Why is Specular Reflectance a better measurement of light output than Total Reflectance?

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

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