itself. The reliable market indicator instead is the adjacent market for plastic laser welding, since any production-scale system requires material and part qualification based on laser transparency.

Commercial market studies put this market at roughly USD 1.5 to 1.7 billion for 2025, growing to about USD 2.3 to 2.7 billion by 2032 – annual growth of roughly 5 to 8 %. The spread between estimates is considerable and results from differing scopes: some count only equipment, others include contract manufacturing, tooling and service. The order of magnitude is informative, the exact figure is not.

More technically relevant than the absolute number is a shift within the value chain:

The causal chain is consistent throughout: miniaturisation and functional integration shrink joint areas and wall thicknesses. That narrows the tolerable energy margin, because the same absolute deviation produces a larger relative error. At the same time, the share of filled and coloured compounds is rising, and their optical properties vary by batch and by position. Together, both effects shift quality assurance from post-weld seam inspection to material testing beforehand.

Regulation weighs most heavily in medical technology: EU Regulation 2017/745 (MDR) requires process validation with documented process limits for processes that cannot be inspected non-destructively – and joining fluidic channels is one of them. The transmittance of the joining partner is one of the few quantities that can be captured non-destructively before joining even takes place.

Laser transmission welding joins two thermoplastics that differ in how they interact optically with the laser radiation. The upper joining partner is largely transparent at the process wavelength; the lower one absorbs it – usually through carbon black, or alternatively through NIR absorbers that are inconspicuous in the visible range.

The radiation passes through the upper partner, is converted to heat in a thin surface layer of the absorbing material at the interface, and locally melts it. The heat then reaches the transparent partner by conduction, which melts as well. Under joint pressure, a material bond forms through interdiffusion of the macromolecules across the interface.

Physically, the balance of radiant fractions holds:

A + R + T = 1

with absorptance A, reflectance R and transmittance T. All three quantities depend on the material, the wavelength and the temperature, and always refer to a specific material thickness. In optically inhomogeneous media – for example due to glass fibres, pigments or crystallites – scattering additionally deflects radiation out of its original direction of propagation without absorbing it.

Typical process wavelengths lie between 800 and 1100 nm, since most unpigmented thermoplastics absorb only weakly in this range. 980 nm has become the reference wavelength for transmission testing because it sits at the centre of the industrially used diode laser range.

The process variant determines how many measurement points a transmission test needs and how strongly local transmission variations propagate into the result.

Among beam sources, diode lasers at 808 to 980 nm dominate because of their high electro-optical efficiency and compact design. Fibre lasers at 1064 nm offer better beam quality and smaller focus diameters. Thulium fibre lasers around 1940 nm address the polymer's own C-H and O-H overtone absorption and allow joining without an absorber; there, the energy deposition shifts from the interface into the volume of both partners.

Important for test planning: transmission testing must be done at the wavelength that will later be used for welding. A value measured at 980 nm does not carry over to a 1940 nm process, because that process deliberately exploits absorption bands that are avoided at 980 nm.

Transmittance T at the process wavelength. It scales the power available at the joint gap directly: P_joint = T · P_laser. A drop from 45 % to 35 % means 22 % less energy at the joint surface – with the system parameters unchanged.

Optical path length. Transmittance is not a material constant, it applies to a specific thickness. Wall thickness transitions, ribs, chamfers and obliquely irradiated regions lengthen the path and reduce T disproportionately once scattering dominates.

Scattering and angular distribution. Scattered radiation is not lost; it reaches the joint plane spread out and with a lower peak intensity. The seam becomes wider, the peak temperature lower. Two parts with identical overall transmittance can therefore produce different weld results.

Absorptivity of the lower partner. The penetration depth of the radiation into the absorbing joint surface determines whether the heat forms across an area or within a very thin layer. Too high an absorber concentration concentrates the energy into a few micrometres and promotes thermal damage and outgassing.

Intensity and interaction time. It is not the energy alone that decides the outcome, but its distribution in time. Intensity I = P/A and interaction time t = d_spot/v jointly determine the peak temperature reached; heat conduction acts as a competing loss mechanism.

Joint pressure and gap. An air gap interrupts heat conduction into the transparent partner almost completely, since the thermal conductivity of air is roughly two orders of magnitude below that of thermoplastics.

