1. Concept of Uniformity

Uniformity encompasses image luminance uniformity and image color uniformity.

Luminance uniformity refers to the consistency of luminance distribution from the center to the peripheral edges of an image. Poor luminance uniformity results in inconsistent luminance from the center to the edges of the image, as shown in Figure 1, where the center appears bright while the periphery is dark, indicating a luminance fall-off issue.

Color uniformity refers to the consistency of color reproduction from the center to the peripheral edges of an image. Poor color uniformity results in color deviations between the center and the edges of the image, such as reddish, yellowish, or bluish casts. As shown in Figure 2, the center appears reddish while the periphery appears greenish, indicating a color shift issue.

Figure 1	Figure 2

2. Analysis of Causes for Poor Uniformity

2.1 Causes of Poor Luminance Uniformity:
Treating the lens as an aperture, the luminous flux at the center is greater than at the edges, resulting in higher illuminance at the center of the sensor compared to the edges.

As shown in the figure, assuming the luminous object is a Lambertian radiator (or cosine radiator), light propagates from the center and the edge of the lens respectively. From the center to the edge, the angle θ in the figure, i.e., the chief ray angle, increases. The solid angle of the lens entrance pupil with respect to the luminous point and the luminous intensity of the luminous point decrease as the angle θ increases. This causes the luminous flux at the center to be greater than that at the edge, ultimately resulting in higher illuminance at the center of the sensor than at the edge.

2.2 Causes of Poor Color Uniformity:
1. When the lens CRA is significantly larger than the sensor CRA, light refracts onto adjacent pixels, causing crosstalk between pixels and resulting in color casts.

2. Thin-film interference infrared cut-off filters experience a shift in their spectral curves due to changes in the chief ray angle, thereby causing color casts in the image.