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Photorealistic rendering is a technique that uses 3D rendering software to generate lifelike images using physically-based virtual lights, cameras, and materials, allowing you to showcase your projects as though they existed in real life. By enabling you to visualize your project in advance, this technology can reduce the price of prototyping, and its appeal spans a wide range of product industries ¨C automotive, for example, as well as general lighting, and even cosmetics.
Realistic rendering is done using optical software such as LightTools and LucidShape, where extensive details about the optical properties of the materials are specified in the software and processed via simulations to obtain a photorealistic rendering.
The goal, of course, is to produce an image that is as close as possible to reality. To achieve this, it's important to know as accurately as possible the optical characteristics of the materials that will be used. That's where we come in. We provide tools that enable you to measure the optical properties ¨C directly and with high precision.
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Synopsys is equipped with a Class 10,000 lab (ISO7), where the optical properties of material can be directly measured. It is in a dark room, where temperature and humidity are controlled, with specialized instruments such as:
- Synopsys REFLET 180S goniophotometer
- Synopsys High specular bench (10 meters)
- Video photometer
- Lux meter
- Luminance meter
- Spectrophotometer
- Integrating spheres: 6¡±
These instruments enable us to accurately characterize the optical properties of many materials.
There are two ways to obtain optical property data from these instruments: using Synopsys SmartStart Measurement Services and the Synopsys SmartStart Library.
SmartStart Measurement Services
Using Synopsys SmartStart Measurement Services, customers can send their samples and choose the measurements they want. We provide assistance for choosing the most appropriate measurement for your samples to meet your needs and expectations.
Measurements can be customized to satisfy customers¡¯ needs ¨C based on specific criteria such as angle of incidence or wavelength, for example. The price for each measurement depends on the measurement type. Results are delivered in an average of three weeks from the time we receive the order and samples.
SmartStart Library
You can also obtain optical property data measured in our labs via the Synopsys SmartStart Library. This option is available to LightTools and LucidShape users as a separately licensed module. In addition to providing access to a large and growing library of materials, this option also allows customers to request measurements for new materials without an additional charge. Note that the sample for which a measurement is requested must belong to a manufacturer's catalog; it cannot be a custom sample or custom measurements.
Examples of Renderings Obtained From Synopsys Measurements
1. Automotive Light Guide
In the automotive field, light guide can be found inside and outside of the vehicle.
The light guide design shown in LucidShape, below, is composed of a transparent, non-diffusive plastic material that is surrounded by a diffusive plastic (more precisely a volume scatterer), which allows the spatial redistribution of light. To complete design, white and black plastic with Lambertian properties are used as a housing.
Transparent Material
For a full characterization of the transparent material, two measurements are needed: refractive index measurements, shown on the left, and spectral transmittance measurements, shown on the right.
For the refractive index measurement, a customized commercial refractometer is used to obtain the spectral dependence of the refractive index as represented in the curve.
The spectral transmittance is equal?to the ratio of transmitted power and incident power when light is shot onto a surface. It takes into account both specular and scattered transmitted light. To measure it, the idea is to collect all transmitted light in all directions. In practice, an integrating sphere connected to a spectral detector is used. The sample is placed at the entrance port of an integrating sphere. Then, the transmitted intensity of the incident light (without anything at the entrance of the sphere) is measured. From this, the spectral transmittance of the sample is calculated.
Diffusive Plastic
For a full characterization of this material, a volume scattering measurement is needed in order to model the scattering introduced by tiny particles suspended in the plastic. It cannot be measured directly; instead, a methodology has been developed in LightTools that determines scattering parameter values for both Gegenbauer and Mie models. This methodology is based on 2D bidirectional transmittance distribution function (BTDF) measurements of the same sample in four different thicknesses. These measurements are done with the Synopsys Reflet 180S goniophotometer. Performance data for this instrument is summarized in the following table.
The LightTools methodology uses these four BTDF measurements to find the parameters needed to simulate scattering that occurs in this material.
We can choose either of the following volume scattering particle types:?
- Gegenbauer model: based on the obtained mean free path, Alpha, and g parameters?from this model.
- Mie Scattering model: based on the obtained radius, density, and refractive index of particles.??
Of course, we also verify that the calculated data provides the same simulation results as the measurements.
Housing
For a full characterization of the two Lambertian plastics (black and white) for the housing, both bidirectional reflectance direction function (BRDF) and reflectance measurements are needed.
The BRDF is performed with the Synopsys REFLET 180S. The BRDF as a function of the scattering angle is obtained as represented on the left in the figure below. As expected, the curve is flat. Of course, the level is lower for the black plastic than for the white plastic: approximately 5% of a level of a perfect Lambertian for the black and almost 95% of a level of a perfect Lambertian for the white.
The reflectance measurement is similar to the transmittance measurement but in the reflected space. It describes how?much?light?is?reflected from?a?surface or an optical element. The measurement consists of the integration of all the reflected signal (scattering + specular) into a sphere, normalized by the total reflected power of the incident beam. In practice, for reflectance, the sample is placed on the sphere with a black on the back to avoid transmission and lighted up by a laser source. Then, the reflected intensity by a Lambertian Spectralon is measured. Knowing the calibrated reference reflectance, reflectance of the sample is calculated, as shown on the right in the figure below.
All of the measurements are exported directly to LucidShape format and applied to our design.
Here are the results with a different scale and different level of luminance for the environment.
2. Automotive Tail Light
Another example of a photorealistic rendering for an automotive application is a tail light, shown in the figure below. It is composed of several materials:
- Transparent plastic: clear plastic for the housing of the lens, orange plastic for the turn lens and position lens, and red plastic for the position and stop lens.
- Glass for the light bulb or specular and Gaussian reflector.
For the transparent plastic and the glass (labeled in green text), the spectral transmittance and the refractive index are performed to characterize those materials.
For the Gaussian reflector, a BRDF measurement with the Synopsys REFLET 180S and a reflectance measurement with the integrating sphere are performed.
For the specular reflector, a BRDF with a small azimuthal step around the specular is performed (generally 10¡ã and 1¡ã around the specular).
Using those measurements, the result of the rendering simulations is shown in the figure below. It looks like a real tail light!
