MgF₂ Coated vs. Uncoated Achromatic Lenses: Performance, Applications, and Buying Considerations

Achromatic lenses are widely used in optical systems to reduce chromatic aberration and improve image quality across multiple wavelengths. When selecting an achromatic lens, one critical decision engineers and buyers must make is whether to choose MgF₂ coated or uncoated optics.

While both options share the same fundamental optical design, their surface treatment significantly affects transmission efficiency, reflection loss, durability, and application suitability. This article provides a comprehensive comparison of MgF₂ coated vs. uncoated achromatic lenses, helping optical designers, OEMs, and procurement teams make informed decisions.

What Is an Achromatic Lens?

An achromatic lens (often called an achromatic doublet) is composed of two optical elements—typically crown glass and flint glass—cemented together to correct chromatic aberration at two wavelengths (commonly red and blue).

Key benefits include:

Reduced color fringing

Improved image sharpness

Better focus consistency across visible wavelengths

Achromatic lenses are commonly used in:

Imaging systems

Laser optics

Machine vision

Scientific instruments

Medical and inspection equipment

However, surface reflections remain a limiting factor—this is where optical coatings, such as MgF₂, become important.

Understanding MgF₂ Optical Coating

Magnesium Fluoride (MgF₂) is one of the most widely used single-layer anti-reflection (AR) coatings in the optics industry.

Key Properties of MgF₂ Coating

Optimized for visible wavelengths (typically ~550 nm)

Reduces surface reflection from ~4% to ~1.5%

High environmental stability

Cost-effective compared to multi-layer AR coatings

MgF₂ coatings are applied via vacuum deposition and are known for their durability and broad compatibility across optical applications.

Uncoated Achromatic Lenses: Characteristics and Limitations

Uncoated achromatic lenses have bare glass surfaces, meaning no anti-reflection treatment is applied.

Advantages of Uncoated Achromatic Lenses

Lower upfront cost

Suitable for non-critical optical paths

Adequate for educational or experimental use

Limitations

Higher Fresnel reflection losses (~4% per surface)

Reduced transmission efficiency

Increased ghosting and stray light

Lower contrast in imaging systems

In systems with multiple optical elements, reflection losses from uncoated lenses can accumulate rapidly, degrading overall system performance.

MgF₂ Coated Achromatic Lenses: Performance Advantages

1. Higher Optical Transmission

MgF₂ coated achromatic lenses significantly improve light throughput by minimizing surface reflections.

Uncoated doublet: ~92% total transmission

MgF₂ coated doublet: ~96–97% total transmission

This difference is especially critical in:

Low-light imaging

Laser delivery systems

Spectroscopy applications

2. Reduced Reflection and Ghost Images

Surface reflections in uncoated lenses can lead to:

Internal reflections

Ghost images

Reduced contrast

MgF₂ coatings suppress these effects, making coated lenses preferable for high-contrast imaging and precision optics.

3. Improved System Signal-to-Noise Ratio

In machine vision, metrology, and scientific imaging, stray reflections introduce background noise. MgF₂ coated lenses help maintain a higher signal-to-noise ratio, improving measurement accuracy.

4. Better Performance in Multi-Lens Assemblies

As optical systems grow more complex, the cumulative impact of reflection losses increases. MgF₂ coatings help maintain consistent performance in:

Compound lens systems

Optical modules

OEM optical assemblies

Durability and Environmental Resistance

MgF₂ coatings offer:

Good hardness

Resistance to humidity

Stability under moderate temperature changes

Compared with uncoated lenses, MgF₂ coated optics are less prone to surface degradation in real-world environments, making them more suitable for industrial and long-term applications.

Typical Applications and Recommended Choices

Choose Uncoated Achromatic Lenses When:

Budget is extremely limited

Optical performance requirements are minimal

Used in educational demonstrations or basic experiments

Choose MgF₂ Coated Achromatic Lenses When:

High transmission is required

Contrast and image clarity matter

Used in laser, imaging, or inspection systems

Optics are part of a multi-element assembly

MgF₂ vs. Multi-Layer AR Coatings: A Quick Note

While MgF₂ coatings offer excellent value, some high-end applications may require broadband multi-layer AR coatings, which provide even lower reflectance across wider wavelength ranges.

However, MgF₂ remains a popular industry standard due to its:

Cost-performance balance

Reliability

Compatibility with visible optics

Buying Considerations for Optical Engineers and OEMs

When sourcing achromatic lenses, consider:

Target wavelength range

Required transmission efficiency

System complexity

Environmental conditions

Total cost of ownership

For most general-purpose visible light applications, MgF₂ coated achromatic lenses provide the optimal balance of performance and cost.

Conclusion: Which Should You Choose?

The choice between MgF₂ coated and uncoated achromatic lenses ultimately depends on application requirements.

Uncoated lenses may suffice for basic, low-performance systems

MgF₂ coated lenses deliver higher transmission, better contrast, and improved system reliability

For industrial, scientific, and commercial optical systems, MgF₂ coated achromatic lenses are generally the preferred and more future-proof solution.