Daylight harvesting light fixture and control system for same
A daylight harvesting light fixture has a housing defining a light aperture, a light emitting diode (LED) in the housing for providing illumination through the aperture, a power supply for supplying drive current to the LED; a photosensor for sensing ambient illumination through the housing aperture, and a control circuit programmed and connected for a) turning off current to the LED for a dark interval imperceptible to the human eye; b) deriving an ambient illumination level signal based on the photosensor output while the LED is turned off; c) calculating an LED light output complementary to measured ambient illumination to achieve a target ambient illumination; and d) restoring drive current to the LED at a level adjusted to produce the calculated complementary LED light output. This sequence may be repeated periodically to maintain a target ambient illumination.
1. An apparatus for controlling a lamp comprising a lamp housing defining an aperture, an electrically powered light source mounted in the housing for illuminating a space through the aperture, a power supply that provides power for the light source, and a photosensor in the housing that is arranged for sensing illumination in the space through the aperture, the apparatus comprising:
a switch connected in a circuit path with the light source and the power supply, the switch being arranged to controllably interrupt and restore power to the light source from the power supply;
a controller coupled to the switch and to the photosensor, wherein the controller is adapted to cause the switch to interrupt power to the light source for a dark interval and to measure an output of the photosensor during the dark interval, the controller further being adapted to control an amount of illumination from the light source based on the measured output; and
a resistor in an output path of the photosensor, wherein the resistor is configured to limit a duration of the dark interval.
2. The apparatus of claim 1 , wherein the controller is further adapted to derive a value representative of existing illumination of said space free of contribution from the light source based on the measured output.
3. The apparatus of claim 2 , wherein the controller is further adapted to estimate a supplemental light output from the light source which in combination with the existing illumination suffices to raise total illumination of said space to a target illumination level.
4. The apparatus of claim 3 , wherein the controller is further coupled to the power supply and adapted to cause the power supply to an adjusted power level corresponding to the supplemental light output.
5. The apparatus of claim 1 , wherein the dark interval has a duration imperceptible to the human eye.
6. The apparatus of claim 5 , wherein the duration is less than one millisecond.
7. The apparatus of claim 5 , wherein the photosensor has a fall time less than the duration.
8. The apparatus of claim 7 , wherein the fall time is less than 200 microseconds.
9. The apparatus of claim 1 , further comprising an amplifier coupled between the photosensor and the controller, wherein the amplifier is adapted to amplify the output from the photosensor before it is received by the controller.
10. The apparatus of claim 1 , wherein the light source comprises a light emitting diode.
11. A method for controlling a light fixture comprising a housing defining a light aperture, a light source in the housing for providing illumination through the aperture, a power supply adapted to supply power to the light source, and a photosensor in the housing for sensing ambient illumination through the aperture, the method comprising:
turning off the power to the light source for a dark interval;
deriving an existing ambient illumination level based on an output of the photosensor output during the dark interval;
calculating a light source output complementary to the existing ambient illumination level needed to achieve a target ambient illumination;
controlling a power level from the power supply so as to cause the light source to produce the complementary light source output; and
configuring a resistor in an output path of the photosensor so as to limit a duration of the dark interval.
12. The method of claim 11 , further comprising waiting for a duration during the dark interval before performing the deriving.
13. The method of claim 12 , wherein the photosensor has a fall time less than the duration.
14. The method of claim 13 , wherein the fall time is less than 200 microseconds.
15. The method of claim 11 , wherein the dark interval has a duration imperceptible to the human eye.
16. The method of claim 15 , wherein the duration is less than one millisecond.
17. The method of claim 11 , wherein the light source comprises a light emitting diode.