The disadvantage of using "luminance" to indicate "brightness" is that if the two LEDs have exactly the same die, the degree of convergence is good. Therefore, when buying LEDs, do not blindly pursue high I values, but also look at the angle of illumination. Many high-value LEDs are not achieved by improving their own emission efficiency, but by narrowing the lens's longer illumination angle. Although useful for LED flashlights, the observable angle is also limited. In addition, the same die LED, the I value of 5mm in diameter is more than double the size of 3mm, but only 1/4 of the diameter of 10mm, because the larger the lens, the better the convergence characteristics. The reason why the flashlight is used to indicate the flashlight or the LED is because the illuminance to the illuminated ground at the same distance is proportional to this. In particular, lx at a distance of 1 m is the cd value. However, in many cases, we need a larger area of illumination, so the ability to use only the intensity of the luminescence does not fully reflect the ability of the flashlight. For example, the same barrel, with a big head (large reflector), the I value increases a lot. Therefore, in many cases we use the luminous flux (unit lumens, see below) to indicate the flashlight. Above we say "bright" and "brightness" with quotes, because this is the brightness we routinely say, not the brightness in the strict sense of photometry, this unit will expand later.
Common light source luminous intensity (cd):
Highlight flashlight, 10000
5mm super bright LED, 15