Light intensity is described by the unit candela, abbreviated as cd. The luminous flux emitted by the light source at a unit solid angle in a given direction is defined as the light intensity of the light source in this direction. The light intensity is specific to the point light source or the situations where the illuminant size is relatively smaller than the irradiation distance.
This parameter shows the convergence of the illuminant in spatial emission. So to speak, the light intensity describes how “bright” a light source can be, because it can describe both the luminous power and the convergence ability.
The higher the light intensity, the brighter the light source will be. Under the same conditions, the object illuminated by the light source with higher light intensity will be brighter. Hence, flashlights often used this parameter in earlier days.
Now the LED light also uses this unit. For instance, a LED is 15000 mcd. 1000 mcd is equal to 1 cd, so 15000 mcd is 15 cd.The LED uses mcd instead of cd 檯燈 as the unit because the earliest LEDs were quite dim. For example, in 1984, the light intensity of a standard 5mm LED was only 0.005cd, which made mcd a much more appropriate measurement.
One disadvantage of using light intensity to describe the “brightness” is that if two LEDs have the same die, the one with better convergence ability would have higher light intensity.
Therefore, the users should not merely pay attention to high intensity value, but also notice the irradiation angle. Many LEDs don’t realize high intensity values by increasing their own emission efficiency, but by lengthening the shot and narrowing the irradiation angle. Though this is applicable to LED flashlights, the viewing angle will be limited.
Additionally, with the same die, the intensity value of the 5mm LED is over two times higher than the 3 mm one, but only one fourth of the 10 mm one. The reason is that the larger the shot, the better the convergence performance.