Part temperature and moisture. The optical properties are temperature-dependent; for polyamides, moisture uptake additionally changes the NIR absorption behaviour, because O-H overtone bands are added. This matters for absorber-free processes; for 980 nm processes it is usually secondary.

This section summarises the misinterpretations encountered most often in practice.

Datasheet values do not apply to the actual part. Compound datasheets state transmission values for injection-moulded standard plaques of defined thickness. In the real part, fibre orientation, weld lines, gate location, mould temperature and local cooling rate change the degree of crystallinity and the fibre distribution. The result is transmission differences of several percentage points within a single part – with nominally identical material.

Nominally sufficient laser power is not a guarantee about the joint plane. The figure quoted is the optical output power of the source. Between the source and the joint surface lie optical losses, Fresnel reflection at two interfaces, volume absorption and scattering. Process approvals based on the set power systematically overestimate the energy actually available.

Electrical input power is not an optical dose. The electro-optical efficiency of diode lasers is typically 40 to 60 % and drops as the junction temperature rises. Process control based on electrical input power captures neither ageing of the diode bars nor contamination of the protective windows.

Directional and diffuse transmission get confused. An integrating sphere captures the hemispherical total transmission and returns the higher value for scattering materials. A narrow detector aperture captures only the quasi-collimated fraction and returns a lower value. Both values are correct and answer different questions: total transmission describes the energy balance, the aperture-limited measurement the intensity concentrated within the seam cross-section. Without stating measurement geometry, aperture size and detector distance, transmittance values for scattering parts are not comparable.

Measuring at the wrong location. A measurement on a flat reference area outside the joint zone captures neither wall thickness transitions nor local fibre accumulation along the seam. What is meaningful is the transmission profile along the planned weld contour, not a single value.

Measuring at the wrong wavelength. The total luminous transmittance determined per ISO 13468 refers to the visible spectral range and is not meaningful for the NIR process wavelength. Conversely, high NIR transmission says nothing about visual appearance – laser-black compounds that stay transparent in the NIR exploit exactly this decoupling.

Reciprocity is assumed where it does not hold. The idea that doubling power while halving exposure time produces the same result only holds while heat conduction is negligible. In slow processes, heat flows into the bulk of the part and the peak temperature actually reached falls short of the calculation; in very fast processes, thermal damage to the absorber limits the permissible intensity.

Transmission is used as the sole release criterion. T describes only the transparent partner. If the absorber concentration in the counterpart varies, the energy deposition shifts without the transmission measurement showing it. A robust material release covers both joining partners.

Positional tolerance for scattering specimens is underestimated. For strongly scattering parts, the displayed transmittance depends noticeably on the position and orientation of the specimen in the measuring head. Without a defined, repeatable part fixture, measurement scatter can exceed the material variation it is meant to detect.

Crystallinity. Semi-crystalline thermoplastics such as PA, PP, POM and PBT contain spherulites whose refractive index differs from that of the amorphous phase. Radiation scatters at these interfaces. Amorphous materials such as PC, PMMA and PS therefore show markedly higher directional transmission. Because crystallinity depends on cooling rate, mould temperature and wall thickness act directly on laser transparency – a moulding process parameter becomes an optical parameter of the welding process.

Glass fibres. Reinforcing fibres reduce transmission mainly through scattering, not absorption. Studies on short-glass-fibre-reinforced PA6 show the transmitted laser energy falling from about 70 % to around 20 % as fibre content rises from 0 to 63 wt.-%; between 0 and 50 wt.-%, the scattered fraction increases by up to about 50 %. Fibre orientation matters in practice too: effective scattering differs between flow-aligned regions and weld-line regions.

Mineral fillers. Talc, chalk and glass beads reduce transmission far more strongly than glass fibres at the same mass fraction. Glass beads can prevent transmission welding even at small wall thicknesses.

Pigments. Opaque white pigments, titanium dioxide in particular, are largely ruled out for laser-transparent parts, since their scattering is only weakly wavelength-dependent and acts similarly in the NIR as in the visible range. Carbon black in the absorbing partner, by contrast, is desired; mass fractions of just 0.05 to 0.5 % already generate sufficient absorption.

