Systems and methods of illuminating plants

1. A system for illuminating a plant in a presence of an additional light source comprising sunlight that has a variable output, wherein the system comprises:

a far-red light source, an output of the far-red light source configured to combine with the variable output of the additional light source to provide an amount of photosynthetically active radiation to illuminate the plant, said amount of photosynthetically active radiation configured to be measured by a light sensor; and

a controller configured to vary the output of the far-red light source using a variable switch, such that, as the output of the additional light source varies, a percentage of total far-red light in the photosynthetically active radiation is no less than a far-red predetermined minimum percentage, wherein the far-red predetermined minimum percentage is below an average percentage of far-red light in daylight;

wherein when the additional light source outputs a percentage of far-red light that is less than the far-red predetermined minimum percentage, the output of the far-red light source is varied such that the percentage of total far-red light in the photosynthetically active radiation is equal to the far-red predetermined minimum percentage; and

wherein when the additional light source outputs a percentage of far-red light that is equal to or greater than the far-red predetermined minimum percentage, the far-red light source is switched off.

2. The system according to claim 1 , wherein the far-red predetermined minimum percentage is 4%, 6%, 8%, or 10%.

3. The system according to claim 1 , further comprising a blue light source, an output of the blue light source configured to combine with the outputs of the far-red light source and the additional light source to provide the amount of photosynthetically active radiation to illuminate the plant, and wherein the controller is configured to vary the output of the blue light source such that, as the output of the additional light source varies, a percentage of total blue light in the photosynthetically active radiation is no less than a blue predetermined minimum percentage.

4. The system according to claim 3 , wherein the blue predetermined minimum percentage is 5%, 7%, 9%, 12%, or 15%.

5. The system according to claim 1 , wherein the light sensor measures an amount of total blue light in the photosynthetically active radiation, an amount of total far-red light in the photosynthetically active radiation, and/or the amount of photosynthetically active radiation, wherein the light sensor is configured to measure:

the amount of total blue light falling within a range of 400 nm to 500 nm,

the amount of total far-red light falling within a range of 700 nm to 800 nm, and/or

the amount of photosynthetically active radiation falling within a range of 400 to 700 nm.

6. The system according to claim 1 , wherein the additional light source further comprises a red light source.

7. The system according to claim 3 , wherein the blue light source, the far-red light source and a red light source each comprise an LED lamp, wherein

the output of the blue light source has a spectrum with a peak wavelength ranging from 400 nm to 500 nm,

the output of the far-red light source has a spectrum with a peak wavelength ranging from 700 nm to 800 nm, and

the red light source has an output having a spectrum with a peak wavelength ranging from 600 nm to 700 nm.

8. A method of illuminating a plant in a presence of an additional light comprising sunlight that has a variable output, comprising:

providing far-red light;

combining the far-red light and the additional light to provide an amount of photosynthetically active radiation to illuminate the plant;

determining by a light sensor an amount of photosynthetically active radiation offered to the plant and a percentage of total far-red light in the photosynthetically active radiation; and

automatically varying the far-red light by controlling an output of the far-red light with a variable switch such that, as the additional light varies, the percentage of total far-red light in the photosynthetically active radiation is no less than a far-red predetermined minimum percentage, wherein the far-red predetermined minimum percentage is below an average percentage of far-red light in daylight;

wherein when the far-red light provided by the additional light amounts to a percentage that is less than the far-red predetermined minimum percentage, the output of the far-red light is varied such that the percentage of total far-red light in the photosynthetically active radiation is equal to the far-red predetermined minimum percentage; and

wherein when the far-red light provided by the additional light amounts to a percentage that is equal to or greater than the far-red predetermined minimum percentage, the far-red light is switched off.

9. The method of claim 8 , further comprising:

providing blue light;

combining the blue light, the far-red light and the additional light to provide an amount of the photosynthetically active radiation to illuminate the plant; and

automatically varying the blue light such that, as the additional light varies, a percentage of total blue light in the photosynthetically active radiation is no less than a blue predetermined minimum percentage.

10. A controller for use with a luminaire comprising a blue light source and a far-red light source, the controller comprising:

a first signal port configured to receive a value corresponding to a percentage of total blue light in a photosynthetically active radiation and a value corresponding to a percentage of total far-red light in the photosynthetically active radiation wherein the blue light, the far-red light and an additional light combine to provide the photosynthetically active radiation, and wherein the additional light has a variable output;

a second signal port configured to control an output of the blue light source and an output of the far-red light source with a variable switch; and

a computer readable program, which when executed, varies:

(i) the output of the blue light source such that, as the additional light varies, the percentage of the total blue light in the photosynthetically active radiation is no less than a blue predetermined minimum percentage, wherein the blue predetermined minimum percentage is below an average percentage of blue light in daylight, and

(ii) the output of the far-red light source such that, as the additional light varies, the percentage of total far-red light in the photosynthetically active radiation is no less than a far-red predetermined minimum percentage.

11. A computer program product including a set of non-transitory computer readable instructions stored on a memory and executable by a processor, the set of non-transitory computer readable instructions arranged to:

control illumination of a plant by providing far-red light in a presence of an additional light comprising sunlight having a variable light output;

control the combination of the far-red light and the additional light to provide an amount of photosynthetically active radiation to illuminate the plant; and

automatically vary the far-red light such that, as the additional light varies, a percentage of total far-red light in the photosynthetically active radiation is no less than a far-red predetermined minimum percentage, wherein the far-red predetermined minimum percentage is below an average percentage of far-red light in daylight;

wherein when the far-red light provided by the additional light amounts to a percentage that is less than the far-red predetermined minimum percentage, the output of the far-red light is varied such that the percentage of total far-red light in the photosynthetically active radiation is equal to the far-red predetermined minimum percentage; and

wherein when the far-red light provided by the additional light amounts to a percentage that is equal to or greater than the far-red predetermined minimum percentage, the far-red light is switched off.