Bio-fluorescence scattering setup
  • Bio-photonics Techniques, Knowledge Center

How to Model Bio-Fluorescence in Optical Design

Bio-fluorescence describes the physical process in which light is absorbed by a material and re-emitted at a different, usually longer, wavelength. This phenomenon is widely used in biomedical imaging, sensing, and life-science optics, and can be effectively modeled using optical design software.

Below, we outline a simple and practical approach to modeling bio-fluorescent material scattering, using a volumetric scattering object and wavelength-shift behavior.

 

System Overview

The example setup (attached at the end of this article) demonstrates the modeling of a bio-fluorescent object using a simplified optical system:

  • A Source Ellipse launches a collimated beam of rays.
  • Rays propagate toward Object 2, a volumetric object where bulk material scattering can occur.
  • During bulk scattering, rays undergo a wavelength shift from an excitation wavelength (λ₁) to an emission wavelength (λ₂).

This configuration allows clear visualization of both scattering behavior and fluorescence-induced wavelength conversion.

 

Bulk Material Scattering with Wavelength Shift

Within Object 2, bulk scattering is defined under the Bulk Material Scattering properties:

  • Scattering model: Angle scattering distribution
  • Mean free path: 0.05 mm
  • Maximum scattering angle: 20°

In this example:

  • All rays start at wavelength #1 (0.780 µm)
  • Any ray that undergoes bulk scattering is shifted to wavelength #2 (0.900 µm)
  • The wavelength-shift probability is set to 100%, meaning every scattered ray fluoresces

In general applications, wavelength-shift behavior is not limited to a single transition, and only a fraction of scattered rays may undergo fluorescence, depending on material properties.

 

Visualizing Fluorescence with Layout Filters

To verify the wavelength-shift behavior, a layout plot filter is used:

X_Waveshift(1,2)

This filter displays only rays that have shifted from wavelength #1 to wavelength #2, allowing clear separation between excitation and emission paths.

 

Wavelength Separation Using a Beamsplitter

A second rectangular volume object is introduced to validate correct wavelength behavior. Its front face is coated with a wavelength-dependent beamsplitter coating:

TABLE SBICBEAMSPLIT
ANGL 45.0
WAVE 0.78 1.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0
WAVE 0.90 0.0 0.0 1.0 1.0 0.0 0.0 0.0 0.0

According to this coating definition:

  • Rays at 0.780 µm are fully reflected
  • Rays at 0.900 µm are fully transmitted

 

Results and Interpretation

From the layout plot, the behavior is clearly observed:

  • Non-scattered rays (no fluorescence) are reflected by the beamsplitter
  • Scattered, wavelength-shifted rays (fluorescence emission) are transmitted

This confirms that:

  • The bulk scattering model is functioning correctly
  • The wavelength-shift tool accurately simulates bio-fluorescence behavior

 

Practical Applications

This modeling approach is useful for:

  • Fluorescence microscopy system design
  • Biomedical imaging simulations
  • Optical sensor development
  • Life-science and diagnostic optics

It provides a controlled and visual method to study how scattering, absorption, and wavelength conversion interact in volumetric materials.

 

Reference Sources

  1. Laikin, Milton. Lens Design. CRC Press, 2007
  2. Zemax Optical Design Program – User’s Guide
  3. Wikipedia – Optical scattering and fluorescence
  4. The design file used in this article is attached. Please download it to explore or modify the bio-fluorescence model. How to design Bio-fluorescence material scattering