It is often found that some enthusiasts are vague about some basic concepts of photometry, such as what is brightness? What is the unit for measuring brightness? How to measure brightness?
This article will try to answer these questions as much as possible without rigorousness. Therefore, the paper lists a large number of examples, and gives the relationship between four quantities and the conversion algorithm, which is not found in most optical books. of.
There are four commonly used quantities related to photometry and light: luminous intensity, luminous flux, illuminance, and brightness. Although these four quantities are related, they are different and cannot be mixed. Just like pressure, gravity, pressure, and quality are different physical quantities.
1. Luminous intensity (I, Intensity), unit candela, ie cd.
Definition: The luminous flux emitted by a light source in a unit solid angle in a given direction is defined as the light intensity (degrees) of the light source in that direction,
Explanation: The luminous intensity is for the point source, or the size of the illuminant is smaller than the irradiation distance. This amount is indicative of the ability of the illuminator to converge in space. It can be said that the luminous intensity describes how much the light source is "bright" because it is a common description of the optical power and convergence ability. The greater the luminous intensity, the brighter the light source appears, and the brighter the object is illuminated by the light source under the same conditions. Therefore, this parameter was used earlier for the description of the flashlight.
Now LED is also described by this unit, for example, an LED is 15000, the unit is mcd, 1000mcd=1cd, so 15000mcd is 15cd.
The reason why LED is expressed in millicd (mcd) instead of cd is because the earliest LED is darker. For example, in 1984, the standard 5mm LED has a luminous intensity of 0.005cd, so it is represented by mcd. Now LED is very powerful. But still follow the original statement.