Laser irradiance
Key Takeaways
- Irradiance = power divided by area, measured in watts per square centimetre at the focal spot.
- Spot size matters more than raw power. A smaller spot concentrates the same watts into far higher intensity.
- Beam quality sets the achievable minimum. A low M2 value allows tighter focusing and higher irradiance.
- Too high damages, too low marks nothing — the usable window is bounded at both ends.
Laser Irradiance
Laser irradiance is optical power divided by the area it is delivered over, expressed in watts per square centimetre (W/cm²). It describes how concentrated the beam is at the workpiece — and on plastics it is irradiance, not the laser’s rated wattage, that determines whether a mark forms, what mechanism forms it, and whether the surrounding material survives.
Irradiance = Power (W) ÷ Spot area (cm²)
Why Rated Power Misleads
Spot area scales with the square of the spot diameter, so halving the focused spot quadruples the irradiance at identical power. Two 30 W marking systems with different beam quality, different beam expanders and different f-theta focal lengths can deliver irradiances that differ by an order of magnitude at the part.
This is the practical reason that a marking recipe cannot be transferred between machines by copying power and speed alone, and why specifying a large marking field “for flexibility” quietly degrades marking capability — a longer focal length produces a bigger spot and a lower power density.
Irradiance and Fluence
The two are routinely confused and describe different things:
| Irradiance | Fluence | |
|---|---|---|
| Quantity | Power per unit area | Energy per unit area |
| Units | W/cm² | J/cm² |
| Describes | How hard the beam hits | Total dose delivered |
| Governs | Whether a threshold is crossed | How much material responds |
For pulsed marking lasers the distinction sharpens further, because peak irradiance within a nanosecond pulse can be many orders of magnitude above the average. A 30 W laser emitting 100 ns pulses at 20 kHz has an average power of 30 W but a peak power in the tens of kilowatts. That peak is what drives the marking mechanism; the average power mostly determines how much waste heat accumulates in the part.
The Process Window on Polymers
Plastics have a narrower usable window than metals, bounded at both ends:
- Below threshold — the surface warms, perhaps discolours faintly, but no durable mark forms. Raising speed or lowering power moves further into this region.
- In the window — controlled carbonization, foaming or colour change produces the intended contrast.
- Above the window — rapid vaporisation, burning, melting, charred halos, internal stress and dimensional distortion.
The window’s position depends on the resin, its additive package and its colourants, which is why a validated recipe belongs to a specific material grade rather than to a polymer family in general.
Controlling Irradiance in Practice
- Focus position is the most immediate lever. Deliberate defocusing enlarges the spot and drops irradiance sharply — a legitimate technique for softening an over-aggressive mark, and a frequent unintentional cause of a weak one when part height varies.
- Beam quality (M²) sets the smallest achievable spot. A low M² is what allows high irradiance without high power.
- Pulse duration, on a MOPA source, changes peak irradiance directly while average power stays constant — the cleanest way to raise intensity without adding heat.
- Average power should generally be the last adjustment, since raising it increases bulk heating along with intensity.
A Worked Example
The reason rated power misleads becomes obvious as soon as the arithmetic is done. Take
a 20 W source and change nothing but the focus:
| Condition | Spot diameter | Spot area | Irradiance from 20 W |
|---|---|---|---|
| Well focused, short focal length lens | 30 µm | 7.1 × 10-6 cm² | About 2.8 × 106 W/cm² |
| Focused, longer focal length lens | 60 µm | 2.8 × 10-5 cm² | About 7.1 × 105 W/cm² |
| Slightly defocused | 120 µm | 1.1 × 10-4 cm² | About 1.8 × 105 W/cm² |
| Badly defocused | 300 µm | 7.1 × 10-4 cm² | About 2.8 × 104 W/cm² |
Doubling the spot diameter quarters the irradiance, because area goes as the square of
the diameter. A 100-fold change in delivered intensity is available on one machine at one
power setting, purely through focus and lens choice. This is why a recipe that works on one
system fails on another of identical rated power, and why focus is the first thing to
check when a mark degrades.
