Understanding How Advanced Shutter Settings Improve Low-Light Performance

The camera dome supports the ability to view color images from extremely low-light situations. This feature is called Open Shutter and is only in effect during low-light situations where an image would not be obtainable otherwise. It does not affect the camera performance in normal or bright light situations.

When the Open Shutter is enabled and the scene illumination is too low to obtain a clear image at the normal video rate, the camera collects luminance information from multiple fields. As it does so, the current video information is retransmitted until new information is available from the camera. Under these conditions, moving objects will appear blurred, and still images may appear blurred, choppy, and with more static than video obtained under normal lighting conditions.

The Shutter Limit value sets the video update time in fractions of a second. Depending on the lighting conditions, the video information may be updated more frequently, but no slower than the limit set.

Figure 3–1illustrates a Shutter Limit of 1/4.

Figure 3–1: Graphical view of Shutter Limit settings

Red Light

Red Light

Red Light

Green Light

250

250

 

180

msec

msec

 

msec

In this example, the dome receives information about the color of the traffic light. While the light is red, the image is relatively static. With the shutter limit set to 1/4 second (250 milliseconds), updated red light information is transmitted at 1/4-second intervals. When the light changes from red to green, updated light color information becomes available. The dome must now transmit information about the green light. This update occurs as soon as the green light information is available. This may occur before the 1/4-second interval elapses. In this example, the light changed to green after 180- milliseconds. Thereafter the green light information is transmitted at 1/4-second intervals until new light color information becomes available.

If the light were to change from red to green halfway through the field integration interval (125 milliseconds), it may appear that both the red and green lights are lit simultaneously. This situation is illustrated in Figure 3–2.

Figure 3–2: Mixed field integration

Red Light

Red Light

Red/Green Light

Green Light

250

125

 

125

msec

msec

 

msec

Adjusting the Shutter Limit sets the update time used to maintain the image quality. If you want to videotape an incident in low-light conditions, you may find that tape quality is not acceptable. To ensure that the videotape quality is acceptable for possible prosecution purposes, you may want to test the Shutter Limit settings under the expected lighting conditions.

3–4

SpeedDome Ultra VII Operator's Manual