How to Design Custom Microscope Objectives for Imaging Applications
What you will learn:
- How to specify numerical aperature, angular field of view, and chromatic correction when designing microscope objectives for custom imaging systems.
- How the choice of waveband of light impacts the imaging system resolution. While a “perfect” lens does not exist, chromatic aberrations can be minimized to reduce changes to image quality.
- How the optical design of the microscope objective impacts the cost and complexity of manufacturing and assembling it.
In microscopy, infinite conjugate microscope objectives have become the industry standard, offering enhanced flexibility and compatibility with supporting optical components such as filters, beamsplitters, and fluorescence cubes. However, while these objectives enable sophisticated optical designs, they also introduce a layer of complexity—particularly when integrating them into custom imaging systems. The reality is that there is no "one-size-fits-all" solution; system requirements vary widely across applications, from biomedical imaging to industrial inspection, each imposing unique constraints on angular field of view, numerical aperture, and chromatic correction. This article will discuss these specifications when designing a custom microscope objective and how to determine what trade-offs to make.
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Design Considerations
Numerical Aperture
Numerical aperture (NA) is arguably one of the most important specifications to define when designing a microscope objective into an imaging system. NA defines an objective’s ability to gather light at a given working distance (Figure 1). It describes the acceptance cone of an objective defined by the angle θ and relates to the objective’s diffraction limit or resolution.
Equation 1 below establishes the relationship between NA and system resolution:
Resolution = λ/(2*NA) [1]
Where:
λ = wavelength
The relationship of resolution and NA can be used to understand what the system diffraction limit is, or what minimum distance at which two distinct point sources of light can be distinguished. This defines what the system resolution is in object space and drives the complexity of a microscope objective design. As the NA grows, the rays (θ) entering the objective become steeper and require more optical elements and advanced assembly techniques to keep aberrations controlled. In a large NA design, the objective will have higher resolving power and can be used for high-end microscopy applications in electronics inspection and life sciences applications.
The angular field of view (AFoV) of a microscope objective describes the maximum viewable area when looking through an eyepiece or with a camera (Figure 2). The AFoV is typically specified as the full angle (in degrees) associated with the horizontal dimension of the imaging sensor the objective is being used with. A standard microscope objective typically has an AFoV of ~4° and increases with sensor coverage. Inversely, shorter focal length objectives will have a larger AFoV, but this introduces challenges with optical design to correct for aberrations such as spherical, which cause the rays to defocus causing image blur.
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Waveband
Similar to a machine vision imaging lens, the design waveband is an important consideration for a custom microscope objective as it impacts the system resolution. These effects from polychromatic light induce chromatic aberrations, which are deviations from “perfect” theoretical performance. While a “perfect” lens does not exist, chromatic aberrations can be minimized to reduce changes to image quality. It is important to understand which wavelengths of light are required for the application when designing a custom microscope objective. For example, the design requirements for an objective to transmit compared to image ultraviolet light are very different. Fluorescence microscopy is a great example application where shorter wavelengths only need to be transmitted to excite a specific fluorophore. This puts less of a constraint on the design as any chromatic aberrations induced by the shorter wavelengths will not be what is imaged on the sensor.
Figure 3 shows the cross-section for an apochromatic (left) and achromatic (right) objective. The apochromatic design has triple the number of elements compared to an achromatic objective. Per ISO 19012-2:2013(E), achromat objectives only consider the focal shift between red and blue wavebands whereas apochromats consider the focal shift between red, blue, and the reference wavelength (typically green for an objective designed for visible light). This difference in design requirements can be related to the chromatic color correction each objective has. Figure 4 compares the degree of color correction between the two designs.
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Manufacturing Considerations
Numerical Aperture
Large numerical apertures demand a lot from the optical design to correct for off-axis aberrations like coma, astigmatism, and lateral color. This results in increased complexity for the assembly and manufacturing of optical and metal components. The typical assembly procedure for a high NA objective involves active alignment where the wavefront of the objective is measured as the optical elements are aligned in the optical cell. This type of assembly requires skilled operators and an increased production time.
