Light measurement often seems complex, presenting a confusing array of units like lumens, lux, and candela on product packaging. The candela, symbolized as ‘cd’, represents the fundamental unit of light measurement within the International System of Units (SI). This unit forms the basis of photometry, the science dedicated to quantifying visible light as perceived by the average human eye.
Defining Candela as Luminous Intensity
The candela is the SI base unit for luminous intensity, which quantifies the concentration of light emitted by a source in one specific direction. Luminous intensity is a directional quantity, indicating the “strength” of the light beam, defined as the luminous power per unit of solid angle, measured in steradians.
A steradian is the three-dimensional equivalent of a two-dimensional angle, representing the cone-shaped spread of light from its source. The candela value is high when a given amount of light is tightly focused into a narrow cone, even if the total light produced is small. Historically, a common wax candle provided the approximate intensity of one candela, which is how the unit received its name.
The modern definition anchors the candela to a physical constant. One candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation at a frequency of 540 terahertz (THz). This light must have a radiant intensity in that same direction of exactly 1/683 watt per steradian.
The Role of Human Visual Perception
Candela is unique among SI base units because it is a psychophysical measure, directly linked to human perception rather than raw physical power (measured in watts). It measures light power weighted by how sensitive the human eye is to different wavelengths, a process defined by the photopic luminosity function, commonly known as the V(\(\lambda\)) curve.
The V(\(\lambda\)) curve shows that the average human eye is not equally sensitive to all colors of light. Under normal daylight conditions, our visual system is most sensitive to light with a wavelength of 555 nanometers (yellowish-green). The candela measurement assigns a significantly higher value to a light source emitting power in the green-yellow spectrum than it would to a source emitting the same power in the deep red or blue spectrum.
This ensures the candela value accurately reflects perceived brightness. If a blue light and a green light have the same radiant intensity in watts, the green light will have a much higher candela value because the eye registers it as brighter.
How Candela Differs from Lumens and Lux
Candela, lumens (lm), and lux (lx) measure three distinct, mathematically related aspects of light. The candela measures luminous intensity, focusing on the concentration of light into a specific beam.
The lumen, or luminous flux, measures the total quantity of light emitted by a source in all directions. Lumens measure the total light output overall, regardless of direction. This can be compared to measuring the total volume of water flowing out of a garden hose per second. A light source with a wide, dispersed beam may have high lumens but a low candela value because the light is not focused.
Lux, or illuminance, measures the amount of light that actually falls onto a specific surface area. One lux is defined as one lumen distributed over one square meter (\(1 \text{ lx} = 1 \text{ lm}/\text{m}^2\)). Carrying the hose analogy forward, lux measures how wet the ground is in a specific spot, which depends on both the total water flow (lumens) and how spread out that water is.
A light source can have a high candela rating, meaning a very tight, strong beam, which results in a high lux reading on a small area far away. Conversely, a standard room light might have high lumens to fill a whole room, but a low candela rating because the light is spread everywhere, resulting in a lower lux reading on any single surface.
Practical Applications of Candela Measurement
The directional nature of the candela makes it the most relevant measurement for applications requiring focused light, such as spotlights and searchlights. When light needs to travel a distance or be concentrated for a specific task, the candela value is more informative than the total lumen output.
Vehicle headlights are a prime example, measured in candelas to ensure sufficient road illumination without blinding oncoming drivers. Signaling lights, such as those used at airports or in marine navigation, rely on high candela values to ensure visibility over long distances and through various atmospheric conditions. In architectural lighting, the candela rating helps designers calculate how much light will reach a specific point, such as highlighting a piece of art or providing task lighting on a desk.