Phi (capital φ, lowercase φ) is the 2nd1Greek letter. Greek lowercase letters, the bend in the upper left corner is open; When used as a symbol, it is generally written as a reduced capital Φ shape (Unicode: U+03D5). F (capital Φ, lowercase Ф) is the 2nd1Bulgarian letter. Φ in capital letters is used for: physical magnetic flux Φ φ= SB. The unit is Weber (Wb) symbol. It is used for: the symbol of golden section of physical fluctuating phase current and voltage; Golden section number, φ = [(√ 5)- 1]/2 Mathematically, the angle between a straight line and the z axis in the three-dimensional coordinates of a complex number represents an empty set mathematically.

Φ 5 or Φ 8 in LED lamp indicates its diameter. In other words, the thickness of the lamp tube is also a necessary factor affecting the light emission.

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