Fluoresence Feature Highlights

Introduction

TracePro includes the modeling of fluorescent material in the Expert Edition. Fluorescence is modeled in TracePro by enhanced material properties and enhanced ray tracing features. Material property enhancements include relative absorption and relative excitation normalized to the peak molar extinction coefficient, and relative emission. All of the values can be created in a material property with variation versus temperature and wavelength. Concentration of the fluorescing material can be set in the model be entering the molar concentration when applying the material property to a solid object. The ray trace enhancement is the implementation of a two-part ray trace. In the first part of the ray trace, rays are traced in the excitation part of the material spectrum. A byproduct of the first ray trace is that TracePro source files are generated containing rays generated by the fluorescent material. The second part of the ray trace uses the source files to trace fluorescent rays.

Enhanced material properties

Material properties are defined and edited in TracePro using the Material Property Editor. This is done by clicking a checkbox in the Material Property Editor. Checking this box will add new data to the material property:
1)    Quantum Efficiency value
2)    Peak molar extinction value
3)   Relative Absorption column on the Absorption Table tab. This column replaces the Absorption Coefficient column, and the tab is renamed Absorption/Excitation Table.
4)   Relative Excitation column on the Absorption/Excitation Table tab. This column is editable.
5)   Excitation Efficiency column labeled on the Absorption/Excitation tab. This column is calculated as the ratio of Relative Excitation to Relative Absorption and is not editable.
6)   New tab labeled Fluorescence Emission table. This table has three columns.
a. Temperature (K).
b.   Emission Wavelength (um). These wavelengths are added via the Add… button.
c.    Relative Emission.

These values are editable when the property is unlocked. The relative absorption, relative excitation, and relative emission values are normalized. An example Absorption/Excitation tab is shown in Figure 1, and an example Fluorescence Emission Table is shown in Figure 2.

Figure 1 . Example Absorption/Excitation Table.

Figure 2 . Example Fluorescence Emission Table.

It is customary, in measuring fluorescence spectra, to express the peak molar extinction in base 10 rather than base e. The base 10 absorption coefficient is then

where K_peak is the peak molar extinction corresponding to the value of 1 in the relative absorption ab(λ) , and C_molar is the molar concentration in the particular sample. The transmittance through a sample of thickness t is then

.

The absorption coefficient used in a non-fluorescent material property in TracePro is related to the base 10 absorption coefficient by

.

This is used for Lambert/Beer Law absorption, in which the transmittance through a thickness t is

The optics absorption coefficient μ_a is computed internally in TracePro for use by the raytrace. The same rule applies to the relative excitation values.

Raytrace Options

The Raytrace Options dialog box, Wavelengths tab, has a choice on the Type list for entering fluorescence wavebands called Fluorescence emission wavebands . These are wavebands for which fluorescence rays are generated, similar to blackbody wavebands. The operation is the same as for entering Continuous Wavebands on the Wavelengths tab. You can enable the generation of fluorescent rays via a checkbox in the Raytrace Options dialog, Options tab. The checkbox is outside the Ray Splitting group, like the Polarization selection. Accompanying the Fluorescence checkbox is a drop-down list with the choices Generate emission sources only and Immediately trace emission wavelengths . There is also an entry for the molar concentration of the fluorescent material.

Fluorescence ray trace

A fluorescence ray trace is done in two parts. First, rays are traced within the excitation band of the fluorescent material(s) in the model, at the wavelengths set in the Wavelength tab of the Raytrace Options dialog box, Discrete Wavelengths selection. Next, rays are traced at the mid-points of the fluorescence wavebands in the Fluorescence emission wavebands, except for the highest waveband. For the highest waveband (λ_N to ∞), the fluorescence wavelength is two times the low-end wavelength, i.e. λ = 2λ_N . You can trace fluorescence rays in either of two ways: 1)      In method one, at the conclusion of the excitation ray-trace, the emission ray-trace will automatically begin, so that emission rays are “mixed in” with excitation rays. All irradiance map features, candela plots, flux report, etc. report the fluorescence wavelength results along with the excitation wavelength results.2)      In method two, the emission ray files are generated, but the emission rays will not be traced. You can trace them later at your discretion, by:

a.      inserting the emission source file(s) into the model (or any model you choose)
b.      adding the discrete wavelengths corresponding to the fluorescence wavebands to the Discrete wavelengths in the Wavelengths tab of the Raytrace Options dialog box
c.       removing the excitation wavelengths from the list of discrete wavelengths
d.      modifying any surface sources in the model to have zero rays (or simply removing the source property altogether)
e.      unchecking the Fluorescence option in the Raytrace Options dialog box
f.       initiating a Surface Source ray trace.