The dynamic range of a USB camera module is an important indicator for measuring its ability to capture light intensity intervals, directly affecting the imaging quality under different lighting conditions. The following is a detailed analysis:
The definition of dynamic range
Dynamic Range refers to the interval of light intensity that a camera can capture, usually expressed as the ratio of the maximum measurable light intensity to the minimum measurable light intensity, with the unit expressed in decibels (dB). It reflects the camera’s ability to capture extreme light and dark differences, Això és, to simultaneously record the brightness ratio between the brightest area (highlights) and the darkest area (shadows). The larger the dynamic range, the better the camera performs under different lighting conditions, and it can handle high-contrast scenes better, thereby improving video quality.
The influence of dynamic range on image quality
High dynamic range scene performance: In scenes with a large contrast between light and dark or backlit conditions, a camera with a small dynamic range will overexpose the bright areas of the entire image and underexpose the darker areas, making it difficult to see the brightest and darkest parts of the image clearly. Per exemple, in scenes such as bank savings offices and entrances and exits of important places, the strong light shining through the window and the soft light from the fluorescent lamps on the ceiling may both cause difficulties in capturing the indoor and outdoor scenes at that time. It is impossible to clearly shoot the indoor and outdoor scenes with a large contrast simultaneously, and the captured images will have overly bright backgrounds and overly dark backgrounds. Or in cases where the background is clear but the foreground is too dark, or the foreground is suitable but the background is too bright.
Image details and layers: The larger the dynamic range, the richer the layers that can be expressed in the image, and the broader the color space it contains. When high-brightness areas under strong light sources (such as sunlight, lamps or reflections) and relatively low-brightness areas like shadows and backlights coexist in an image, a camera with a high dynamic range can better preserve the details and layers of the image.
Factors affecting the dynamic range
Image sensor: The dynamic range of the image sensor is one of the key factors determining the dynamic range of the camera. The dynamic range of an image sensor is defined as the ratio of the saturation charge Qs to the minimum detectable charge Qd, which is limited by factors such as the sensitivity of the photosensitive device, the full well capacity, and the noise level. En general parlant, the larger the pixel area and fill ratio, the higher the sensitivity of the CCD, the higher the potential well capacity, and the larger the dynamic range. However, under the condition that the sensor area is fixed, increasing the number of pixels is contradictory to increasing the dynamic range. Per aquesta raó, without reducing the number of pixels in the image sensor, increasing the dynamic range requires increasing the area of the image sensor, which will increase the cost and may also affect the structure of the imaging system.
Image bit depth: Currently, due to the limitations of software and hardware, the digital image processing part of ordinary output/display devices is mostly based on 8-bit, which can only represent 256 gray levels, resulting in a significant difference between the brightness level of the image and that of the real scene. If the dynamic range of image sensors is different, the actual brightness difference reflected by the 256 levels of gray scale they provide is also different. That is, the larger the dynamic range, the better it can display the brightness changes in nature.
Techniques for enhancing the dynamic range
Hardware expansion
Altering the optical system structure or adding other components: By changing the optical system structure or adding other components, the received light energy of the image sensor is altered, and the dynamic range is expanded through post-processing. If a beam splitting prism is used to form images of the incident light on different target surfaces, sensors with different sensitivities are adopted for reception or different circuit gains are set, and then different images obtained from multiple sensors are used to generate a high dynamic range image. However, expanding the dynamic range of the system from a hardware perspective poses extremely high technical difficulties. Currently, only a few CCD/CMOS manufacturers are conducting research, and there is no mature and reliable solution yet.
Logarithmic response technology: By changing the signal output curve of pixels, it delays the time required for pixels to reach saturation, thereby achieving the purpose of increasing the charge capacity of pixels and expanding the dynamic range.
Software algorithm
Multiple exposure technology: Without improving the pixel photodiode, traditional low dynamic range image sensors can obtain high dynamic range images by synthesizing multiple exposure images.
High Dynamic Range Imaging (HDR) technology: In the field of digital imaging and digital photography, HDR technology is a set of techniques used to achieve a wider exposure range and higher dynamic range (i.e., greater light and dark differences) than ordinary digital image technology, which can better reflect the visual effects in the real environment. The problem that HDR technology aims to solve is to represent a wide brightness range within a limited brightness range.