Here's why:
* Candela (cd) is the base unit of luminous intensity in the International System of Units (SI). It measures the power emitted by a light source in a specific direction within the visible spectrum.
* Visible light intensity refers to the brightness of a light source, which is a subjective perception and not a directly measurable physical quantity.
While candela measures the objective physical power of a light source, visible light intensity is influenced by factors like:
* Spectral distribution of the light source: Our eyes are most sensitive to green light, so a source with a strong green component will appear brighter than one with a strong red or blue component, even if they have the same candela rating.
* Adaptation of the human eye: Our eyes adjust their sensitivity to light levels, making a source appear brighter in low light and dimmer in high light.
Instead of a direct equivalence, we can relate candela to other units:
* Luminous flux (lumens): One lumen is the luminous flux emitted by a point source with a luminous intensity of one candela within a solid angle of one steradian.
* Illuminance (lux): One lux is equal to one lumen per square meter.
In conclusion: While candela is a fundamental unit of light intensity, visible light intensity is a subjective perception influenced by various factors and doesn't have a direct unit equivalent to candela.
