With HMI bulbs, color temperature varies significantly with lamp age. A new bulb generally will output at a color temperature close to 15,000 K during its first few hours. As the bulb ages, the color temperature reaches its nominal value of around 5600 K or 6000 K. With age, the arc length becomes larger as more of the electrodes burn away. This requires greater voltage to sustain the arc, and as voltage increases, color temperature decreases proportionately at a rate of approximately 0.5–1 kelvin for every hour burnt. For this reason, and other safety reasons, HMI bulbs are not recommended to be used past half their lifetime.