Exploring Reflective Objectives

Microscope objectives are among the most recognizable and critical components of microscope design. They magnify images so that they can be viewed by the human eye through an eyepiece or captured by an imaging system (such as an imaging lens plus a camera).

The most common traditional objective design is refractive, meaning it uses one or more transparent optical lenses to bend (refract) light. These are well-suited for applications in a specific wavelength band—usually the visible spectrum.

However, certain applications require high magnification focusing optics that are chromatically corrected from the deep-ultraviolet (UV) to the far-infrared (IR). In these cases, reflective (mirror-based) objectives are often the superior choice.

Reflective objectives focus light using mirrors rather than lenses. This removes chromatic aberration entirely because mirrors reflect all wavelengths equally, avoiding the wavelength-dependent “splitting” of light that occurs with refraction.

Types of Reflective Objectives

Figure 2 and Figure 3: Edmund Optics design

Key Advantages Over Refractive Objectives

Feature	Reflective Objectives	Refractive Objectives
Main optical element	Mirrors	Lenses
Chromatic aberration	None (broadband performance from deep-UV to far-IR)	Present, unless corrected for a specific range
Wavelength range	Deep-UV → far-IR	Optimized for visible, performance drops outside design range
Thermal stability	High	Depends on glass type
Typical applications	FTIR spectroscopy, laser focusing, ellipsometry, broadband imaging	Biological microscopy, industrial inspection, visible-light imaging

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Important Specifications for Reflective Objectives

• Obscuration
  Occurs when parts of the objective block light, such as:
  • The central non-reflective area of the primary mirror.
  • The diameter of the secondary mirror.
  • The thickness of the spider mount legs.
    Good designs minimize these effects; some manufacturers quote only central obscuration, others (like Edmund Optics®) include all sources.
• Transmitted Wavefront Error
  The difference between the wavefront at system entry and exit.
  Modern manufacturing can achieve mirrors with λ/20 peak-to-valley (P-V) surface accuracy, leading to objectives with ≤ λ/4 P-V transmitted wavefront.
  High-precision models can reach λ/10 RMS for diffraction-limited imaging.
  Performance can be assessed using interferometers such as Zygo GPI-XP.

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Diffraction-Limited Performance

Diffraction-limited systems can focus down to the Airy Disk size: Min. spot diameter=1.22λ / NA; where λλ is the wavelength, NA is the numerical aperture.

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Other microscopy objective brands in our portfolio

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