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The efficiency of solar thermal collectors is primarily influenced by the angle of sunlight received. This is because the amount of solar energy that a collector can absorb directly depends on its orientation relative to the sun. When the sun's rays strike the collector at a perpendicular angle, maximum absorption occurs, leading to higher efficiency.

If the angle of incidence is not optimal, meaning that sunlight strikes the collector at a slant, the effective area for light absorption decreases, reducing the energy collected. Factors such as seasonal changes in solar angle and the geographic location can affect how sunlight reaches the solar thermal collectors throughout the year.

Other options do not have as significant an impact. Building materials might influence heat retention or insulation but do not affect the collector's direct interaction with sunlight. The color of the building exterior may relate to heat absorption in terms of aesthetics or overall energy efficiency, but it is not a primary factor for the collectors themselves. The age of the collector system might introduce degradation over time, yet this factor is secondary compared to the direct influence of sunlight angle on current operational efficiency.