I. 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 darker, 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.

II. 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 edges 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 edges, ultimately resulting in higher illuminance at the center of the sensor than at the edges.

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 cast.

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 captured image.
III. Uniformity Testing

3.2 Test Equipment:
LIS-CCR150-220 Multi-CCT Integrating Sphere Light Source, MIG-S2 Image Capture Card, lifting table, fixture, image capture software, and RIQA.

The LIS-CCR150-220 Multi-CCT Integrating Sphere Light Source combines the high brightness, long lifespan, and high energy efficiency of LEDs with the high uniformity of an integrating sphere. It supports precise color temperature adjustment from 2300K to 10000K and a wide illuminance adjustment range of 50-10,000 lux, making it suitable for luminance uniformity and color uniformity testing of wide-angle and fisheye cameras. The infrared version supports 850nm and 940nm bands, which can be turned on individually or mixed, specifically designed for infrared camera testing.

3.3 Test Procedure:

1. Environment Setup: In a dark environment, set the light source color temperature to 6500K. The illuminance must be adjusted to ensure that the luminance of the G channel in the central area is close to 80% of saturation.

2. Sample Image Capture: Place the DUT in the COC fixture, view the live feed, and adjust the optical axis of the module to be perpendicular to the light-emitting surface so that the light-emitting surface fills the entire frame. Once the image stabilizes, capture and save the image.

3. RIQA Analysis:

1. Open the RIQA software. Select the Uniformity module for luminance uniformity and the Uniformity(Color) module for color uniformity. Here, the RIQA operation for luminance uniformity is used as an example.

2. Click the “Add +” button to import the image to be tested, click the “Start” button to begin testing, adjust the ROI area, and click the “Analyze” button to start the analysis.

3. The center of the software displays plots such as the luminance map of the test sample and measured data for multiple metrics. Right-click the image in the upper right corner to view the ROI selection. Click the “Generate Report” button.

4. You will be redirected to the report generation interface, where you need to select the output file format, enter the file name and file path, and click the “Export” button. A prompt will indicate successful report generation once the report is successfully generated.

4. Result Interpretation:
Luminance Uniformity:
The sampled maximum luminance and sampled minimum luminance are the maximum and minimum values among all sampled blocks; the center luminance is the luminance of the center block; the minimum edge luminance is the minimum value among the eight sampled edge blocks.
Uniformity (Sampled) is the ratio of the minimum value to the maximum value among the multiple sampled blocks.
Uniformity (Edge) is the ratio of the minimum value among the eight edge blocks to the maximum value among the multiple sampled blocks.
Uniformity (lin) is the ratio of the difference between the maximum and minimum luminance values of the multiple sampled blocks to the maximum value.

Color Uniformity:
The test report displays the channel data for each sampled block of RGBY, as well as the color channel ratios and the ratios between different sampled areas.

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