Part thickness. For scattering materials, transmission loss grows faster than a pure absorption model would predict, because multiple scattering disproportionately increases the mean photon path length. For strongly scattering materials such as POM or PBT, this limits the thickness of the transparent partner in practice to a few millimetres; for PEEK the limit is even lower.

Geometry and angle of incidence. At curved surfaces and radial seams the radiation strikes at an angle. The irradiated path length grows with 1/cos θ, while the Fresnel reflectance simultaneously rises with the angle of incidence. Both effects reduce the power arriving at the joint without any change in material. Transmission tests on rotationally symmetric parts should therefore be carried out in the actual process geometry.

The simplest quantitative description of transmission combines Fresnel interface reflection with Beer-Lambert volume absorption:

T = (1 − R)² · e^{(−α · d)}

where

• T transmittance (dimensionless)
• R reflectance of one interface at normal incidence, R = ((n − 1)/(n + 1))²
• n refractive index of the polymer (PA6: ≈ 1.53; PC: ≈ 1.58; PMMA: ≈ 1.49)
• α absorption coefficient in mm⁻¹ at the process wavelength
• d irradiated thickness in mm

Consequence of the Fresnel terms. For PA6 with n = 1.53, R ≈ 4.4 % per interface. Even a perfectly absorption-free, non-scattering part therefore reaches only T ≈ 91.4 %. Transmission requirements above this value are physically unachievable for uncoated plastic surfaces.

Assumptions of the model. The Beer-Lambert law assumes a homogeneous, non-scattering medium, monochromatic radiation, collimated propagation and a linear interaction. It therefore applies approximately to amorphous, unfilled thermoplastics.

Limits. As soon as scattering occurs – which means practically every semi-crystalline, filled or pigmented material – α no longer describes absorption alone, but becomes an effective extinction coefficient µ_ext = µ_a + µ_s combining absorption and scattering. The distinction is process-critical: absorbed radiation is lost to the joint plane, scattered radiation still reaches it, only spread out spatially. A measurement that captures only the collimated fraction cannot reconstruct the energy balance. A physically correct description requires the radiative transport equation – approximated in practice by two-flux models of the Kubelka-Munk type, or by Monte Carlo and ray-tracing methods – to split the radiation into absorbed, forward- and back-scattered fractions.

Practical consequence. For scattering materials, a single transmittance value is only a meaningful process parameter if the measurement geometry resembles the process geometry: the same wavelength range, a comparable beam diameter, a defined detector aperture, and a defined distance between the underside of the part and the detector. This is exactly why process standards mandate stating the measurement principle – power measurement, photometric or spectral measurement.

1. Assumptions. Contour welding of a housing made of PA6-GF30, wall thickness 2 mm, 980 nm diode laser, optical output power P₀ = 40 W, spot diameter at the joint plane d_S = 1.5 mm, feed rate v = 100 mm/s. Release value of transmittance T₁ = 42 %; in a region with local fibre accumulation, T₂ = 30 % is measured.

2. Model.
P_F = T · P₀  ·  E_l = P_F / v  ·  I = P_F / A  ·  t = d_S / v  ·  H = I · t

3. Calculation

4. Result. A 12-percentage-point drop in transmission reduces the energy available at the joint plane by roughly 29 %.

5. Technical interpretation. On paper, the loss could be offset by raising the output power to about 56 W or reducing the feed rate to about 71 mm/s. The two routes are not equivalent, however: higher power raises the peak temperature in the absorber and with it the risk of thermal damage and outgassing; a lower feed rate lengthens the heat-conduction time and widens the heat-affected zone. Nor is the compensation exactly linear, since melt-layer thickness also depends on heat conduction. Rather than a blanket increase in the parameters, a spatially resolved transmission test is the better route – it either enables position-dependent power control or allows the part to be sorted out before joining.

Automotive and e-mobility. Sensor housings, ECU covers, expansion tanks and fluid manifolds are predominantly joined from glass-fibre-reinforced polyamides and PBT. What matters here is batch and positional variation in fibre content, because the same system parameterisation has to run for months across changing material batches. The critical process quantity is the transmission profile along the circumferential sealing seam, not the average.

