Transmissive Properties

Image courtesy of Toni Fresnedo (www.tonifresnedo.com)

Introduction

To create a transparent material in Maxwell you must first set the surface roughness (see Surface Properties) low enough for a typical clear transparent material (roughness 0 - 5) or rough glass (5-25), and then use:

• the parameters Transmittance and Attenuation to control the color and transparency of the material.
• the ND parameter to control the index of refraction of the material. A list of typical refractive values can be found here
• the Abbe parameter if you also want to render dispersion effects (see below)

Transmittance

This parameter controls the color of the light when it passes through a transparent material. Choose a Transmittance color by clicking on the color picker, or specify a texture by clicking on the texture button. The Transmittance color represents the color of the light when it has reached the Attenuation distance (see below).

Attenuation 

Glass, water, or even air are transparent when thin, but become opaque when a specific thickness is reached (different for each material). This is because as light travels through a material, it loses energy. The Attenuation distance parameter allows you to specify how far light can move through an object before losing half its energy. For example, if you have a 2cm thick glass window and you set the attenuation distance to 2cm, the light shining through the glass on the other side will be half as bright.

Attenuation distance and Transmittance work together. You must set the Transmittance color higher than RGB 0 for the Attenuation distance to become active. If your Attenuation distance is very small (for example 1nm) the object will remain opaque because the light can only travel a very small distance into the object, and will not come out on the other side. On the other hand, if you have a 1cm thick glass window and you set the attenuation distance very high (for example 900 meters), and you set the Transmittance color to blue, the glass will not be colored blue at all. It will be completely transparent because there was not enough distance in the 1cm thick glass to cause any attenuation and reveal the Transmittance color. Attenuation is ruled by an exponential curve, therefore: the thicker the object, the more attenuated the light will be.

The cube is 10cm thick. Only the attenuation parameter was changed. From left to right: 100cm, 10cm, 5cm.

To better understand the concept of Attenuation, consider sea water. When the water layer is very thin (like water in the palm of your hand) you do not see attenuation: the water looks transparent. When you have more water thickness, you see a typical sea color (grey, dark-blue or light blue-green, depending on deep or shallow waters). The Transmittance color represents the color that you want to get approximately at the Attenuation distance. Beyond this distance, the light is more and more attenuated, it loses more and more energy, and the view eventually turns opaque if the volume is thick enough.

ND

The ND determines the index of refraction of the transparent material. As light enters an object, it bends, or refracts, because the light slows down as it passes from a less dense medium such as air into a thicker medium such as glass. The higher the index of refraction the more the light bends as it enters the object and produces the typical distortions of objects seen through it.

ND changing from left to right: 1.1, 1.3, 1.5, 2.0. Note also that the surface reflections are stronger with increasing ND

Force Fresnel

If the Force Fresnel parameter is checked, and the surface roughness is low (0 to around 15) then the Reflectance colors will not matter - they can be set to any color you want or even at black. The surface reflectance will instead be determined purely by the ND (higher ND will produce stronger surface reflections). This is because the ND governs the balance between how much of the incoming light gets reflected off the surface and how much of it gets transmitted through the object. But this "auto balance" is only really valid for surfaces that have a low roughness. As the roughness increases it is no longer possible to define the surface reflectance based purely on the ND so you have to adjust manually the reflectance colors to produce the look you want. Usually it's a matter of brightening the reflectance colors a bit (if they were black) to bring back some surface reflections for very rough transparent materials:

All parameters the same except roughness. Above: roughness 0, Left: roughness 50, Right: roughness 50 with changed reflectance colors

Abbe (dispersion)

Different wavelengths of light can be refracted at slightly different angles as they pass through a material, and this is what causes dispersion, the effect seen when a beam of light passes through a prism and is split up into the different wavelengths of light.

The name Abbe comes from the German physicist Ernst Abbe, who defined the Abbe number. Abbe value controls the amount of dispersion. The higher the Abbe number, the less dispersion will be visible in the render. An Abbe number higher than 60-70 will render as if dispersion was not activated.

Glass material with Dispersion calculations disabled (left) and enabled (right)

Courtesy of Thomas Anagnostou