Axial Resolution: Key Parameter for OCT System Selection

01 Axial Resolution – Definition and Significance

Optical Coherence Tomography (OCT), with its advantages of non-contact operation, high resolution, and real-time imaging, has been widely adopted in medical diagnostics, industrial inspection, materials science, and other fields. As the core parameter for selecting an OCT spectrometer, axial resolution directly determines the system's ability to resolve fine structures along the depth direction of a sample.

The so-called axial resolution refers to the minimum distance along the depth direction (i.e., the direction of light propagation) at which two adjacent structures can be clearly distinguished. It is typically expressed in micrometers (μm). In simple terms, for a bulk tissue, axial resolution acts like a "dissecting knife" – the higher the axial resolution (the smaller the numerical value), the thinner the slice that can be obtained, capturing more subtle structural differences. In applications such as ophthalmology, dermatology, and material inspection, axial resolution directly determines the accuracy and reliability of the measurements.

Higher axial resolution enables thinner "slices" and richer detail.

02 Calculation of Axial Resolution

Axial resolution is not a fixed value; it is determined by multiple parameters of the OCT system. Understanding the influencing factors enables the selection of optimal parameters for a given application, balancing performance requirements without incurring unnecessary costs from over‑specifying resolution.

The core factors affecting axial resolution are the optical properties of the light source, among which the most critical are the central wavelength (λ₀) and the spectral bandwidth (Δλ). These two parameters directly determine the theoretical axial resolution. A commonly used simplified formula in the industry is:

Δz = (2ln2/π)·(λ₀²/Δλ) = 0.44 * (λ₀²/Δλ)

Where Δz is the axial resolution, λ₀ is the central wavelength of the light source, and Δλ is the spectral bandwidth. The formula shows intuitively that:

• The broader the spectral bandwidth (Δλ), the higher the axial resolution (smaller Δz).
• The shorter the central wavelength (λ₀), the higher the axial resolution; conversely, a longer wavelength reduces the resolution.

For example:

• A light source with a central wavelength of 850 nm and a bandwidth of 50 nm provides a theoretical axial resolution of approximately 6.36 μm – sufficient for retinal layer observation in ophthalmology.
• An 850 nm source with a bandwidth of 100 nm achieves a theoretical resolution of about 3.2 μm – suitable for demanding research at the cellular level.
• With a central wavelength of 1300 nm and the same bandwidth, the axial resolution drops to 8–12 μm, which is more appropriate for applications requiring deeper penetration.

The retina consists of 10 layers, with a total thickness of roughly 10–15 μm; the thinnest layer is only about 1 μm, and the thickest around 40 μm.

In an OCT system, not only should the light source bandwidth meet the axial resolution requirement, but the spectrometer bandwidth must also satisfy it. JINSP offers spectrometers with various bandwidths to meet different needs:

As seen in the table above, axial resolution and detection depth are in a trade‑off relationship: improving one inherently degrades the other. This can be understood intuitively with a camera analogy – you cannot simultaneously capture a larger field of view and finer details; you must balance according to the actual needs, either sacrificing some field of view or some detail. Therefore, when designing and selecting an OCT system, it is essential to focus on the primary requirements and make appropriate trade‑offs.

03 Effect of Refractive Index on Axial Resolution

In addition to the source parameters, axial resolution is also affected by the refractive index (n) of the medium, optical aberrations, detector sampling rate, and other factors. In practical applications, the refractive index of the medium modifies the axial resolution. The corrected formula can be simplified as:

Δz = 0.44/n · (λ₀²/Δλ)

Where n is the refractive index of the sample medium (e.g., for biological tissues, n ≈ 1.33). This is a key consideration for OCT system selection in the medical field. Using the previous example of an 850 nm source with 50 nm bandwidth, the theoretical axial resolution in air is about 6.36 μm, but in tissue it becomes approximately 4.8 μm.

Furthermore, optical aberrations and detector sampling rate can reduce the actual axial resolution by 20–30% compared to the theoretical value, so a margin should be reserved during system selection.