If a beam of light with a uniformly distributed intensity enters an objective from the rear,
the beam is condensed to a spot of finite size.
This size is known as the spot diameter.
The approximate value of a spot diameter is calculated from the following expression:

øSpot = µm = 1,22 * λ / NA
Examples for green light (λ=0,55 µm (550nm )):

However, the above expression cannot be applied if the light source is a laser beam of which the intensity forms a Gaussian distribution on the cross section.
The diameter of a laser beam is generally indicated by 1/e*e of the peak value, i.e. 13,5% of the peak value.
The spot diameter of a laser beam is calculated from the following expression:

øµm = 4 * λ * FL / π * D