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Besides challenges with assembly, the optical design itself has the potential to require lens elements with sharp curvatures or very short radii. This makes manufacturing these elements more difficult, requiring longer machining times. Aspheres are sometimes used in very complex designs to reduce the number of optical elements but also requires specific manufacturing techniques.
Angular Field of View
Designing for a large angular field of view requires the same precision manufacturing and assembly techniques that a large NA objective entail. Active alignment and increased processing time are needed to achieve lens positioning with single-digit micron precision. Similar to large NA, the glass types used in the design of a larger AFoV objective can have high indexes, making them more difficult to manufacture. There is also the potential for supply chain challenges when sourcing materials that are more exotic in the optical glass catalog.
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Waveband
When the design waveband of a microscope objective is very broad, this can exponentially increase the complexity of the design compared to a narrowband objective. For example, a design that is optimized to cover 400-1000nm (VIS-NIR) compared to a design that only covers 400-700nm (VIS) will likely be available at very different price points. This is due to specialized glass being required to transmit different wavebands, especially in the ultraviolet and infrared bands. Chromatic aberrations, known as axial and lateral color, are also affected by the waveband and require different considerations for the glass type and lens geometry.
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Impact on Application
Numerical Aperture
Using an objective with a smaller numerical aperture can be useful in applications that would benefit from a large depth of field. When the application involves inspecting an object that will have varying heights (semiconductor wafers, electronics pieces, biological samples, and microfluidic devices), having a large depth of field will alleviate the need for refocusing during sampling. Figure 5 shows a comparison of the depth of field of an objective with a small NA (top) versus a large NA (bottom). The center of each cone is representative of an object that is in focus and as the cones get larger, the image blurs faster.
When depth of field is not as critical as light throughput and resolution, using a large NA objective is the better solution. Large NA objectives will collect significantly more light and are ideal options for low-light applications like fluorescence microscopy. Referring back to equation 1, a larger NA will theoretically allow an imaging system to resolve smaller feature sizes on the object under observation. However, it is important to consider that the larger NA objectives come at a higher price point due to the design and manufacturing challenges that were previously discussed.
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Angular Field of View
If a compact system is ideal and additional optics like filters or beamsplitters are required, then using a large AFoV infinite conjugate objective could be a suitable option. The overall system length of a larger AFoV objective is shorter than that of a typical ~4° AFoV objective. As seen in Figure 6, the larger AFoV objective has a smaller system length, but at the cost of larger optics. The objective and tube lens will increase in size and, as a result, additional optics in the beam path must grow so no vignetting occurs.
Waveband
Besides having an objective that is color corrected over the application waveband, using bandpass filters will improve the image quality of the system. When using polychromatic light, off-axis chromatic aberrations, like lateral color, are difficult to correct for. Figure 7 shows that when polychromatic light is used, lateral color causes different wavebands to focus at different points on the image plane. This type of aberration can be detrimental for life science applications where color is critical to identify a specific cell structure. When a bandpass filter is used, only a small waveband of light is imaged through the objective and lateral color will be significantly reduced.
Conclusion
Designing custom microscope objectives demands consideration of the tradeoffs between numerical aperture, waveband, and angular field of view. For example, increasing NA improves resolution and light-gathering capability but often comes at the cost of reduced AFoV and more complex chromatic correction across a given waveband. Similarly, broadening the spectral range can introduce significant chromatic aberrations if not controlled for in the optical lens design, while expanding AFoV may introduce off-axis aberrations that challenge conventional lens design strategies. Ultimately, optimizing a custom objective is an exercise in prioritization—matching the optical configuration to the specific application requirements while acknowledging that gains in one parameter often require compromises in another.
About the Author
Rebecca Charboneau
Rebecca Charboneau is an optical engineer at Edmund Optics (Barrington, NJ, USA). She is part of Edmund Optics’ Engineering Leadership Program and is currently working with the imaging team at the company’s Cherry Hill, N.J. office. She joined Edmund Optics in 2021 and has been working closely with both machine vision and life science customers to provide them with custom optical solutions based around their system needs.