About Video Gain, Shutter Speed, And Spectral Response; Figure C-1 Video Gain Equation - Honeywell ACUIX User Manual

Understanding Automatic Exposure and NightShot

About Video Gain, Shutter Speed, and Spectral Response

98
The terms auto iris, manual iris, and iris control are used to describe the general
concept of exposure control. Exposure control includes both auto exposure (AE)
and manual exposure control. Most modern camera blocks use video gain,
shutter speed and spectral response controls, in addition to iris control, to
accomplish exposure control.
Video Gain
Video gain is the amount that a video signal is amplified and is measured in
decibels (dB). The video gain number usually specified is the amount of
adjustable gain being used. The normal gain measurement is 0 dB. The equation
in Figure C-1 is used to determine the video gain value, where:
• Vi is the video voltage input to the amplifier
• Vo is the video voltage output of the amplifier
• GdB is logarithmic video gain in dB
Figure C-1 Video Gain Equation
The video gain increases after the lens aperture reaches its maximum size (the
smallest f-number) in order to maintain a video output level with a decreasing
light level. Every 6 dB of gain change represents a doubling (or halving) of the
output video level in relation to the input video level. Increasing video gain
increases video noise.
Shutter Speed
The shutter speed (or shutter time) is a measure of the time that the video sensor
integrates light. The normal shutter speed is 1/60 s for NTSC cameras and 1/50 s
for PAL cameras. The shutter time decreases after the lens aperture reaches its
minimum size (the largest f-number) in order to maintain a video output level with
an increasing light level. If the slow shutter function is enabled, the shutter time
increases after the lens aperture reaches its maximum size (the smallest
f-number) and the video gain reaches its maximum value in order to maintain a
video output level with a decreasing light level.
When shutter times are less than the normal shutter time, this decreases the
blurring or smearing of moving objects. Conversely, when shutter times are
greater than the normal shutter time, this increases the blurring or smearing of
the image of moving objects.
V
o
=
G 20 log
dB V
i