UV microscope Objective
  • Imaging Techniques

Microscope Objective Lens Design Principles and Tips

Microscope objectives are among the most demanding optical systems to design. They are essentially diffraction-limited devices, where performance is governed primarily by numerical aperture (NA), wavelength, and aberration control rather than by field of view alone.

Designing a high-performance microscope objective requires careful consideration of:

  • Numerical aperture (NA)
  • Magnification
  • Tube length or infinity correction
  • Cover glass and immersion medium
  • Chromatic and spherical aberrations

This article explains the fundamental design principles of microscope objectives, highlights classical and modern configurations, and shares practical design tips used in real optical systems.

 

Classical 160 mm Tube-Length Objective Design

Many traditional microscope objectives are based on the 160 mm tube-length standard.

Key features:

  • Distance from the image to a principal plane in the objective: 160 mm
  • Approximate object-to-image distance: 180 mm
  • Designed to mount on a precision shoulder: 35.68 mm from the specimen
  • Specimen covered by: 0.18 mm cover glass

 

Cover Glass Considerations

  • Material:

    • Fused silica (UV objectives)

    • Chemically resistant soda-lime glass

  • Refractive index close to N-K5, which is commonly used in designs

  • For oil-immersion objectives:

    • Additional 0.14 mm oil layer

    • Oil fills the gap between the cover glass and first lens surface

 

Mechanical Interface Standards

  • Objective thread: Whitworth

    • Included angle: 55°

    • Thread pitch: 36 threads/inch

    • Major diameter: 0.796 in

  • Typical laboratory microscope:

    • Shoulder-to-specimen distance: 45 mm

These mechanical constraints must be respected during optical and mechanical co-design.

 

Infinity-Corrected Microscope Objectives

Modern microscope systems increasingly use infinity-corrected objectives.

Advantages:

  • Output beam is collimated
  • Enables easy insertion of beam splitters, filters, polarizers, fluorescence modules
  • Greater system flexibility without re-optimizing the objective

All designs discussed here assume:

  • 180 mm object-to-image distance
  • 16 mm image diameter

 

Resolution Limit and Numerical Aperture

According to the Rayleigh criterion, the minimum resolvable distance is:

Z = 0.61λ / NA

Where:

  • Z = minimum resolvable separation
  • λ = wavelength
  • NA = n \sin \Theta
  • n = refractive index of the immersion medium

 

How to Improve Resolution

  • Decrease wavelength (e.g. UV illumination)
  • Increase NA
  • Use oil immersion to push NA beyond 1.0

This is why:

  • UV objectives offer higher resolution
  • Oil-immersion objectives (e.g. 98×) outperform dry objectives

Refer to diagram below:

 

Values of NA given in the following prescriptions are paraxial, as shown below:

 

4-mm EFL Apochromatic Objective (NA = 0.91)

This example demonstrates a high-performance apochromatic objective:

  • Effective focal length (EFL): 4 mm
  • Actual focal length: 3.76 mm
  • Numerical aperture: 0.91
  • Magnification: 47.5×
  • Small residual primary color
  • Minimal flare at pupil edge

Notable design feature:

  • The next-to-last lens surface is nearly concentric with the image plane
  • This eliminates spherical aberration contribution at that surface

 

References

  1. Laikin, Milton. Lens Design. CRC Press, 2007.
  2. https://www.zemax.com/
  3. The design file used in this article is attached. 4-mm EFL apochromatic objective / UV reflective objective 53x