Medical technology and in-vitro diagnostics. Microfluidic cartridges contain channels a few hundred micrometres wide. A melt zone that is too wide narrows or blocks the channel. Because the seam cannot be fully assessed non-destructively after joining, quality assurance shifts to material release. What matters is the spatial resolution of the measurement and the traceability of the measurement chain.

Electronics and sensors. For hermetically sealed housings, seam quality determines the ingress protection rating and with it resistance to moisture and media – reproducibility of energy input along the full contour is critical in electronics and semiconductor manufacturing as well, particularly at corners, where interaction time changes with path deceleration.

Optics and photonics. For light guides, lens holders and covers made of PC or PMMA, a dual requirement applies: high NIR transmission for the joining process and defined transmission in the intended spectral range – the same dual requirement seen in optical precision components. Both quantities must be tested separately, since they concern different wavelength ranges.

Packaging and consumer goods. When joining thin films and semi-finished products, thickness dependence dominates: for PA6/PP film laminates, achievable weld speed drops markedly as film thickness increases, because more material has to melt and the low thermal conductivity of thermoplastics limits the process time.

Research and materials development. When developing laser-transparent compounds, spectrally resolved transmission across 600 to 2000 nm is the central evaluation quantity, since it shows whether a formulation change altered absorption or scattering.

Adjacent application fields: UV bonding, potting and encapsulation, electronics and semiconductors, optics and precision components cover neighbouring joining processes and part classes.

Two transparent joining partners. Absorber-free welding at wavelengths between about 1500 and 2000 nm exploits the polymer's own absorption. The energy is then deposited not at the interface but across a volume; focus position determines the location of the melt zone. Transmission testing must be carried out at the actual process wavelength here, since the optical properties differ fundamentally from those at 980 nm. Alternatively, NIR absorbers that are transparent in the visible range are used as a thin interlayer in the joint gap.

Visually black, transparent parts. Colourant systems without carbon black allow visually deep-black parts with high NIR transmission. Release of such compounds is only possible through an NIR transmission measurement; a visual assessment is systematically misleading.

Strongly scattering materials. For POM, PBT and highly filled compounds, the thickness of the transparent partner is limited in practice to one or two millimetres. Adapted irradiation strategies – such as beam shaping to a flat-top or double-peaked intensity profile, or multiple passes – partly compensate for the scattering-induced widening.

Large variations in wall thickness. For parts with a varying wall thickness along the seam, position-dependent power control is worthwhile. It relies on a transmission profile measured beforehand and fed into the path controller.

The reasoning runs from the application to the measurement quantity, not the other way round.

Which quantity? What is needed is the transmittance of the laser-transparent joining partner at the process wavelength, stated together with the part thickness at the measurement location. For materials development and failure analysis, spectrally resolved transmission is additionally required; for series testing, a single-wavelength measurement is usually sufficient – the underlying quantities are summarised in the overview of radiometric quantities.

Where to measure? On the part, in the joining geometry, and along the planned weld contour. A measurement grid along the seam reveals fibre accumulation, weld lines and wall thickness transitions that a single measurement point would miss.

Which measurement uncertainties matter?

• Drift of the light source from ageing and temperature – limitable with a dual-beam design and an internal reference diode
• Ambient light in the production environment – reducible through modulated sources and synchronous detection
• Positioning uncertainty of the specimen – often the dominant contribution for scattering parts, controllable only with a part-specific, repeatable fixture
• Definition of detector aperture and distance – determines which scattered fraction is captured
• Traceability of the reading – via reference targets of known transmittance and calibration in a laboratory accredited to DIN EN ISO/IEC 17025

When is a broadband or single-wavelength measurement enough? For released compounds with a known spectral profile, measuring at the process wavelength is sufficient, since only deviations from a known state need to be detected.

When is a spectral measurement required? On a material or formulation change, colourant changes, complaint analysis, and absorber-free processes. Only a spectrum shows whether a change in transmission stems from additional absorption bands or from altered scattering – the first can be avoided by choosing a different wavelength, the second cannot.

When are spatially or time-resolved measurements needed? Spatially resolved for any part with uneven fibre distribution or variable wall thickness, and for seam widths under about one millimetre. Time-resolved for processes with thermal feedback, where the optical properties change during welding.

