2022/04/29Photo-Response Non-Uniformity (PRNU) represents the uniformity of a camera’s response to light, and is particularly important in high-light applications. PRNU quantifies variations in the gain—the ratio of detected photo-electrons to the corresponding digital greyscale value (ADU)—across the pixels in a camera sensor.
1. PRNU in Imaging
1) Basic Concept of PRNU
When a camera detects light, each pixel captures a certain number of photo-electrons during exposure, which is then converted into a digital greyscale value (ADU) by the analogue-to-digital converter (ADC). This conversion follows a ratio of ADU per electron, known as the conversion gain, and a fixed offset value (typically around 100 ADU). These values are determined by the ADC and the amplifier used for the conversion.
CMOS Cameras achieve high-speed and low-noise performance by operating in parallel, with one or more ADCs per column and one amplifier per pixel. This parallel operation allows for rapid data processing but can introduce small variations in gain and offset values across pixels.
2) PRNU and Fixed Pattern Noise
The PRNU represents any variations in the gain (the ratio of electrons to ADU) from one pixel to another. While the dark signal non-uniformity (DSNU) represents variations in the offset value (resulting in fixed pattern noise in low-light), PRNU accounts for variations in the gain itself. These variations in gain can also cause fixed pattern noise, but it is typically more noticeable at high-light levels.
2. PRNU and Image Quality
1) PRNU and Low-Light Imaging
For low- and medium-light imaging (where the photon signal is relatively weak, typically in the 1000e- or less range), the PRNU variation is generally insignificant compared to other noise sources, such as read noise or dark signal noise (DSNU). In these scenarios, the variation in PRNU has little impact on the overall image quality.
2) PRNU and High-Light Imaging
In high-light imaging applications, the impact of PRNU on image quality is also minimal, especially when the light intensity is high enough that other sources of noise, such as photon shot noise, dominate the image quality. However, in applications requiring high measurement precision (e.g., frame-averaging or frame-summing techniques), low PRNU can be beneficial to ensure greater accuracy and reduce subtle inconsistencies in light response across the sensor.
3. Typical PRNU Values
Typical PRNU values for most CMOS sensors are typically less than 1%. For imaging applications in low- and medium-light conditions, the variation in PRNU is usually too small to affect the image significantly.
4. PRNU in Practical Applications
PRNU is an important factor in applications where measurement precision is critical, such as:
Scientific imaging: Where small variations in light levels can affect the results.
Industrial inspections: Where precise measurements are required, particularly in semiconductor inspection or quality control. The article "Why DSNU/PRNU Correction Matters in Semiconductor Inspection" explains the significance of PRNU in practical applications.
In these cases, ensuring a low PRNU (less than 1%) can help to improve quantitative accuracy and reduce fixed pattern noise, especially when using techniques like frame averaging or summed exposures to enhance signal quality.
