a shorter lifespan and sometimes catastrophic failure. Special electronic ballasts (integrated or separate) are required for dimming service. CFL light output is roughly proportional to phosphor surface area, and high output CFLs are often larger than their incandescent equivalents. This means that the CFL may not fit well in existing light fixtures. To fit enough phosphor coated area within the approximate overall dimensions of an incandescent lamp, standard shapes of CFL tube are a helix with one or more turns, multiple parallel tubes, circular arc, or a butterfly. Some CFLs are labeled not to be run base up, since heat will shorten the ballast's life. Such CFLs are unsuitable for use in pendant lamps and especially unsuitable for recessed light fixtures. CFLs designed for use in such fixtures are available. Current recommendations for fully enclosed, unventilated light fixtures (such as those recessed into insulated ceilings), are either to use "reflector CFLs" (R-CFL), cold-cathode CFLs or to replace such fixtures with those designed for CFLs. A CFL will thrive in areas that have good airflow, such as in a table lamp. Characteristics Spectrum of light CFLs emit light from a mix of phosphors, each emitting one band of color with some bands still in the ultraviolet range as can be seen on the light spectrum. Modern phosphor designs balance the emitted light color, energy efficiency, and cost. Every extra phosphor added to the coating mix improves color rendering but decreases efficiency and increases cost. Good quality consumer CFLs use three or four phosphors to achieve a "white" light with a color rendering index (CRI) of about 80, where the maximum 100 represents the appearance of colors under daylight or other sources of black-body radiation such as an incandescent light bulb (depending on the correlated color temperature). Color temperature can be indicated in kelvins or mireds (1 million divided by the color temperature in kelvins). The color temperature of a light source is the temperature of a black body that has the same chromaticity (i.e. color) as the light source. A notional temperature, the correlated color temperature, the temperature of a black body that emits light of a hue that to human color perception most closely matches the light from the lamp, is assigned. The color temperature is characteristic of black-body radiation; practical white light sources approximate the radiation of a black body at a given temperature, but will not have an