The Crucial Role of Reflectance and Transmittance Measurements in Optical Design

Optical Solutions Editorial Team

Dec 08, 2022 / 4 min read

Developments in optical technologies place higher and sometimes completely new demands on the quality of materials, surfaces, components, and systems. For optics in automotive applications, materials that facilitate high levels of transmittance and scattering can be important for optical system efficiency and appearance. For optics in astronomy and space-borne systems, material requirements can be quite different; for example, materials that limit reflectivity and reduce stray light can be very important.

Demands on materials used in optical technologies require analysis methods that include sufficiently precise data about a material’s characteristics.

Including reflectance, transmittance, and absorption measurements in your optical design key information about material and surface properties. This helps you evaluate the impact of materials on your optical product performance prior to physical prototyping and fabrication.

Here are some examples of how reflectance and transmittance measurements can improve your optical product design with the help of the Synopsys TIS Pro instrument.

Definition of Reflectance and Transmittance Measurements

Accurate optical system simulation results often rely upon precise material and surface properties. Geometry alone does not determine light distribution ; optical properties are needed to characterize how the energy and light ray directions change. We will describe two measurement quantities that can be used to characterize optical properties of surfaces and materials.

The first quantity is the angular spread of light. This is also called BSDF (Bidirectional Scattering Distribution Function). BSDF describes where light is scattered in the angular space, both in transmittance and/or reflectance when incident light is incoming.

The second physical quantity is the reflectance/transmittance (R/T) ratio. For such measurements, no matter where light is scattered, the quantity integrates the entire reflected or transmitted light when the light hits surfaces or materials.

Reflectance or transmittance is then defined by the ratio of total reflected or transmitted light compared to the input flux.

Benefits of Optical Measurements

Quantifying the overall appearance of an object is complicated because appearance is a subjective quality. However, there are specific aspects of appearance that can be quantified with optical scattering measurements:

  • Color from the spectral distribution of light reflected by the material
  • Shine from the geometric distribution of light reflected by the material
  • Texture from the spatial distribution of light reflected by the material

The quantification of appearance requires establishing correlations between physical quantities and the observer’s response obtained by visual measurements. BSDF is the physical quantity that integrates this information

Reflectance and transmittance values can be computed from BSDF, but this strongly depends on instrument accuracy and measurement sampling or resolution.

For this reason, having a separate measurement for R/T values on top of BSDF can enhance the accuracy of optical simulations.

How Synopsys TIS Pro Works

Synopsys REFLET and Mini-Diff products provide BSDF measurements.

Synopsys TIS Pro is a new product that allows users to measure reflectance/transmittance ratios of surfaces and materials. Synopsys TIS Pro features an integrated sphere and spectral detector assembled in a housing that controls stray light to ensure accurate measurement results.

To use Synopsys TIS Pro, you first complete a calibration step using a standard reference that is included with the instrument.

You can then measure your optical surface. There are two measurement configurations: one is for reflectance measurements (shown in Figure 1) where the sample is placed in the center of the integrating sphere; the second one (shown in Figure 2) is for transmittance, where the sample is placed on an entrance port.

Synopsys TIS Pro setup for reflectance measurements

Figure 1: Synopsys TIS Pro setup for reflectance measurements

Synopsys TIS Pro setup for transmittance measurements

Figure 2: Synopsys TIS Pro setup for transmittance measurements

Once you specify the angles of incidence to measure, you can start the measurement process. The rotation stages from the source and sample will rotate accordingly. Synopsys TIS Pro will then aim light onto the sample and its spectral detector will collect the signal exiting the integrating sphere for the specified positions. Synopsys TIS Pro compares that signal to the calibrated signal and then computes the reflectance, transmittance, or absorption values.

You can export the measurement results to optical simulation software, such as LightTools.

Application Examples

Synopsys TIS Pro is ideal for assessing the effects of surfaces and materials in optical systems. You can use the measurement results to:

  • Characterize reflector/diffuser materials for automotive design or general lighting systems
  • Evaluate quality controls in production
  • Analyze stray light suppression from coatings used in aerospace optics
  • Measure spectral behavior to incorporate in photorealistic renderings
  • Study optical properties of cosmetics
  • Characterize materials for many incident angles
Application Example
Synopsys TIS Pro measurement results

Figure 3: Synopsys TIS Pro measurement results

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