What Is Meant By Candle Power?

Definition of Candle Power

Candle power is a measurement of light intensity, which refers to how bright a light source appears. It is based on an early standard dating back to the late 1800s, when candles were commonly used for illumination.

One candle power is defined as the light produced by a single candle emitting one candela of luminous intensity. A candela is the base unit of luminous intensity in the International System of Units (SI).

So in simple terms, the candle power of a light source indicates how many times brighter that light is than the light from one standard candle. For example, a 100 candle power bulb emits light that appears 100 times brighter than a single candle.

Candle power has historically been used to specify and compare the brightness of different light sources like bulbs and lamps. The higher the candle power, the greater the luminous intensity. Even today, you may see the candle power rating on some light bulb packages to give an approximation of their brightness.

History of Candle Power

The term “candlepower” originates from early photometric measurements where light sources were compared to a standard candle. Candles were once used as a standard source of light for testing the intensity of other lights. The candle adopted as the international standard was a spermaceti candle that burned at a specific rate and brightness.

The concept of using a candle as a standard unit emerged in the 18th century. By the late 19th century, a candle burning spermaceti wax at a specified rate of 120 grains per hour became the accepted standard for regulators and the scientific community. The luminescence produced by this standard candle burning at a precise rate came to be known as one candlepower.

Candlepower provided a simple way for people to understand and compare the brightness of artificial light sources in the early days of electric lighting. It was easy to grasp that a 60-candlepower light bulb produced light equivalent to 60 standard candles. The candlepower rating system enabled new electric lamps to be marketed to the public with an easily understood brightness measurement.

How Candle Power is Measured

measuring the candlepower of a lightbulb

Candle power is measured in candela, which is the modern SI unit for luminous intensity. One candela is defined as the luminous intensity produced by a light source emitting monochromatic radiation of frequency 540 x 1012 Hz and with a radiant intensity of 1/683 watt per steradian in the same direction.

To measure candlepower, a light source is placed at the center of a sphere with a light meter on the outer surface. The intensity of light is measured at various points on the sphere and used to calculate the total luminous intensity emitted by the light source. This measurement of luminous intensity in candela is equivalent to the candlepower rating. Higher candlepower indicates a more intense, concentrated light beam.

Using Candle Power

Candle power ratings are primarily used for:

  • Automotive headlights and interior lights to measure the brightness and range of illumination. The candle power rating indicates how far and how bright the headlights will shine.

  • Flashlights and other portable lighting to rate and compare brightness. Brighter flashlights with higher candle power ratings will illuminate farther distances.

  • Spotlights and floodlights since candle power measures the intensity of a focused beam of light. Higher candle power spotlights can illuminate objects at farther distances.

  • Older light bulb packaging and specifications since candle power was historically used as a brightness rating before lumens. Some older incandescent and halogen bulbs still list the candle power.

  • Stage lighting and theater spotlights where a narrow focused beam is important. The candle power rating indicates beam intensity.

  • Photography lighting like strobes and flashes to compare power and guide positioning for proper exposure.

  • Security and surveillance lighting to determine the distance and area illuminated. Higher candle power floodlights cover larger outdoor areas.

Candle Power vs Lumen

Candlepower and lumen are both units used to measure the intensity of light, but they measure different properties.

Candlepower, also known as candela, measures light intensity in a specific direction. It measures how much light is emitted in a certain direction, like a beam. So candlepower tells you how bright and focused the beam of light is.

In contrast, lumen measures the total amount of visible light emitted in all directions. Lumens measure total light output, rather than directional intensity. So a light source can have a high lumen value but a low candlepower, meaning it emits a lot of overall light but not a focused beam.

For example, an ordinary 40W incandescent light bulb produces about 450 lumens but only 10-15 candlepower. Whereas a tactical flashlight may produce 300 lumens but 45,000 candlepower. The flashlight beam is far more intense and focused, even though the light bulb produces more overall light output.

In summary, candlepower measures beam intensity, while lumen measures total light output. Candlepower tells you how bright a directed beam is, while lumens indicate how much light in total is being emitted by a source.

Candle Power vs Lux

Candlepower and lux measure different characteristics of light. Candlepower (also known as candela) measures light intensity, specifically the intensity of light from a source in a particular direction. It considers only the quantity of light emitted. Lux, on the other hand, measures illuminance, or the amount of light falling on a surface. It takes into account the intensity of the light source as well as how far away it is from the illuminated surface and the angle of incidence.

