BACKGROUND

Traditional sources of light such as the Sun (and later incandescent lights) may exhibit the characteristics of a black body radiator. Such light sources typically emit a relatively continuous-spectrum of light, and the continuous emissions range the entire bandwidth of the visible light spectrum (e.g., light with wavelengths between approximately 390 nm and 700 nm). The human eye has grown accustomed to operating in the presence of black body radiators and has evolved to be able to distinguish a large variety of colors when emissions from a black body radiator are reflected off of an object of interest.

Further, the frequency or wavelength of the continuous light spectrum emitted by a black body radiator may be dependent on the temperature of the black body radiator. Plank's law states that a black body radiator in thermal equilibrium will emit a continuous-spectrum of light that is dependent on the equilibrium temperature of the black body.FIG. 1illustrates the Black Body Radiator Curve according to Plank's law.

As shown inFIG. 1, as the temperature of the black body radiator increases, the frequency of the peak of the emitted spectrum shifts to higher frequencies. At room temperature (e.g., roughly 300 Kelvin (K)), the frequency peak is typically within the infrared portion of the spectrum and thus is imperceptible to the human eye. However, when the temperature is increased to approximately 700-750 K, the blackbody radiator will begin to emit light in the visible range of the electromagnetic spectrum.

Typically, as the temperature of the black body radiator decreases, the wavelength of the emitted light increases and the frequency decreases, such that the emitted light appears “redder”. As the temperature increases, the peak of the emitted spectrum become “bluer” or decreases in wavelength (e.g., increases in frequency). For black body radiators, this relationship between temperature and wavelength/frequency of the emitted light is inseparable—higher temperature radiators appear bluer and lower temperature radiators appear redder.

Thus, various wavelengths/frequencies of the visible light spectrum may be associated with a given “color temperature” of a black body radiator.FIG. 2illustrates an example comparison of the colors associated with different color temperature values. The color temperature of a light source may refer to the temperature of an ideal black body radiator that radiates light of comparable hue to that of the light source. As shown inFIG. 2, candlelight, tungsten light (e.g., from an incandescent bulb), early sunrise, and/or household light bulbs may appear to have relatively low color temperatures, for example on the range of 1,000-3,000 K. Noon daylight, direct sun (e.g., sunlight above the atmosphere), and/or electronic flash bulbs may appear to have color temperature values on the order of 4,000-5,000 K and may have a greenish blue hue. An overcast day may appear to have a color temperature of approximately 7,000 K and may be even bluer than noon daylight. North light may be bluer still, appearing to have a color temperature on the range of 10,000 K.

Color temperatures over 5,000 K are often referred to as cool colors (e.g., bluish white to deep blue), while lower color temperatures (e.g., 2,700-3,000 K) are often referred to as warm colors (e.g., red through yellowish white).

Incandescent and halogen lamps typically act as black body radiators. For example, a current is passed through a wire (e.g., a filament), causing the wire to increase in temperature. When the wire reaches a critical temperature, it begins to radiate light in the visible spectrum. The color temperature of the radiated light is dictated by Plank's law. When an incandescent or halogen light is dimmed, the temperature (and color temperature) is decreased, meaning that the emitter light becomes redder (e.g., higher wavelength, lower frequency). Thus, humans are accustomed to dimmed lights having a redder hue.

Recently, non-incandescent light sources such as fluorescent lights (e.g., compact fluorescent lights or CFLs) and light emitting diodes (LEDs) have become more widely available due to their relative power savings as compared to traditional incandescent lamps. Typically light from CFLs or LEDs does not exhibit the properties of a black body radiator. Instead, the emitted light is often more discrete in nature due to the differing mechanisms by which CFLs and/or LEDs generate light as compared to an incandescent or Halogen light bulbs. Since fluorescents and LEDs do not emit relatively constant amounts of light across the visible light spectrum (e.g., instead having peaked intensities at one or more discrete points within the visible spectrum), fluorescents and LEDs are often referred to as discrete-spectrum light sources.

The wavelength/frequency profile of a light source may be dependent on the device or technique used to generate the light. For example, light from fluorescent lamps is produced by electrically exciting mercury within a glass tube. The applied voltage causes the mercury to become a plasma that emits light in the ultraviolet (UV) frequency range. Typically, the glass tube is coated with a phosphorus-based material that absorbs the radiated UV light and then emits light in the visible frequency range. The wavelength shift from UV to the visible range is referred to as Stokes shift. Depending on the properties of the phosphorus-based material, the wavelength/frequency of the light emitted may be at different points within the visible spectrum.FIG. 3illustrates the discrete-spectrum emitted by an example CFL as compared to an example continuous light source such as an incandescent lamp. For example, the line SPDISC-FLUOR310may represent the relative intensity of light emitted at various wavelengths by an example CFL, and the line SPCONT320may represent the relative intensity of light emitted at the same wavelengths by an example incandescent lamp. As may be seen inFIG. 3, the fluorescent light source may be characterized by one or more “bursts” of emissions at discrete frequencies/wavelengths.

