Lighting system

1. An apparatus for controlling the light intensity received at a plant canopy comprising:

a frame;

an artificial light source supported by the frame and positioned at a distance generally above a plant canopy;

a light intensity sensor positioned to measure a light intensity, the light intensity sensor positioned at one of the plant canopy and a distance above the plant canopy;

a first height adjustment unit configured to adjust the distance between the artificial light source and plant canopy; and

a distance sensor supported by the frame and moveable with the artificial light source, the distance sensor measuring a distance between the distance sensor and the plant canopy, wherein the first height adjustment unit adjusts the distance between the artificial light source and the plant canopy in response to at least one of the light intensity measured by the light intensity sensor and the distance measured by the distance sensor further comprising a second height adjustment unit configured to adjust the relative position of the light intensity sensor to the artificial light source, wherein the second height adjustment unit adjusts the relative distance between the artificial light source and the light intensity sensor to maintain the light intensity sensor at a predetermined distance above the plant canopy.

2. The apparatus of claim 1 , wherein the second height adjustment unit is supported by the frame.

3. The apparatus of claim 1 wherein the apparatus is located in a greenhouse having an internal frame, and the first height adjustment unit is attached to the internal frame.

4. The apparatus of claim 1 wherein the first height adjustment unit comprises a rotatable member, a drive for rotating the rotatable member, and a plurality of cables having a first end wrapped around the rotatable member and a second end attached to the frame, the frame being rigidly attached to the artificial light source, the plurality of cables raising the frame when the drive rotates the rotatable member in a first direction and lowering the frame when the drive rotates the rotatable member in a second direction.

5. The apparatus of claim 1 wherein the distance sensor is an ultrasound sensor.

6. The apparatus of claim 1 , wherein the first height adjustment unit adjusts the distance between the artificial light source and the plant canopy in a first direction, and the second height adjustment unit adjusts the relative position of the light intensity sensor to the artificial light source in a second direction, the second direction being opposite of the first direction.

7. The apparatus of claim 1 , wherein the light intensity sensor is positioned below the artificial light source.

8. The apparatus of claim 1 , wherein the second height adjustment unit is separate from the first height adjustment unit.

9. A method of controlling the light intensity received at a plant canopy comprising the steps of:

providing a plurality of plants defining a plant canopy;

providing artificial light from an artificial light source positioned above the plant canopy;

measuring a light intensity with a light intensity sensor at one of the plant canopy and a predetermined distance above the plant canopy;

adjusting the distance between the plant canopy and artificial light source in response to the measured light intensity;

determining the distance between the light intensity sensor and the plant canopy with a distance sensor; and

adjusting the position of the light intensity sensor relative to the artificial light source in response to the measured distance to position the light intensity sensor at the predetermined distance above the plant canopy.

10. The method of claim 9 further comprising dimming the artificial light source in response to the measured light intensity.

11. The method of claim 9 further said adjusting step further comprises the steps of:

(a) providing a target light intensity;

(b) comparing the measured light intensity with the target light intensity;

(c) increasing the distance between the plant canopy and artificial light source when the measured light intensity is greater than the target light intensity; and

(d) decreasing the distance between the plant canopy and artificial light source when the measured light intensity is less than the target light intensity.

12. The method of claim 11 , wherein steps (a)-(d) are repeated at predetermined intervals.

13. The method of claim 12 , wherein the target light intensity at each predetermined interval is the same.

14. The method of claim 12 , wherein the target light intensity at each predetermined interval is cumulative of the target light intensity of previous periods and the measured light intensity at each predetermined interval is cumulative of the target light intensity of previous periods.