1. Definition
CMS (Camera Monitor System), commonly referred to in the industry as an electronic rearview mirror, uses a camera and monitor combination to replace traditional optical rearview mirrors. In this display mode, images captured by external cameras are processed and displayed on an in-cabin screen. It can also integrate features such as blind-spot warnings and obstacle alerts.

CMS horizontal uniformity refers to the luminance consistency across different areas of the CMS monitor. Luminance inconsistency can easily lead to vignetting, where the center of the image is bright and the periphery is dark. By measuring the full-screen visual perception from a single viewing angle to ensure it falls within a reasonable range, it guarantees that there is no vignetting when the driver views the monitor straight on, thereby not affecting road condition judgment.

The non-uniformity mainly stems from two aspects:
- Imaging end factors: During imaging, the illuminance at the center of the sensor is higher than at the edges, resulting in luminance inconsistency between the center and the periphery of the image.
- Display end factors: Caused by factors such as the light-emitting units, panel structure, and manufacturing processes of the monitor.

2. Test Method
Test KPI:
According to the international standard ISO 16505: the ratio of the maximum luminance difference among the measurement points to the maximum luminance for CMS horizontal uniformity should be less than 35%. The calculation formula is as follows:
$$\frac{\max\left\{ \left( L_{j/\text{white}}(\Theta, \Phi) \right) \right\} - \min\left\{ \left( L_{j/\text{white}}(\Theta, \Phi) \right) \right\}}{\max\left\{ \left( L_{j/\text{white}}(\Theta, \Phi) \right) \right\}} < 35\%\tag{1}$$

Measurement Points:
CMS horizontal uniformity is based on the measurement results at 9 points on the monitor screen. These measurement points are positioned at various percentages based on the defined horizontal/vertical dimensions of the monitor, covering the center, four corners, and edge midpoints of the screen. During testing, the optical axis of the luminance meter should be coaxial with the normal to the screen at the measurement points.

Test Procedure:
Display a 70% gray level image on the monitor, align the optical axis of the luminance meter coaxially with the normal to the screen at the measurement points, and capture the screen after the image stabilizes. Analyze the luminance distribution at the nine points using RIQA software and calculate the luminance deviation ratio. A ratio of less than 35% indicates compliance with the standard.

Environment Setup:
Camera end: The standard has no specific requirements. Switch the test chart to a 70% gray chart. The recommended light source color temperature is 6500K±1500K, with an illuminance of 800±80 lx (if the screen is self-illuminating, there is no need to change the chart or add supplemental lighting);
Display end: The standard has no illuminance requirements. A dark environment with less than 10 lx is recommended, with the imaging luminance meter facing the monitor screen directly.

Recommended Equipment:
Automated Test Chart Switching Bracket System, Multi-CCT LED Supplemental Light Source (Visible and NIR), 70% Light Gray Test Chart, 2D Imaging Luminance Meter, RIQA-CMS Image Quality Analysis Software, Mobile Bracket for Monitor and Luminance Meter Result Analysis:
A luminance deviation ratio of less than 35% indicates compliance with the standard.