The Luminous Intensity Units calls candela , that is cd.
It is desfined as the solid angle of the emitted light in the direction of the light intensity among Light source in a given direction space. but the Luminous intensity only make sense for point light source or the small compariation occasions between the size of the ojbect and the iradiation distance. This indicates the capabilities of the lminous object convergency in space. We can say that the luminous intensity degree is described as how much bright it is.
because it is a common decription of Optical power and convergency capabilities. The larger the light emission intensity is , the brighter the light source appears ,while under the same conditions the object look more bright by light. Thefefore, in earlier dates, the flashlight are using this parameter.
Now The LED units are also used to describe , for example, a LED is 15,000 , the unit is mcd,
1000mcd = 1cd, therefore 15000mcd is 15cd. the reason why we use LED Milli cd(mcd) rather than directly with cd to represent is that the earliest first LED is very dark. For example, for 1984 year’s 5mm LED standard, the luminous intensity was 0.005cd. therefore, we always use mcd as units. Now LED is very Powerful, but the industry still follow the original rules.
However, the luminous intensity by means of “brightness” disavantage is that if two identical die LED, the converging is good, then the luminous intensity is very high. Thefefore, When buying LED’s, Do not blindly pursue the High I value, but also consider the irradiation angle. Many LED with High I value is not achieved by improving their emission efficiency, but achieving by making the mirror head longer and irradiation angle narrows, which although useful for led flashlight, but the viewing angle is limited. Furthermore, Same Die LED, the I value of diameter 5mm is twice as larger than 3mm’s, but only is a 1/4 of diameter of 10mm,since the larger the lense get, the better converging charateristics get.