light falling onto the surface would be difficult to use because the focused light is concentrated by a factor of 10 to 15 and saturation or overheating of the light sensor is hard to avoid thus an approach using two temperature sensors to control thermal heat collection which addresses all of these issues is shown in fig7 two temperature sensors 402 and 404 are mounted on a sensor bracket 406 which is attached to one of the receiver tubes 300 because all of the mirrors move in concert it is only necessary to instrument one of the tubes the sensor bracket 406 is placed in such a position that is completely shadowed by the adjacent mirror so that no direct sunlight strikes the bracket the sensor bracket is made of a thin metal sheet which is thermally isolated from the receiver tube 300 upon which it is mounted the thermal insulation 408 is important to decouple the temperature sensors 402 and 404 from both the thermal lag of the receiver tube 300 to and the temperature disturbances coming from the fluid itself the face of the sensor bracket 406 that faces the concentrated light is painted a medium gray color with a light absorbance factor of about 0 3 if the mirror 204 is perfectly focused and all of the light is falling on the receiver tube 300 the temperature of the upper and lower temperature sensors 402 and 404 will be equal as the sun moves across the sky or something else causes an error in the pointing some of the concentrated light will either spill high or low.