Paraxial Optics | Categories | OptoWiki Knowledge Base

is a mapping invariant of paraxial optics, given by the product

$\lambda := n \cdot \sigma \cdot y$

where n is the refraction index, $\sigma$ is the aperture angle, and y is the object height.

$\lambda$ is also called “etendue”

This value doesn’t change, if the object side values are replaced by the corresponding image side values:

$\lambda = n \cdot \sigma \cdot y = n' \cdot \sigma' \cdot y'$

From this we get the paraxial Magnification:

$\beta' = \frac{y'}{y} = \frac{n \cdot \sigma}{ n' \cdot \sigma'}$

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