First, let us distinguish two concepts: lens illuminance and relative illuminance.

Lens illuminance refers to the brightness at the optical axis of the lens, namely the center of the image plane, which depends on the F-number (aperture value).

The relative illuminance of a lens is defined as the ratio of the illuminance at each field position on the image plane to the illuminance at the central field of view. Affected by lens vignetting and the cosine fourth law, the peripheral field of view becomes darker than the image center.

Today we mainly introduce the key factors that affect the relative illuminance of industrial lenses:

Incident light parallel to the lens optical axis converges and forms an image at the image center. Assume the image illuminance on the optical axis is E0′.

For oblique light rays that are not parallel to the optical axis and form an object field angle ω with the optical axis, the image plane illuminance Eω′ is:Eω′=E

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