2022/02/25The camera pixel size refers to the physical dimensions (width and height) of each individual pixel on the camera sensor, typically measured in micro-meters (μm). It is a crucial specification that influences both the sensitivity of the camera and its ability to capture fine details. However, the image pixel size (resulting from the sensor pixel size and optical system magnification) plays a more direct role in determining many imaging properties.
1. The Relationship Between Sensor Pixel Size and Image Pixel Size
Sensor Pixel Size vs. Image Pixel Size:
The sensor pixel size is the physical size of a single pixel on the sensor. However, the image pixel size is determined by the sensor pixel size divided by the system magnification.
The optical system magnification (which is determined by the optical components like lenses or microscope objectives) plays a crucial role in the effective image pixel size.
Impact of Optical Systems:
Fixed Focal Plane Systems (e.g., microscope objectives) vs. Focusable Systems (e.g., conventional camera lenses):
In fixed focal plane systems, the pixel size is directly related to the magnification, and larger pixels can collect more light, offering greater sensitivity.
In focusable systems, magnification can be adjusted by changing the distance to the subject or using zoom lenses, which in turn alters the effective image pixel size.
2. Sensitivity and Pixel Size
Larger Pixels and Sensitivity:
Larger pixels can collect more light, improving the camera's sensitivity, especially in low-light conditions.
Analogy: Larger pixels are like a bucket compared to a cup for collecting rainwater—the larger area collects more photons, which increases sensitivity.
For example, if a pixel’s size doubles in both the X and Y directions, the pixel area increases four times, meaning it can collect four times more photons.
Low-Light Imaging Benefits:
Increased pixel size significantly improves the camera's ability to capture weak light signals, reducing the required exposure time or light level.
3. Pixel Size vs. Image Resolution
Resolution Trade-Off:
Larger pixels may improve sensitivity, but they can also reduce the ability to resolve fine details.
Example: A 1 μm pixel will struggle to resolve details smaller than 2 μm, as the neighboring features will blur into each other.
The degree of pixilation increases with larger pixels, meaning finer details in the image will become indistinct.
Optical System Limitations:
Optical system resolution is also a limiting factor in resolving fine details. Each optical system has a limit beyond which reducing pixel size won't enhance resolution but will reduce sensitivity.
For microscope-objective-based systems, the Numerical Aperture (NA) primarily determines this resolution limit.
4. Matching Pixel Size to Optical System
Ideal Pixel Size for High-NA Microscopes:
A camera with a 6.5 μm pixel size is ideally matched with 60x high-NA microscope objectives.
Cameras with 10 or 11 μm pixels are best suited for 100x high-NA objectives.
Larger pixels can yield higher sensitivity, but smaller pixels do not necessarily improve detail resolution; they are more useful for capturing finer image details only if the optical system can support such resolution.
5. Conclusion: Balancing Pixel Size, Sensitivity, and Resolution
Pixel Size Trade-Off:
Larger pixels are better for low-light imaging and improving sensitivity but sacrifice the ability to resolve fine image details.
Smaller pixels improve image resolution but may reduce sensitivity, especially in low-light conditions.
System Matching:
The ideal camera pixel size depends on the optical system being used. For systems with higher magnification (like high-NA microscopes), the pixel size must be balanced with the optical system’s resolving power and the intended imaging application.
