Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields. The color temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable [[hue]] to that of the light source. In practice, color temperature is only meaningful for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e. those on a line from reddish/orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of e.g. a green or a purple light. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K.

Color temperatures over 5,000K are called cool colors (blueish white), while lower color temperatures (2,7003,000 K) are called warm colors (yellowish white through red).(cite)  This relation, however, is a psychological one in contrast to the physical relation implied by Wien's displacement law, according to which the spectral peak is shifted towards shorter wavelengths (resulting in a more blueish white) for higher temperatures.

The Sun's effective temperature, defined by the total radiative power per square unit, is about 5,780 K.(cite)

As the Sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing color of the sun over the course of the day is mainly a result of scattering of light, and is not due to changes in black body radiation. The blue color of the sky is caused by Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red light.

Daylight has a spectrum similar to that of a black body with a correlated color temperature of 6,500 K (D65 viewing standard) or 5,500 K (daylight-balanced photographic film standard).

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For the purpose of indoor gardening/hydroponics, most growers want "sun-similar", which generally means a color temperature of 6200K or higher during the mass growing stages (vegetive stage) to simulate the sun during summer when it is high in the sky, and a lower color temperature (below 4000K) during the flowering stage, to simulate the sun in the late summer and fall, when it has a more yellow/red hue. This is primarily for plants grown for their flowers and fruits rather than ornamental purposes, such as common house plants. This is also why many growers use special color temperature metal halides while the plant is growing mass, then switch to special color temperature high pressure sodium lamps when it is time to begin the actual flowering, to better simulate the natural sun cycles and light hues.

For enthusiasts wanting the most sun-similar experience for their gardening, this poses a problem since the electronics system for high pressure sodium and metal halide are very different, as they use the same size base, but have very different starting requirements. Fortunately, this is easily overcome by using digital ballasts that are auto-sensing and will work with both lamps. This allows a gardener to start cuttings and/or germination using fluorescent lamps, switch to metal halide for the growth stages, then just change the bulb to a high pressure sodium vapor lamp for flowering. Note that HP sodium vapor lamps will produce about 20%-30% more light per watt than metal halide lamps, as they are significantly more efficient.

Dual purpose digital ballasts are very price competitive with traditional ballasts, are smaller, lighter and generate less waste heat. If you are replacing an older ballast, digital is the way to go simply due to the flexibility this will give you. Some also have the option of running at 50%, 75% or 100% of power, which adds flexibility for those who are using them as supplements to natural sunlight in a greenhouse.

The lede image is based on an image in the public domain and is provided courtesy of User:PAR at Wikipedia.