Global exposure and roller shutter exposure of industrial cameras

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What is a shutter

Global exposure and roller shutter exposure are common camera exposure methods . Generally speaking ,CCD The camera is global exposure , and CMOS The camera has roller shutter exposure . that , Which of these two ways is better or worse ？ Or say , What is the difference between them ？ that , Let's start with the definition of both .

The shutter is a component used by the camera to control the effective exposure time of the photosensitive film , It is an important part of the camera , Shutter structure / Form and function are important factors that determine the grade of camera . Generally speaking , The larger the time range of the shutter, the better . Low seconds are suitable for shooting moving objects . When we shoot the traffic at night , When light painting , The shutter time will be lengthened . Empathy , The common silky water in the photos can only be photographed with a slow shutter .

Roller shutter and global shutter

Rolling shutter , Usually used for CMOS The sensor is exposed line by line . At the beginning of the exposure , The sensor scans line by line for exposure , Until all pixels are exposed . All actions are completed in a very short time . The exposure time of different row pixels is different .

Global shutter is achieved by exposing the whole scene at the same time . sensor （Sensor） All pixels collect light at the same time , Exposure at the same time . At the beginning of the exposure , The sensor starts to collect light . At the end of the exposure , The light collection circuit is cut off . Then the sensor reads it out as a picture .CCD Is to use the global shutter mode , All pixels are exposed at the same time .

For camera manufacturers , Roller shutter can achieve higher frame rate , But when the exposure is wrong or the object moves faster , There is a deviation between what the shutter records and what we see .

Generally speaking ,CCD Sensor multi-purpose global shutter ,CMOS Sensor multi-purpose shutter . Manufacturers need to consider many factors when choosing a certain shutter speed for sensors , Such as ： Processing speed , Battery consumption , Manufacturing costs and overall complexity . So , It's not a problem for static photography , But when shooting video, it is important to choose which shutter , Especially when shooting high-speed objects .

Why does this happen

For most cameras , Shutter “ Rolling curtains ” The velocity is 1/30 second , The motion changes of most objects are not obvious . But when we shoot high-speed moving objects , Such as the propeller of an airplane ,“ The shooting effect ” It will be very obvious .

But shooting high-speed objects is not the only problem . But we are sitting in a high-speed car , Shoot outside through the window , There will also be a shutter effect . Now we simulate a piece 10️10 The effect of the sensor .

That explains , We sat on the high-speed rail and photographed why the trees outside were a little inclined .

Interesting effect

In fact, roller shutter is not a “ shortcoming ”. Although in most cases we don't want to see “ Roller shutter effect ”, However, we can still use it to capture some interesting pictures .

Global exposure

The way of global exposure is relatively simple . That is to say, after the aperture is opened , The whole image chip is exposed at the same time . therefore , The exposure time is related to the switching speed of the machine . Since it is related to mechanical motion , therefore , There is a theoretical minimum exposure time

advantage ： All pixels are exposed at the same time

shortcoming ： Exposure time is limited , Minimum exposure time with mechanical limit

Roller shutter exposure

seeing the name of a thing one thinks of its function , The way of exposure of roller shutter may be related to the concept of roller shutter . This exposure method is when the aperture is opened , There are also roller shutters with certain intervals to control the exposure time of the sensor . Be careful , As shown in the figure below , The way of rolling the curtain is from left to right . therefore , The length of exposure time depends entirely on the opening size of the roller shutter and the movement speed of the roller shutter . in other words , The faster the shutter moves , The smaller the roller shutter spacing , The smaller the exposure time of the sensor . therefore , The roller shutter exposure method can have a smaller exposure time .

Its rolling shutter movement mode is shown in the figure below ： The roller shutter is in motion , The sensor of its roller shutter opening can receive light .

A certain position during the movement of the roller shutter

The last position of the roller shutter in the moving process

advantage ： With smaller exposure time

shortcoming ： Line by line exposure , There is a drag , Not suitable for shooting moving objects

Drag analysis

Professional description ： The smear is at the time of exposure , There is relative motion between the shooting target and the camera system , Because of this Relative motion causes the image formed on the chip to change all the time , The pixels of each part are affected by the imaging from different positions of the object in the process of exposure , The resulting picture is a superposition of pictures in a continuously changing image space .

About the exposure time , In fact, the camera image acquisition process includes two parts , The first part is exposure (exposure), The second part is after the exposure , Read data from the register of the sensor and transmit it (Readout). When the camera is 1 Shoot fast moving objects at the position of the No A when , If the exposure time of the camera is too long （ That is to collect 1 The time of dot image is too long ）, At this time, the object A But displacement has occurred , Therefore, the acquired image is not completely 1 Image at point , At this time, the acquired image is acquired during the movement ( Slightly deviate 1 Dot image and 1 Superposition of point images ), The resulting drag .

The longer the exposure time , The slower the shooting speed , But the shorter the exposure time , The amount of light entering will become less , At this time, you need to increase the aperture , Increase light , To ensure the brightness of the image . therefore , For shooting moving objects , The exposure time should be set reasonably , The exposure time should not be set too large blindly due to the need of image brightness effect , Otherwise, it is easy to produce smear , The image brightness caused by small exposure time is poor , At this time, it should be adjusted by enlarging the aperture and increasing the illumination .

Line by line exposure

  chart 1 Progressive exposure mode

Line by line exposure sensor The implementation is as shown in the figure 1 The progressive exposure mode is shown . Unlike global exposure , Line by line exposure starts from the first line , The second line is exposed after one line cycle . By analogy , after N-1 After the line N The line begins to be exposed . After the exposure of the first line is completed, read out the data , Reading a line requires a line cycle time （ Including line blanking time ）. After the first line is completely read , The second line just begins to read , By analogy , When the first N-1 After reading the line , The first N Line starts reading , Until the whole image is completely read . Exposed line by line sensor The technical difficulty is more than global exposure sensor low , Cheapness , And the resolution is large , For some still image shooting is a good choice .

Global exposure

chart 2  Global exposure mode

Global exposure sensor The implementation is as shown in the figure 2 Shown , Sensor All lines of begin to be exposed at the same time , And end the exposure at the same time , After the exposure ,Sensor Transfer all electrons from the photosensitive area to the storage area , Then read out the pixel data line by line . The advantage of this exposure is that the exposure time of each line of the image is relatively consistent , And when shooting moving objects, the image will not appear offset and skew .

notes ： The change of exposure time of electronic shutter , Just changed CCD The induction time of photosensitive pixel to external light , It doesn't change CCD Internal video readout cycle .

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