Direct and Total Solar-Energy Transmittance
Direct solar-energy transmittance and total solar-energy transmittance are two measurements of how incident solar energy reaches the cabin through glazing. The total value includes an inward heat path that the direct value omits.
The two measurements
- Direct solar-energy transmittance is the proportion of incident solar radiation that passes directly through the glazing. It describes the transmitted short-wave solar energy, not the glass's visible darkness alone.
- Total solar-energy transmittance adds the share of absorbed solar energy that the warmed glazing subsequently transfers inward by long-wave radiation and convection. Pilkington Global Glass Handbook: Solar-energy terms In glazing terminology, the total value is also called the solar factor or g-value; solar heat gain coefficient (SHGC) is a related name used in North America. Pilkington glass glossary: Solar transmittance terms
ISO 13837 specifies methods for determining direct and total solar transmittance for monolithic or laminated, clear or tinted automotive safety glazing. It treats them separately from luminous transmittance and colorimetry. ISO 13837:2021: Automotive glazing transmittance methods
Why the distinction matters
Absorbing glazing can reduce direct transmission while warming in sunlight. Some of that absorbed energy then moves toward the cabin and some toward the outside, so the total value is higher than the direct component alone. Reflective and absorbing constructions can therefore produce different total results even when another headline metric looks similar.
Neither value can be inferred from visible light transmission, ultraviolet rejection, or a narrowly defined infrared-rejection figure. A useful comparison names the metric, test method, sample construction, and whether the value describes direct or total solar energy.
See Solar-Control and Infrared-Reflective Glass for the main solar-control constructions and Solar-Control and Acoustic Automotive Glazing for the wider vehicle-system effects.