I.Definition
Depth of Field (DoF) of a CMS (Camera–Monitor System) refers to the object-distance range behind the vehicle over which targets can be sharply imaged.

In optical imaging, object points outside the focal plane produce defocused blur on the image sensor, typically described by the circle of confusion (CoC). When the CoC exceeds an acceptable threshold, the resulting blur surpasses the critical limit for clear recognition, making reliable target identification impossible.

Fig. 1: Optical diagram of circle of confusion formation, showing how out - of - focus points create blur on the image plane.

Conventional photography sets the allowable CoC based on human visual criteria. In contrast, the CMS industry adopts MTF10 as an objective quantitative criterion. It converts the boundary of perceived image clarity into a computable metric, allowing the resolution at different object distances across the entire depth range to be quantitatively evaluated, thereby verifying whether the system complies with vehicle-mounted regulatory requirements.

Fig. 2: Real - world CMS display comparison at 4m and 10m, highlighting resolution degradation with distance.

II.Measurement Requirements
Per regulatory standard ISO 16505, Camera-Monitor Systems (CMS) shall meet specified \(MTF10_{(1:1)}\) resolution thresholds at different object distances, to ensure drivers can clearly identify rear surrounding targets.

• For Target Point 1 (10 m, simulating infinity) and Target Point 2 (6 m, representing mid-range object distance):
  

$$MTF10_{(1:1)} \geq 0.9^*MTF10_{MIN(1:1)} \left[ \text{LW/PH} \right]\tag{1}$$

• For Target Point 3 (4 m, representing near-range object distance):
  

$$MTF10_{(1:1)} \geq \frac{1}{2} MTF10_{MIN(1:1)} \left[ \text{LW/PH} \right]]\tag{2}$$

where:

• $MTF10_{(1:1)}$ is the spatial frequency where the modulation drops to 10% under 1:1 native display conditions.
  

Test Method:
Environment Setup:
Actual Scene:
Camera side: Light source color temperature 6500K±1500K, illuminance 800±80 lx;
Monitor side: Dark environment with 0-10 lx.

Main Equipment Used:
Automated Chart Switching Bracket Test System, Multi-CCT LED Fill Light Source (Visible and NIR), Color Reproduction Test Chart, 2D Imaging Luminance Meter, RIQA-CMS Image Quality Analysis Software, Mobile Bracket for Monitor and Luminance Meter

Operation Procedure:
Camera side:

1. Switch the chart to the depth of field chart;
2. Turn on the fill light source, adjust the light brightness, and set the color temperature to 6500K;  
3. Adjust the camera so that its optical axis is perpendicular to the chart and the chart is centered in the image; the camera can be adjusted to capture resolution in different directions;
4. Adjust the distance between the camera and the chart, and capture images at 4m, 6m, and 10m respectively;
(For wide-angle mirrors replacing a radius of curvature r≤400mm, the measurement at 10m can be omitted; if the focal length is less than 6m, the distance d from the camera entrance pupil to the chart should be less than or equal to 6m);

Monitor side:

1. The capture lens of the imaging luminance meter is perpendicular to the monitor;
2. Turn off other light sources to ensure the ambient luminance on the monitor side is below 10 lx;
3. Capture images using the 2D imaging luminance meter.

Analysis and Interpretation:
Use RIQA Software (an image quality software independently developed by Yanding) to analyze the test samples.
The depth of field results require determining whether the $MTF10$ meets regulatory requirements. MTF10 (P*K) is calculated based on the visually determined position of blurred line pairs (P value) combined with the coefficient K. As long as this value meets the standard, the sample complies with regulatory requirements.