When you're developing medical imaging systems, one of the biggest challenges is understanding how light behaves when it travels through human tissue. While water might seem like a reasonable starting point for testing, real tissue is far more complex—and that complexity can make or break your imaging performance.
Light interaction with tissue has been well studied for nearly half a century. In this post, we will provide a brief overview of the framework for understanding light transport in tissue and how we can measure it to guide optical sensing device development.
Biological tissues are messy. They absorb light (thanks to hemoglobin, melanin, water), scatter it off tissue cellular structures (lipids and proteins), and vary massively from patient to patient. For your device to work reliably having a quantitative basis for photon transport (i.e., absorption coefficient [μₐ] reduced scattering coefficient [μₛ']) across all relevant light colors helps understand and inform device design. This starts with measuring the reflectance and transmission spectra from samples of interest.
Shorter wavelengths (e.g., blue, green light) are easily seen by eye, but quickly attenuated, thus limiting their effectiveness to superficial tissue layers (< 400 μm). Chromophores in tissue can impact their observed color. Light scattering increases exponentially as the wavelength of light gets shorter. This is why many fluorescence imaging applications using fluorophores that emit in the blue-green range are difficult to detect at depth.
Red and near-infrared light (600-1000nm wavelengths), on the other hand, can penetrate much deeper into tissue (~1-5 mm) because they absorb and scatter less in tissue. While they're difficult to see by eye, This deeper photon penetration capability makes these wavelengths particularly valuable for applications like fluorescence-guided surgery, where you need to visualize structures beneath the surface.
These optical properties directly impact your device’s technical specs and real-world performance. How deep does your light penetrate? What's your signal-to-noise ratio? How much light power do you need for your measurements?
Without proper characterization, you're flying blind. And when clinical validation rolls around, you'll pay for it. It all starts with capturing accurate optical properties of the tissues you expect to measure in.
Molecules that absorb and scatter light across different light colors (i.e., wavelengths) are called chromophores. The key players in tissue optics—hemoglobin, melanin, lipids, and water—each contribute to how light behaves in tissue:
Understanding these interactions helps engineers design systems optimized for specific imaging depths and clinical applications.
There are a few ways to characterize optical properties. (1) Spectrophotometry, (2) Fiber-optic diffuse reflectance spectroscopy, (3) Photon Time-of-flight, and (4) Spatial Frequency Domain Imaging. These methods are summarized in the table below:
| Methods | Description | ✅ Benefits | ⚠️ Drawbacks |
| Spectrophotometry | The gold standard method. It measures the reflectance and transmittance from thin sections of scattering samples with radiative transport modeling to calculate absorption and scattering properties. |
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| Fiber Optic Diffuse Reflectance Spectroscopy |
Probe-based adaptation of spectrophotometry with added sampling flexibility.(e.g., live tissue). |
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| Photon Time-of-Flight |
Clocks the time for photons to scatter through a sample. |
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| Spatial Frequency Domain Imaging |
Provides 2D images of absorption and reduced scattering coefficients at distinct wavelengths by measuring spatial effects of reflected illumination patterns in samples. |
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QUEL Imaging relies primarily on spectrophotometry for our material analysis and optical property tuning, but we have experience with all of these measurement methods.
Did you know we can also help you understand and measure optical properties for your device development needs? Reach out today to learn how we can help your team with decades of tissue optical expertise.