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Illumination Spot Size Calculator

Determine the illuminated spot size resulting from a collimated light source passing through a singlet based on the input beam diameter, the lens focal length, and the propagation distance. Calculator assumes a thin lens and uses the paraxial approximation. The calculator works for three possible scenarios: f > 0 and the propagation distance > f, f > 0 and the propagation distance < f, and f < 0. The larger the difference between propagation distance and focal length the greater the spot size will be.

Geometrical Spot Size Calculator

Spot Size (mm) : --     

db (mm):


f (mm):


x (mm):

Note: if f<0, be sure type in a negative sign into a calculator.

Equations and corresponding legend

$$ \text{when} \, f > 0 \, \, \text{and} \, \, x > f: \, \, \text{Spot Size} = \frac{d_b}{f} \times \left( x - f \right) $$
Spot Size Represents the resulting geometrical spot size after the beam has exited the lens
db Represents the diameter of the beam that enters the lens (units are in mm)
f Represents the focal length of the lens
x Represents the propagation distance from the output side of the lens to the illuminated spot

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