The law of conservation of energy requires us to account for all energy when light transfers from one medium to another. A system’s total energy can be divided into four parts: transmission, reflection, absorption, and scatter. All four of these parameters are functions of the materials chemistry and surface finish. Different types of optics are engineered to maximize or minimize the energy that goes into one or more of these parameters.
For a mirror: transmission, absorption, and scatter are minimized, while reflection is maximized.
For a lens: scatter, reflection, and absorption are minimized, while transmission is maximized.
In the case of White Diffusing Glass, scatter is maximized, while absorption and transmission are balanced by adjusting the thickness of the diffuser.
White Diffusing Glass is a colloidal material that utilizes the Tyndall effect to scatter light. Tyndall scattering requires that the particles contributing to scatter be roughly the same size as the wavelength of light being scattered. Thus, if you can you visually see the material scattering the particles must be roughly the same size as the wavelength of visible light. As the wavelength of incident light increases, the intensity of the scattered light will decrease while the intensity of light transmitting through the material with its part unaltered will increase.
To make a diffuser that is efficient at diffusing both visible and longer wavelength light you can simply add a second mechanism of scatter with different wavelength dependencies.
Geometric scattering occurs when the feature scattering light is much larger than the wavelength of light. Put simply, if light hits a surface whose surface normal changes randomly, then light will refract at random angles along the surface. This is the mechanism at work for ground glass diffusers. When you add a sandblasted surface to colloidal material, you get a diffuser that is effective across a wide wavelength range.
The three other forms of energy transfer must be taken into account to maximize a systems performance. In the case of White Diffusing Glass, the material is polished to a 1.25mm thickness to balance transmission and absorption. WDG can be polished to different thickness to match the performance requirements of any application.
The below figure shows transmission scans associated with WDG polished to various thicknesses.