The candela - (symbol: cd)

The candela (symbol: cd) is the base unit of luminous intensity. That is, luminous power per unit solid angle emitted by a point light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths). A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres. The human eye, when adapted for bright conditions, is most sensitive near this frequency. Under these conditions, photopic vision dominates the visual perception of our eyes over the scotopic vision. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye.

In simpler terms, the candela measures the brightness of light as perceived by the human eye. It's used to quantify the intensity of light emitted by various light sources, such as light bulbs, LEDs, and lasers. The candela is an important unit in various fields like lighting design, photography, and vision science, as it helps describe the amount of light emitted by a source and how it's perceived by humans.