Candlepower is an obsolete term that was once used to express the intensity of light at the source. Today, the term has been replaced by "candela," but many people continue to use the original word. This term is in especially common use on the packaging for flashlights. It is applied inconsistently, however, so a term like "200 candlepower" may not be very helpful for determining how bright something is at the source.
Like other measurements, candlepower revolves around a standardized and easily replicated base. When people were originally standardizing the term, they used a basic candle as the basis for measurements. Over time, the standard base was adjusted several times to different light sources. In 1948, this unit of measurement was abandoned in favor of the candela. The candela is based on a physical process and can be precisely calibrated.
Expressing intensity in this unit of measurement is designed to convey how many candles would be required to generate light of the same intensity. As science and technology progressed, one of the things that developed was increasingly powerful and flexible lighting technology. This provided access to sources of illumination significantly brighter than candles, in addition to also being safer because they did not require open flames to function.
When measured by the last standard used, one candlepower is equal to .981 candela. Some people and manufacturers use the terms "candela" and "candlepower" interchangeably, however, giving candlepower a slight upgrade. This can create confusion when comparing the intensity of light sources. It also makes it difficult to convert between this and other measures, as formulas used for converting do not work when people do not know which version of candlepower is being referred to.
Products advertised in terms of candlepower may have additional information about the intensity of their light, such as a measure in candelas that will provide a more accurate description. If consumers are not sure about how bright a given product is, they can contact the manufacturer for more information. Staff at stores where lighting products are sold may also know how manufacturers of specific products measure brightness and are usually happy to offer assistance when asked.
It is important to know that candlepower is an expression of intensity of light at the source. It does not provide information about how strong the light is at a distance or how much light is perceived by the human eye. This measurement also cannot be used to determine how much electricity a given light source uses.