The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of…

🧠 The Fundamental Unit Check The definition of the Candela is based on a specific frequency where the human eye is most sensitive.

🗺️ The Definition Map

1. Frequency ($A$): According to the SI definition, the monochromatic radiation frequency is $540 \times 10^{12} \text{ Hz}$. So, $A = 540$.
2. Intensity ($1/B$): The radiant intensity is exactly $\mathbf{1/683} \text{ watt per steradian}$. So, $B = 683$.

⚡ The "555-683" Memory Hook $540 \text{ THz}$ corresponds to light near $555 \text{ nm}$ (Green). Memorize $555$ and $683$ together—they are the DNA of luminous intensity.

⚠️ Common Traps Don't swap the numbers! $683$ is the denominator for intensity, not the frequency.

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