Exploring Refractive 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.
There are multiple refractive objective designs each utilizing different optical configurations.
The designs can range from two elements in basic achromatic objectives (an achromatic lens and a meniscus lens) to fifteen elements in plan-apochromatic objectives (Figure 1 Source Edmund Optics). Plan-apochromatic objectives are the most complex, high-end objective design with chromatic and flat field correction done within the objective itself.
Types of Refractive Objectives
Infinity-Corrected Reflective Objectives
- Designed for focusing applications.
- Collimated light enters through the central aperture in the primary mirror and focuses at a specific working distance.
- Ideal for focusing broadband or multi-laser sources (UV, visible, IR) to a single spot.
- Common use: laser focusing (e.g., Nd:YAG with a visible alignment beam).
Infinity-Corrected Reflective ObjFinite-Conjugate Reflective Objectives
- Designed for direct imaging without additional optics.
- Offer excellent resolution and can often replace refractive objectives in compatible setups.
- Infinity-corrected versions can be used in imaging when paired with a tube lens, providing flexibility to insert beam manipulation elements into the path.
Figure 2 and Figure 3: Edmund Optics design
Characteristics
- Excellent for visible-light imaging.
- Can deliver high resolution, but performance declines outside their design range.
- More affordable and widely used in general microscopy.
Key Parameters Reflective vs. 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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| Producer | Model | Style | Magni- fication | Coating | Numerical Aperture NA: | Diameter of Small Mirror (mm): | Aperture Diameter (mm): | Horizontal Field of View, 1/2" Sensor (mm): | Horizontal Field of View, 2/3" Sensor (mm): | Focal Length (mm) | Obscu- ration (%): | Working Distance (mm): | Specification |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Edmund Optics TECHSPEC | High Performance ReflX™ Objectives #59-884  | Infinity Corrected | 15X | DUV Enhanced Aluminum (150-11000nm) | 0.28 | 8.8 | 8.5 | 0.43 | 0.59 | 13.3 | 27.00 | 23.75 | Specification |
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