Light modelling, in general, involves three aspects:

• Global illumination model
• Light sources
• Local illumination model

where as the Global illumination model is doing the actual light computation, the Light sources are the light emitting elements, and Local illumination model defines the optical properties of the scene objects.

For each aspect, computer graphics knows plenty of alternatives.

Several of them are implemented in GroIMP as ready-to-use tools.

GroIMP integrates two two main light model implementations, namely:

• Twilight, a CPU-based implementation
• GPUFlux, a GPU-based implementation

Both implementing different global illumination model for rendering and for light computation.

In the following, only light computation or light modelling is discussed.

Regarding light sources, GroIMP provides the whole set of possible implementations. They all implementing the Light and LightBase interface what makes them handled equally and so easy to exchange.

For the Local illumination model, that defines the optical properties of the scene objects, i.e. values for absorption, transmission and reflection, so-called shader are used.

GroIMP provides a set of standard shader implementations, e.g., for Lambert, and Phong shading. Whereas the Lambertian model supports only diffuse reflection, the Phong reflection model (B.T. Phong, 1973) is a combination of ambient, diffuse, and specular light reflection.

These three core aspects of light simulation, the global and local illumination model, and light sources, are the base for any light simulation. When it now comes to spectral light simulations, specialized implementations of the above mentioned aspects are required, able to simulate not only one or three light channels - as typical for common light models - but moreover able to simulate the whole light spectrum for different wavelengths.

Spectral light simulations now deals not only with the pure calculation of light distributions, but moreover includes aspects of principal characteristics of light, i.e. light quality, quantity, and duration.

The main factor to influence the light quality is the light spectral composition - trivially called colour. So, the compositions of different intensities of different wavelengths, that together form the final light spectrum or colour. Below are the light spectra of typical sun light, of a common HPS-lamp (high-pressure sodium lamp, as they are used for instance as additional light sources within greenhouses), and a red LED lamp.

In GroIMP, the GPUFlux model allows us to simulate spectral light between about 380 to 720 nm (default values), up to 1nm buckets if wanted.

Note: The implementation basically allows to set individual limits for min and maxLambda extending the range of the visible light spectrum, i.e. infra-red and ultra violet. When the wavelength deviates too much from visual light, ray optics is no longer the most adequate tool for describing flow of electromagnetic radiation as other effects become quite significant. Therefore, from a physical point of view, the results will become not meaningful any more. One reason for this choice - the default range of 380-72nm - is that the model uses smith's conversion from RGB colours to full spectra. This conversion assumes that the whole spectrum is somehow covered by the RGB colour, thus restricting the spectrum to the visual range. But if your model only uses spectral colours as inputs then the interval could safely be extended.

public void grow ()
[
 Bud ==>Shoot Bud;
]

Special thanks to Dietger van Antwerpen, who implemented the GPUFlux light model for GroIMP!