National Instruments NI Vision NI 17xx 用户手册

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This chapter contains information about acquiring images with the 
NI Smart Camera and explains the relationships between triggering, 
lighting, and exposure.

The NI Smart Camera provides control of the image sensor exposure time 
through software. The exposure time is the amount of time that light is 
allowed to strike the sensor to produce an image. When light strikes the 
surface of the sensor, it dislodges electrons. As more light strikes the 
sensor, more electrons are freed, creating a charge on the sensor.

For a given amount of light, the sensor collects more charge during a longer 
exposure time than a shorter exposure time. Because the charge is what is 
read out to produce the image, it is important to have an optimal amount of 
light and exposure time for your application.

Exposing the image sensor for too short of a time relative to the amount of 
light in the environment results in a dark, low contrast image. Exposing the 
image sensor for too long of a time relative to the amount of light in the 
environment results in a bright, low contrast image. When the image sensor 
is exposed for an appropriate amount of time relative to the light in the 
environment, acquired images will exhibit appropriate contrast to easily 
distinguish both dark and light features. Contrast is a key factor in obtaining 
good results from image processing algorithms.

In applications where the object under inspection is moving, the exposure 
time must be carefully considered. If the object moves significantly during 
the exposure, the resulting image is blurry and unsuitable for processing.

The maximum exposure time for imaging a moving object without blurring 
depends on the per pixel spatial resolution and the rate of motion of the 
object. The per pixel spatial resolution is the field of view, calculated in 
the

 section of Chapter 3, 

divided by the number of pixels in the sensor. Together, this information