In the modern age of digital cameras and rolling shutter sensor designs the variables that affect the luminal quality of the picture have shifted from their traditional set of aperture, shutter speed, film sensitivity (ISO) into Black Level, Gains, and “exposure” (the “exposure” is in quotes because its value doesn't affect the physical blocking of light but rather how the chip reads and re-sets data values in individual sensor elements).

Below are a few sections that provide a bit of information on what effect each of the 3 types of digital settings have on the output image

Black level basically tells the sensor which level of brightness to consider “true black”. If you take a look at the brightness histogram of a given image the value of the Black Level will directly correspond to the gap at the left edge of that histogram, where there are no pixels reported to be of such low brightness value. Increasing the Black Level too high will cause the image to look washed out while using a too low of a setting causes some grey pixels to be mistakenly read as pitch black.

Remember: the brighter the pixel the lower it's read charge. So increasing the Black Level causes more pixels to be considered “brighter than black” and lowering the Black Level value causes more off-black (grey) pixels to be reported as solid black.

Gains are constant values that are applied to the analog data read from the sensor before running through an ADC and can be either additive or multiplicative.

There are 3 individual Gain settings for each of the 3 color channels (Red, Green, Blue) and those are applied to pixel data corresponding to that color. Naturally changing these values has a profound effect on the output color.

Global Gain is a Gain that is applied to every pixel. Adjusting this setting greatly affects the brightness of the picture while only minimally affecting the color balance.

The timing generator outputs clocks to access the rows of the imaging array, precharging and sampling the rows of the array sequentially. In the time between precharging and sampling a row, the charge in the pixels decrease with exposure to incident light. This is the exposure time in rolling shutter architecture.

The exposure time is controlled by adjusting the time interval between pre-charging and sampling. After the data of the pixels in the row has been sampled, it is processed through analog circuitry to correct the offset and multiply the data with corresponding Gain. Following analog processing is the ADC which outputs 10, 12, or 14 bits¹⁾ of data for each pixel in the array.

Ignoring the effects of UVC's “Brightness”, “Contrast”, and “Gamma” controls we are only concentrating on the following sensor chip level adjustments when trying to configure the settings for an optimal picture luminocity: Black level, Global Gain, Exposure.

So what do we mean when we set off to adjust the exposure? In short - we are trying to hit these vague goals:

1. No overexposed pixels²⁾
2. “Bright enough” overall picture
3. Natural progression from dark to bright

In terms of quantification of the above goals we are striving to:

1. Limit the number of overexposed pixels to less than a given percentage, say 2%
2. The global brightness average (arithmetic mean) should be in a comfortable zone around 90-100 (assuming [0..255] range for brightness)
3. Black level should be set to correctly reflect the visual perception of the scene and the image's rendering on a display

As the sensor processes input its electrical characteristics could fluctuate (due to, for example, heating up) requiring a manual human intervention to set an appropriate Black level (or an automated “Black level Calibration” procedure, if implemented by the sensor chip).