相机捕获中可检测到的光强度

介绍

相机与我们的眼睛非常相似，因为它将光子流转换为包含信息的图像。相机中的成像传感器包含一个像素网格。其中每个像素都对在读取阶段或曝光过程中接收的光子进行计数，并输出强度分数。

相机的动态范围定义了从黑色到白色的最小和最大可测量光强度之间的比率，是摄影中的一个基本属性。

信噪比解释

Light Intensity in Imaging Sensors

For a single image acquisition, a camera sensor measures light intensity for each pixel within a specific range. Typical sensors can detect between 256 and 4096 light intensity levels, corresponding to 8 to 12 bits of data per pixel.

Signal and Noise in Light Measurement

For illustration, let us consider the figure below which shows a typical signal with a sine-wave shape, growing amplitude, combined with thermal noise.

信噪比解释

The intensity measurement process works as follows:

  • Noise Floor (Left): The lowest measurable intensity is limited by noise, such as read and quantization noise. Light intensity must exceed this noise floor to be detected.

  • Usable Signal Range (Middle): Region with low and high SNR is the range where the signal is strong enough to measure accurately and distinguish intensity levels.

  • Saturation (Right): When light intensity is too high, the pixel becomes saturated, causing clipping and loss of information.

The full range, from the lowest detectable intensity to the highest measurable intensity, is the sensor’s dynamic range (DR). The ratio of useful signal to noise is called the signal-to-noise ratio (SNR).

The graph below shows the relationship between signal level, noise, and light intensity.

信号幅度、噪声和光强度

Clipping and High-Contrast Scenes

The imaging sensor often cannot capture the complete dynamic range of the scene, resulting in an optical phenomenon called clipping. Clipping is when the intensity in a particular area falls outside the minimum and maximum intensity that the sensor can represent. Such loss of detail occurs in high contrast scenes, where both very bright and very dark are present.

剪裁高光和阴影

Adjusting Exposure

A camera sensor’s upper and lower readout limits are roughly fixed in terms of photon count However, adjusting exposure changes the the number of photons entering the camera, hitting the pixel, and getting read

  • Increasing exposure: More photons hit the pixel, improving the readout of dark objects.

  • Decreasing exposure: Fewer photons hit the pixel, preventing saturation for bright objects.

Adjusting exposure ensures that the sensor operates in its optimal range, providing a good signal-to-noise ratio (SNR) for the given region of interest. This helps achieve balanced intensity levels for both bright and dark areas of the scene.

进一步阅读

要了解有关曝光的更多信息，我们建议您阅读我们的下一篇文章 Stops（曝光级数）简介。