Focal Length Characteristics of Field Lenses in Pulsed Laser Cleaning Machine

Analysis and Application Scenarios of Different Scan Lenses

In pulsed laser cleaning machine, the laser cleaning head is not a passive component that simply delivers laser output.

Instead, together with laser parameters such as single pulse energy, repetition frequency, and pulse width, the field lens (scan lens) inside the cleaning head plays a decisive role in determining:

• How laser energy is distributed on the surface
• Whether the cleaning process is stable and repeatable
• Whether the substrate remains within a safe thermal window
• How efficient and controllable the overall operation is

Among all optical parameters of a pulsed laser cleaning system, the focal length of the field lens is one of the most frequently overlooked, yet most influential factors from an engineering perspective.

What Is the Focal Length of a Field Lens in Pulsed Laser Cleaning?

The focal length of a field lens refers to:

The designed working distance at which the scanning laser beam is focused onto the workpiece surface, producing the intended spot size and energy distribution across the cleaning field.

It is determined by the optical design of the scan lens and is typically specified in millimeters, such as:

• F150
• F200
• F300
• F400

Important clarification:
The focal length of a field lens is not an arbitrary adjustment. It is tightly coupled with:

• The scan lens optical structure
• The galvanometer scanning system
• The designed scan field size
• The intended working distance

The Significance of Field Lens Focal Length

From an engineering standpoint, the focal length of the field lens directly influences four critical performance aspects:

• Laser spot size on the workpiece
• Energy density per unit area
• Process safety window
• Cleaning consistency across the scan field

The core relationship is straightforward but fundamental:

With all other parameters unchanged:

• Shorter focal length → smaller spot → higher energy density
• Longer focal length → larger spot → lower energy density

A practical engineering analogy:

• Short focal length field lens = cleaning with a sharp, concentrated tool
• Long focal length field lens = cleaning with a wider, more uniform tool

Typical Field Lens Focal Lengths and Their Characteristics

Short Focal Length Field Lenses (F150–F200)

Engineering characteristics:

• Small laser spot size
• Very high energy density
• Strong stripping capability
• Narrow focus tolerance

Cleaning behavior:

• Aggressive material removal
• Visible cleaning traces
• Lower margin for parameter error

Typical applications:

• Heavy rust
• Thick oxide scale
• Thick coatings and paint layers
• Localized or spot-intensive cleaning tasks

Engineering note:
Short focal length field lenses are well suited for difficult contamination, but they are not ideal for large-area or precision-sensitive surfaces.

Medium Focal Length Field Lenses (F200–F300)

Characteristics:

• Balanced spot size and energy density
• Larger operational tolerance
• Stable and repeatable cleaning performance

Cleaning behavior:

• Good balance between efficiency and substrate protection
• Uniform surface appearance
• Operator-friendly setup

Typical applications:

• General industrial rust removal
• Pre-welding surface preparation
• Routine mold maintenance
• Stainless steel and carbon steel surface treatment

Engineering assessment:
This focal length range is the most widely used and versatile in pulsed laser cleaning systems.

Long Focal Length Field Lenses (F300–F400 and Above)

Characteristics:

• Large laser spot size
• Highly uniform energy distribution
• Lower energy density per unit area
• Widest safety window

Cleaning behavior:

• Gentle and controlled cleaning action
• Minimal thermal impact on the substrate
• Excellent surface consistency

Applications:

• Aluminum alloys
• Lithium battery materials
• Precision molds
• Large-area surface cleaning

Engineering insight:
A long focal length field lens does not mean weak cleaning. It means higher controllability, better uniformity, and increased substrate safety.

Interaction Between Field Lens Focal Length, Power, and Single Pulse Energy

In real-world applications, the field lens focal length must always be matched with other system parameters, including:

• Laser power level (100 W, 200 W, 500 W, 1000 W)
• Single pulse energy
• Repetition frequency
• Scanning speed

Typical engineering logic:

• High power + short focal length field lens
  • Extremely high energy density
  • Suitable only for very heavy contamination
• High power + long focal length field lens
  • Energy distributed over a wider area
  • Ideal for high-efficiency, large-area cleaning

This is why higher-power pulsed laser cleaning systems are commonly paired with medium or long focal length field lenses rather than very short ones.

Field Lens Selection Recommendations by Application

Engineering Notes on Adjustable or Multi-Focal Field Lens Systems

Some advanced pulsed laser cleaning systems support:

• Adjustable field lens configurations
• Rapid switching between multiple focal lengths

Advantages:

• One system covers a wider range of applications
• Reduced need for optical replacement
• Improved on-site flexibility

Engineering considerations:

• Higher optical and mechanical complexity
• Greater setup and operator training requirements

Common Misunderstandings

Incorrect assumptions:

• Shorter focal length always means better cleaning
• Longer focal length always reduces efficiency

Correct engineering understanding:

• Field lens focal length determines energy distribution
• Laser power determines total available energy
• Single pulse energy determines whether contamination can be removed

Summary

In pulsed laser cleaning systems:

• Field lens focal length determines how cleaning energy is distributed
• Laser power defines the upper limit of cleaning capacity
• Both must be matched to contamination type and substrate characteristics

Proper field lens selection is essential for achieving stable, safe, and repeatable pulsed laser cleaning results.