What Does Daylight, Cool & Warm Mean in Light Bulbs?

Color temperature of light bulbs falls in a spectrum along the Kelvin absolute temperature scale. The greater the Kelvin number, the cooler the light appears. In this system, the following definitions apply:

• Warm white light bulbs have a color temperature ranging from 2500K up to 3000K. Warm whites have a yellow tone, which is considered a "warm" color.
• Cool white bulbs fall in the range of 3100K to 4500K. The light appears as a bright, neutral white that shades into blue at the upper end of this range.
• Daylight bulbs have color temperatures that begin at 4600K and can range up 6500K or higher. These bulbs give off light that appears distinctly shaded with blue and simulates the natural color of daylight, which is 5600K on a sunny day at noon.

A light bulb's color temperature can affect mood as well as how colors appear in the room.

Warm white is familiar to most people because it's the color of light produced by most incandescent bulbs. Warm white lighting exudes the feeling of warmth, calmness and relaxation, making it ideal for bedrooms, dining rooms and living rooms. This color of light enhances warm paint colors but can make cool colors look dull. For example, some shades of purple can look brown under warm light.

Cool white light has a lively feeling and enhances cool colors such as whites, blues and greens. The crispness of this type of light makes it suitable for bathrooms and work areas such as kitchens, home offices, basements and garages.

Lighting with daylight bulbs produces a vibrant and intense illumination that is often perceived as overly harsh for home use. However, it is ideal for a home workshop, sewing or craft room, because it provides clear illumination for detail-oriented work. Daylight bulbs are also suitable for lighting a display area, such as shelves holding books or collectibles, and for security lighting.

When accurate color perception is important, the ability of light bulbs to render color must be considered. This characteristic, which is separate from color temperature, depends on the bulb's color rendering index, or CRI. The CRI is a number ranging from 0 to 100, with the higher numbers indicating the best light bulbs for color rendering. Most incandescent and halogen bulbs are close to 100 CRI. Fluorescent bulbs can range from 75 to 90 CRI, even if they're bulbs with the same color temperature. If you are doing work that requires accurate color rendering as well as bright illumination for detail, look for a light bulb with a color temperature close to 5000K and a CRI of 90 or above.