Light from LEDs is produced due to the physical properties of a semiconducting material. For example, when a voltage is applied across a semiconductor junction that has different levels of electron doping across the boundary, an electric current is induced. When an electron from one side of the device recombines with an electron hole on the other, a photon is emitted. Depending on the semiconductor design, the photons may be emitted at various wavelengths/frequencies within the visible light spectrum. Like fluorescents, Stokes shift may cause the frequency of the emitted photons to be lowered to achieve a desired light frequency output.FIG. 4illustrates the discrete-spectrum emitted by an example LED as compared to an example continuous light source such as an incandescent lamp. For example, the line SPDISC-LED410may represent the relative intensity of light emitted at various wavelengths by an example LED, and the line SPCONT420may represent the relative intensity of light emitted at the same wavelengths by an example incandescent lamp. Like the emissions from the fluorescent lamp, the LED light may also be relatively discrete in nature.

When discrete-spectrum light sources are dimmed, their color temperature may not change in the same manner as black body radiators. For example, when incandescents and halogens are dimmed, their temperature is decreased and the emitted light transitions to a lower color temperature value (e.g., becomes redder) according to Plank's law. However, since discrete-spectrum light sources are not black body radiators, Plank's law may not apply. For example, both fluorescent lamps and LEDs may maintain a relatively constant color temperature even in the presence of dimming (e.g., and may actually become slightly bluer or higher frequency as they are dimmed). Such an effect may be unnatural to the human eye, which may expect the color temperature to shift to a redder temperature as the light dims. Moreover, when discrete-spectrum light sources are placed in the vicinity of other light sources, for example sources of light whose color temperature may change over time, the discrete-spectrum light sources can appear unnatural or distracting.

SUMMARY

Methods and systems are disclosed for controlling the color temperature of one or more light sources based on environmental criteria and/or user preferences. For example, a composite lighting load including at least one discrete-spectrum light source and at least one additional light source may be controlled by a load control system in order to vary the color temperature of the light emitted by the composite lighting load. For example, the color temperature of the at least one discrete-spectrum light source may be determined and/or adjusted based on one or more of the ambient color temperature of a space, the actual temperature of the space, the relative brightness of the space, the occupancy of the space, a time clock, a demand response command (e.g., from an electrical utility), the absolute location of the discrete-spectrum light source, the location of the discrete-spectrum light source relative to other light sources, inputs from a camera or other external devices, the operation of appliances or other machines in the vicinity of the discrete-spectrum light source, media content being utilized in the vicinity of the discrete-spectrum light source, and/or other sensor inputs. The light emitted from the composite lighting load may be made redder (e.g., higher wavelength, lower frequency) in response to a first set of criteria and/or the discrete-spectrum light source may be made bluer (e.g., lower wavelength, higher frequency) in response to a second set of criteria.

For example, a system controller may be configured to control the color temperature of one or more controllable-color-temperature lighting loads. Controllable-color-temperature lighting loads (CCTLLs) may also be referred to as color temperature controllable lighting loads. The system controller may receive one or more input signals (e.g., ambient color temperature of a space, actual temperature of a space, the relative brightness of the space, the occupancy of the space, etc.) and may send a signal indicating a determined color temperature and/or a determined change in color temperature to the one or more controllable-color-temperature lighting loads. The indications may be specific to each respective controllable-color-temperature lighting load. The controllable-color-temperature lighting loads may change their respective color temperature levels based on the signal received from the system controller.

In an example, the controllable-color-temperature lighting load may include a control circuit and two or more discrete-spectrum light sources. The discrete-spectrum light sources may be operably coupled to a color temperature load regulation system that is configured to vary the intensity of one or more of the discrete-spectrum light sources in order to vary the color temperature of the controllable-color-temperature lighting load. For example, the color temperature load regulation system may include load regulation circuitry configured such that the control circuit may vary the color temperature of the combined emissions from the discrete-spectrum light sources. The control circuit may also be operably coupled to a communication circuit for communicating with the system controller. In an example, rather than or in addition to utilizing a system controller to control each of the controllable-color-temperature lighting loads in the system, the controllable-color-temperature lighting loads may be configured to communicate with each other and operate in an ad hoc manner.