Only after settling these points does the choice of instrument follow: clamp-style process photometers for production-line, single-wavelength testing of flat part areas, spectrophotometers with an integrating sphere for determining directional and diffuse transmission on material samples, and accredited calibration to trace the whole measurement chain.

In series production, transmission testing is increasingly not a laboratory step but a manufacturing step ahead of joining.

Upstream part inspection. A measurement system traces the weld contour – defined manually or taken from CAD data – and produces a transmission profile. The technical significance lies not in the data transfer, but in the fact that a part can be sorted out before the irreversible joining step. That avoids bonding two parts together when one of them is not weldable.

Adaptive power control. If the transmission profile is passed to the system controller, laser power can be adjusted position by position so that line energy at the joint plane stays constant, instead of the set output power. That shifts the control variable from a system parameter to the physically effective quantity.

In-process monitoring. Pyrometry and thermal imaging capture surface temperature during welding. For absorber-free processes with longer-wavelength radiation this is limited, because the process radiation spectrally overlaps with the thermal radiation being measured. Joint-path measurement in quasi-simultaneous welding instead provides a direct mechanical measure of the melted volume.

Data feedback and traceability. Linking part identification, transmission profile and weld parameters enables statistical process control across batches and reveals material drift before it produces scrap. For regulated industries, this is also part of the required process validation.

Absorber-free welding with thulium fibre lasers. Sources around 1940 nm exploit the polymer's own absorption. Technical consequence: the test wavelength must move with it; a 980 nm reading loses its meaning.

Beam shaping instead of raising power. Adapted intensity profiles produce more uniform joint zones than Gaussian profiles for non-scattering materials. For strongly scattering materials, volume scattering largely evens out the profile differences – a reason to measure the scattering properties before designing the optics.

Simulation-driven process design. Coupled ray-tracing and FEM models capture scattering and heat conduction together. Their input quantities are measured absorption and scattering coefficients; simulation quality therefore depends directly on the quality of the optical characterisation.

Laser-transparent black colouring. Carbon-black-free colourant systems decouple visual appearance from NIR transmission. Technical consequence: incoming-goods inspection can no longer be done visually.

Spatially resolved 100 % inspection. Multi-point and area measurement systems are replacing sample testing. Technical consequence: the test step becomes relevant to cycle time, which requires short measurement times and high repeatability.

Additive manufacturing as a new material class. In 3D-printed joining partners, layer boundaries and pores create additional scattering centres with pronounced directional dependence. Consequence: transmittance values only carry over between parts printed with the same build orientation.

Traceability as a procurement criterion. Automotive and medical-technology customers increasingly demand transmission readings traceable to DIN EN ISO/IEC 17025 rather than manufacturer-internal reference values.

Scattering as the dominant loss mechanism in filled thermoplastics. This study examined how part thickness, glass fibre content and crystallinity change light scattering during laser transmission welding. It separates absorption and scattering fractions experimentally and shows that the loss of transmission in reinforced polyamides is predominantly due to scattering rather than absorption – a result that explains why simply raising the power does not restore seam quality. Xu, Parkinson, Bates, Zak (2015), Optics & Laser Technology 75, 123–131

Irradiation strategies for strongly scattering materials. This study examined adapted irradiation strategies for polymers with high volume scattering. It demonstrates that the process window can be widened by adapting the intensity distribution once the scattering properties of the transparent partner are known in advance. Frick, Schkutow (2018), Procedia CIRP 74, 538–543

A systematic review of irradiation variants. This review ranks contour, quasi-simultaneous, simultaneous and mask welding, along with newer strategies, by their energetic and thermal characteristics, and works out which material and geometry conditions favour which variant. Acherjee (2021), Optics & Laser Technology 137, 106737

A materials and process overview with optical parameters. This overview summarises beam sources, materials, process variants and quality characteristics, and includes comparative penetration-depth figures for various thermoplastics at 940 nm, 1064 nm and 1550 nm – a useful basis for choosing a wavelength. Gonçalves, Duarte, Martins, Paiva (2021), Infrared Physics & Technology 119, 103931

Modelling the temperature field and scattering. This review evaluates numerical models for temperature field, stress field, melt flow and thermal degradation, and shows that models which ignore light scattering systematically overestimate peak intensity and underestimate seam width. Hu, Li, Zuo (2023), Polymers 15, 2125

