The reflection of light is extremely predictable. When light strikes a surface, it is either absorbed or reflected. Total reflectance indicates the percentage of light that is reflected, regardless of the direction it travels after leaving the surface. A surface’s total reflectance can consist of both specular and diffuse components. To illustrate this, imagine a ray of light as a tight bundle of smaller, individual rays, all traveling parallel to each other. In accordance with the Law of Reflection, rays that strike a smooth surface will reflect and remain in a concentrated bundle. This is called specular reflection and represents a “mirror-like” reflection of light. On the other hand, rays that strike a rough, or diffuse, surface will reflect and scatter in many directions through diffuse reflection.
While a diffuse surface can have a high total reflectance, the reflected light is scattered, which prevents it from being transmitted in a consistent and tightly-controlled fashion. For tubular daylighting devices (TDDs), a high specular reflectance is imperative so that light can be efficiently transferred through the system. A tubing system with a diffuse reflectance component should be avoided since that would result in light being scattered and lost back out through the top of the system.
The Spectralight Infinity Tubing used in Solatube Daylighting Systems is the most reflective material used in TDDs. With an extremely high specular reflectance of up to 99.7%* over the visible portion (ranging between 380-760 nanometers (nm)) of the electromagnet spectrum, it ensures that very little light is lost as it travels through the optical tubing system.
Achieving a high specular reflectance is critical since even small differences in reflectance values can significantly impact light output. That’s because every time a ray of light is reflected, some amount of that light is either scattered or lost through absorption. The difference of just a few percentage points in specular reflectivity makes a huge difference in overall performance.
Contrary to total reflectance, specular reflectance is a key factor in determining an optical tube’s efficiency in transferring light, and manufacturers should provide specular reflectance data to document the true efficiency of their TDD systems.
*Specular reflectance greater than 99%, with wavelength-specific reflectances up to 99.7% for the visible spectrum.