To compare candlepower and lux:

  • Candlepower focuses only on the light source, lux considers the full lighting context including distance and angle.
  • Candlepower measures intensity looking out from the source, lux measures brightness looking onto an illuminated surface.
  • Candlepower is an absolute measure of emission, lux depends on distance and orientation.

So candlepower quantifies how much light leaves a source, while lux quantifies how much arrives at a destination. They are complementary metrics used for different purposes in lighting design. Converting between them requires calculating the geometry between the source and illuminated surfaces.

Candle Power vs Watts

Candlepower and watts are two different units used to measure light. The key differences between candlepower and watts are:

Candlepower (CP or cd) measures light intensity – how concentrated or focused a light source is. It quantifies luminous intensity, or how much light is emitted in a certain direction. The higher the candlepower, the brighter the beam of light. Candlepower does not take into account the coverage area of the light.

Watts measure light power and energy consumption. A light bulb’s wattage rating indicates how much electrical power it consumes. Watts measure total light output, taking into account all directions.

For example, a laser pointer may have a very high candlepower rating because it emits a very focused, narrow beam. However, it consumes very little energy and has a low wattage. On the other hand, a 100W light bulb consumes a lot of energy but emits light in all directions so its candlepower in any one direction is relatively low.

In summary, candlepower measures beam intensity, while watts measure energy usage and total light emission. To choose an appropriate light source, both candlepower and watts should be considered based on the lighting application and needs.

Candle Power Rating of Common Light Sources

Candlepower (cp) ratings are still commonly used to measure the intensity of different electric light sources. Here are some typical candlepower levels:

  • Standard 40W incandescent light bulb – 10 to 15 cp

  • 100W incandescent light bulb – 19 to 24 cp

  • Compact fluorescent light bulb (CFL) – 60 to 70 cp

  • LED light bulb – 80 to 100 cp

  • Halogen headlights – 700 to 1500 cp

  • High intensity discharge (HID) headlights – 3000 to 3500 cp

  • Streetlights – 5000 to 15,000 cp

  • Stadium lights – 100,000 to 200,000 cp

  • Lighthouse beam – Up to 20,000,000 cp

As you can see, candlepower spans a huge range depending on the intensity and focus of the light source. While it is an older unit, candlepower is still useful for comparing the relative intensity of different electric lighting.

Limitations of Candle Power

While candle power was useful for comparing brightnesses of light sources in the past, it has some significant limitations and issues that led to it being replaced by modern standards like lumens and lux:

Candle power is highly directional, meaning the measurement can vary greatly depending on the angle the light is measured from. The candle power directly in front of a light source can be very different than at wider angles. This makes it difficult to accurately compare overall light output between sources.

The measurement is based on the human eye’s response to light, which can vary from person to person. This introduces subjectivity and variability from one measurement to the next.

As electrical lighting advanced, candle power comparisons to wax candles became less useful. Comparing an incandescent bulb to a candle provides limited practical guidance on actual brightness.

Candle power does not take into account the area or distance illuminated by a light source. Two lamps can have equal candle power but distribute light very differently. Other units like lumens better capture total light output.

Over time, the many limitations of candle power became increasingly apparent. More advanced and reliable ways of measuring light output were required as lighting technology evolved, ultimately leading to the adoption of better standards.

Modern Replacements for Candle Power

As technology advanced, more accurate ways of measuring light intensity were developed to replace the outdated candlepower unit. Here are some of the key modern replacements:

Lumen: The lumen (lm) measures the total quantity of visible light emitted by a source, taking into account all directions. It better represents the perceived brightness than candlepower.

Lux: The lux (lx) measures the intensity of light falling on a surface. It considers how the emitted light spreads out over an area, unlike candlepower which only measures intensity in one direction.

Watts: The watt (W) measures the electrical power consumption of a light source. For LED and efficient sources, you can estimate lumens based on wattage.

Candela: The candela (cd) measures luminous intensity in a given direction, similar to candlepower. However, it is defined using absolute photometric standards and is thus more precise.

Overall, modern light measurement units like lumens and lux provide far more accuracy than the outdated candlepower. They give a truer representation of perceived brightness and light coverage for lighting design.

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