Candela

The candela (cd) is one of the 7 base SI units: the SI unit of luminous intensity. It is a measure of power emitted from a light source. The current definition for a candela from May of 2019 is:

The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz, K_cd, to be 683 when expressed in the unit lm⋅W⁻¹, which is equal to cd⋅sr⋅W⁻¹, or cd⋅sr⋅kg⁻¹⋅m⁻²⋅s³, where the kilogram, meter and second are defined in terms of h, c and Δν_Cs.

Candela is Latin for the word 'candle', and a typical candle's luminous intensity is actually about equal to one candela.

Significance of the Candela (K_cd)

K_cd is one of the seven defining constants for the SI base units, and is equal to 683 lumens per watt. This value makes the modern candela roughly equivalent to the previously defined candela. The amount of power needed to generate a candle's brightness is approximately 1/683 watt per steradian (notice, the units are power per "solid angle"). Interestingly, this value is not tied to any particular fundamental physical effect, any other being with different visual sensitivity will see candles at different intensities. The constant itself is in that sense arbitrary, and simply convenient for human perception.

Candela vs. Lumen vs. Watt

The candela is the base unit for photometry, which is the science of measuring light as perceived by the human visual system. The unit of candela specifically measures: "the light we can see, coming directly from a source we can see, straight into our eyes." This means that the candela is only used to measure wavelengths within the human visual spectrum, and only light coming directly from the source to one's eyes.

Figure 2. A visual representation of the difference between lumens and candelas.

The lumen is different because it represents the entire (total) amount of light shining from an object. And watts are different because they can be used to describe other types of light (electromagnetic radiation) that human eyes cannot detect, such as radio waves and X-rays.

Measuring only the light coming directly from the source to one's eyes (solid angle)

Figure 3. Plane angle (left) & solid angle (right).

The idea of a "solid angle" is used to convey the radiation of light into three-dimensional space. It is defining a field of view. The units are steradians (sr), as opposed to radians (rad) used for two-dimensional angles. Using these three-dimensional angles specifies the light that shines in a particular direction.

When "brightness" is measured in lumens on a lightbulb, the value tells you much that light radiates in total, meaning how much light is radiating in all directions. On the other hand, the candela measures how bright the light source will be if you look directly at it.

Human eye sensitivity to different wavelengths of light

Figure 4. A graph of the eye's sensitivity to different colours.

The 1979 definition of the Candela specified one particular frequency of light instead of the whole visible spectrum, why? While humans can see the whole visible spectrum, we have varying sensitivities to different colours. In the daytime, our eyes are most sensitive to a greenish-yellow light, the frequency 540 × 10¹² hertz (equivalent to a wavelength of approximately 555 nm) is a hue of green.

For other colours of light, the International Commission on Illumination (CIE) created a method to account for the human eye's greater sensitivity to certain hues. The method grants more influence to colours which the eye registers more strongly. By taking into account the colour's influence on the intensity of light, you find the light source's overall luminous intensity.

While “radiant intensity” is the intensity of light without adjustments for human vision, luminous intensity is a “photometric” unit that compensates for human visual sensitivity.

For Further Reading

• Visit Dr. Rowlett's unit dictionary to learn more about the candela and other units.
• Electromagnetic radiation
• Lumen
• Watt

• Or explore a random page