Why Measure Sharpness in CMS?

As an electronic replacement for traditional optical rearview mirrors, the CMS (Camera Monitor System) provides the driver with an exterior view through a combination of external cameras and in-cabin displays.

Image sharpness directly determines the CMS's ability to render details, which is critical for driving safety. It directly affects whether the driver can clearly perceive the details of approaching vehicles from behind displayed on the monitor (e.g., headlight outlines, license plates, pedestrians, road signs, etc.).
Comparison of sharpness differences (Left: lower sharpness; Right: higher sharpness)

If the sharpness does not meet the requirements, the image will appear blurry, increasing driving risks. Therefore, CMS regulatory standards such as ISO 16505, UN-R46, and GB 15084-2022 all have explicit requirements for CMS sharpness.

Quantitative Metrics

CMS sharpness testing primarily uses the Modulation Transfer Function (MTF) for measurement. MTF50 is a widely used quantitative metric, representing the spatial frequency at which the MTF drops to 50%. A higher MTF50 value indicates a stronger ability of the system to transfer details and a sharper image.

According to the GB 15084 regulatory standard, CMS sharpness must meet the following requirements:
The horizontal and vertical sharpness at the center of the image shall comply with the following formula:
$$\text{MTF}50_{\langle 1:1 \rangle} \geq \frac{1}{2}\text{MTF}10_{\text{MIN}\langle 1:1 \rangle} \left[ \frac{\text{LW}}{\text{PH}} \right]$$

The horizontal and vertical sharpness at the four corners (at 70% field of view) shall comply with the following formula:
$$\text{MTF}50_{\langle 1:1 \rangle} \geq \frac{1}{2} \times \left( \frac{1}{2} \text{MTF}10_{\text{MIN}\langle 1:1 \rangle} \right) \left[ \frac{\text{LW}}{\text{PH}} \right]$$

Where:
$MTF50_{(1:1)}$ is the distinguishable spatial frequency corresponding to when the MTF drops to 50% for a monitor with a 1:1 aspect ratio;
$\text{MTF}10_{(1:1)} \left[ \frac{\text{LW}}{\text{PH}} \right]$ is the distinguishable spatial frequency corresponding to when the MTF drops to 10% within the minimum specified field of view for a monitor with a defined 1:1 aspect ratio;

Key Tools
CMS sharpness is measured by obtaining the Spatial Frequency Response (SFR) from a slanted edge chart to calculate the $MTF10_{(1:1)}$ value. This requires a slanted edge test chart and professional image quality analysis software.

Other recommended tools: Automated Chart Switching Mount Test System, Multi-CCT LED Supplemental Light Source (Visible and NIR), SFR Resolution Test Chart, 2D Imaging Luminance Meter, RIQA-CMS Image Quality Analysis Software, Monitor and Luminance Meter Mobile Stand

Test Procedure
(1) Environment Setup
Camera side: Light source CCT 6500K±1500K, illuminance 800±80 lx, camera facing the chart directly at a distance of 6 m;
Monitor side: Dark environment of 0-10 lx, imaging luminance meter facing the screen directly.

(2) Image Capture
Camera side:
1. Adjust the camera so that its optical axis is perpendicular to the chart;
2. Adjust the distance between the camera and the sharpness chart to 6 meters (if the camera's focus distance is less than 6 meters, measure at the actual focal distance);
3. After capturing the pattern at the center position, adjust the position and angle so that the slanted edge is located at 70% image height with an angle of approximately 5°, and capture the image again.
Monitor side:
1. Adjust the monitor and the reference camera so that the reference camera is perpendicular to the monitor;
2. Capture using the 2D imaging luminance meter.

(3) Analysis and Interpretation
Use professional analysis software (here taking RIQA Image Quality Analysis Software as an example) to automatically analyze the slanted edges of the black squares on the chart, and calculate the horizontal and vertical $MTF10_{(1:1)}$ values for the center and the four corners, respectively.

Software operation:
Here, the center position is taken as an example. The analysis for the four corner positions follows the same procedure by selecting the corresponding test images.

Result interpretation:
In the figure above, the measured horizontal and vertical $MTF10_{(1:1)}$ values at the center both satisfy $\geq \frac{1}{2}\text{MTF}10_{\text{MIN}\langle 1:1 \rangle} \left[ \frac{\text{LW}}{\text{PH}} \right]$. Therefore, the sharpness of this CMS meets the regulatory standard requirements.