1. DVS 2243 (2014): Laser beam welding of thermoplastics (German only). DVS Media, Düsseldorf – the process standard covering equipment, process technology and quality assurance.
2. DVS 2243 Supplement 1 (2007): Determination of the transmittance of the laser-transparent joining partner in laser transmission welding of thermoplastics (German only). DVS Media, Düsseldorf – defines the measurement principles of power measurement, photometric and spectral measurement, and the quantities R, T and A.
3. DIN EN ISO 13468-1/-2: Plastics – Determination of the total luminous transmittance of transparent materials. Normative basis for transmission measurements in the visible spectral range; only partly transferable to NIR process wavelengths.
4. DIN EN ISO/IEC 17025:2018: General requirements for the competence of testing and calibration laboratories. The basis for traceability of transmission readings and reference standards.
5. Acherjee (2021), Optics & Laser Technology 137, 106737, and Xu et al. (2015), Optics & Laser Technology 75, 123–131 – peer-reviewed background on irradiation strategies and scattering.
6. Gonçalves et al. (2021), Infrared Physics & Technology 119, 103931, and Hu, Li, Zuo (2023), Polymers 15, 2125 – review and modelling work on materials, wavelength choice and scattering models.

Market figures in the market-context section are based on commercial market studies on plastic laser welding, referenced to 2025 with a forecast horizon to 2032; they serve as an order of magnitude, not an exact figure.

How is transmittance measured in laser transmission welding?
What is measured is the ratio of the radiant power passing through the part to the incident power, at the process wavelength. Common methods are power measurement, single-wavelength photometric measurement, and spectral measurement. The reading is valid only for the specific part thickness, measurement geometry and detector aperture used, and is not comparable without stating them.

Which wavelength is right for transmission testing?
What matters is the later process wavelength. For diode-laser processes, 980 nm has become the reference because it falls within the industrially dominant range of 800 to 1100 nm. For absorber-free processes at 1500 to 2000 nm, a measurement at 980 nm is not meaningful, since different absorption mechanisms apply there.

What transmittance is required?
There is no universal threshold, since the required transmittance depends on laser power, feed rate, spot size and absorber properties. In practice, values above 20 to 30 % are often targeted. It is more meaningful to derive the requirement from the line energy needed at the joint plane than from a blanket figure.

Why does the laser fail to weld despite sufficient power?
In most cases, too small a fraction of the radiation reaches the joint plane. Causes include scattering from crystallites, glass fibres or pigments, too great an irradiated wall thickness, Fresnel reflections at the interfaces, or a joint gap that interrupts heat conduction into the transparent partner. The set output power says nothing about any of this.

What is the difference between directional and diffuse transmission?
Directional transmission captures only the near-undeflected fraction; total transmission additionally includes the scattered fraction, measured with an integrating sphere. For scattering materials the two values diverge substantially. Total transmission describes the energy balance; the aperture-limited measurement describes the intensity effective within the seam cross-section.

How do glass fibres affect laser transparency?
Glass fibres reduce transmission mainly through scattering. In short-glass-fibre-reinforced PA6, transmitted laser energy falls from about 70 % to around 20 % as fibre content rises to 63 wt.-%. Local fibre orientation adds a further effect, which is why transmission differences occur within a single injection-moulded part.

Why is a datasheet value no substitute for a part-level measurement?
Datasheets give values for standard plaques of defined thickness. In the actual part, gate location, flow path, weld lines, mould temperature and wall thickness variation change crystallinity and fibre distribution. The result is transmission differences within a single part that only a spatially resolved measurement along the weld contour reveals.

When is a spectral transmission measurement necessary?
On material or formulation changes, colourant changes, complaint analysis, and for absorber-free processes. Only a spectrum shows whether a change in transmission results from additional absorption or from altered scattering. Absorption can be avoided by choosing a different wavelength; scattering can only be addressed through formulation, processing or part thickness.

Not sure how much laser power actually reaches the joint gap – and how much is lost to scattering and reflection?

The transmittance of the laser-transparent joining partner can be measured at the relevant process wavelength, in the real part geometry, and along the planned weld contour. For designing a suitable measurement chain – from choosing the wavelength through the measurement geometry to traceable calibration – the application laboratory is available. Get in touch.