Measuring What You Are Actually Delivering
Irradiance is calculated, not read off the front panel, so both terms in the calculation
need to be known rather than assumed:
- Measure power at the workpiece, not at the source. A power meter under
the scan head accounts for transmission losses through the beam expander, the galvanometer
mirrors, the f-theta lens and the protective window — which together are rarely
negligible and grow worse as optics foul. - Determine the spot size rather than quoting the specification. A beam
profiler gives it directly. Failing that, a burn-print series on thermal paper or anodised
aluminium through focus locates the waist and gives a workable estimate. - Account for beam quality. The achievable minimum spot scales with
M², so two sources of equal power and equal focal length do not produce equal
irradiance if their beam quality differs. - Recheck after any optical change. A replaced lens, a new protective
window or a re-calibrated field all move the delivered irradiance without changing a single
number in the marking recipe. - Distinguish average from peak. On a pulsed source, peak irradiance
during the pulse is what drives the material response, and it can be orders of magnitude
above the average that the power meter reports.
Diagnosing Marks Through Irradiance
| Symptom | Irradiance interpretation | Action |
|---|---|---|
| No mark, or a faint inconsistent one | Below the material threshold | Check focus before adding power. A defocused beam is the most common cause and the cheapest to fix. |
| Mark good at the field centre, weak at the edges | Irradiance falling off with field position | Field flatness or focal plane error. On a three-dimensional part, a fixed focus cannot hold irradiance across the surface. |
| Burning, melting or blistering | Above the useful window | Raise speed or defocus slightly rather than cutting power, which may push the mark below threshold elsewhere. |
| Recipe fails when moved to an identical machine | Different spot size or optical transmission | Measure power at the workpiece and spot size on both machines. Rated power being equal proves nothing. |
| Gradual degradation over months | Transmission losses as optics foul | Measured power at the workpiece will show the decline that the front panel does not. |
| Mark quality varies with part height in the fixture | Parts sitting outside the depth of focus | Tighten fixture height control, or specify a three-axis head with dynamic focus. |
Related Terms and Reading
- Process parameters affecting mark quality
- Beam steered lasers
- Fiber lasers
- Plastics laser marking solutions
Applying this in production
The Sabreen Group provides independent engineering support for laser process window development and parameter transfer between marking systems. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What is laser irradiance?
The power of a laser beam per unit area, typically expressed in watts per square centimetre and measured at the focal point or beam spot. It determines the intensity of energy delivered to the material and therefore what happens when the beam meets the surface.
What is the difference between irradiance and fluence?
Irradiance is power per unit area, essentially intensity. Fluence is energy per unit area, measured in joules per square centimetre, and describes the total dose delivered. Irradiance governs how hard the beam hits; fluence governs how much energy is transferred.
What factors change irradiance?
Laser power, focal spot size, beam quality and wavelength. More power raises irradiance, a smaller spot concentrates it, and a low M2 value permits tighter focusing. Wavelength matters because it affects both absorption and the achievable focus.
Why does raw laser wattage mislead?
Because power density at the mark surface is what forms the mark, and that is different from the laser’s rated output. Two 50 W systems with different beam quality and optics can deliver very different irradiance at the workpiece.
What happens if irradiance is set wrong?
Too high causes rapid heating and vaporisation, leading to cracking, excessive burning or damage beyond the intended mark. Too low may produce a slow, weak mark or no visible mark at all. Both failure modes are common in unoptimised processes.
Why does the same wattage give different results on two machines?
Because irradiance depends on spot area, not on rated power. Doubling the spot diameter quarters the delivered intensity, so lens focal length, beam quality and focus position can produce a hundredfold difference in what actually reaches the material at one power setting. Two machines of identical rated power are not equivalent unless their delivered spot size and optical transmission match.
How is delivered irradiance actually measured?
Measure power at the workpiece with a meter under the scan head, which captures losses through the beam expander, mirrors, f-theta lens and protective window. Then determine spot size with a beam profiler, or estimate it from a burn-print series through focus. Both terms have to be measured — quoting the source rating and the lens specification gives a number that can be far from reality.
What should be checked first when a mark degrades?
Focus. It is the largest single lever on irradiance and the most common thing to drift, whether through fixture wear, part height variation or an optical change. Adding power to compensate for a defocused beam works briefly and then makes everything worse, because it widens the gap between the field centre and the field edges.
