USB (universal serial bus) video cameras in microscopy – quite simply far too slow!
USB cameras
are often offered as eyepiece cameras (video eyepieces) or with a
C-mount connection. The components and quality vary greatly between
different manufacturers, but all of them have one thing in common: they
have a bandwidth problem! Digital video cameras are able to take
pictures in high resolution, but unfortunately they cannot transport
the data stream to the computer quickly enough. This results in data
blockages and the number of images transmitted per second falls off
sharply. The current USB connections (USB 1.1 and 2.0) pose a
bottleneck for the data stream. The currently fastest USB 2.0
connection would need to be at least 10 times faster in order to
transfer a video sequence fluently in real time to the computer. In the
entertainment industry, for a 2 megapixel resolution or full HD with
1920 x 1080 pixels with 30 pictures per second, a data transfer rate of
3960 Mbit/second is planned.
The required
bandwidth is calculated as follows:
1920
pixels x 1080 pixels x 36 bit (colour depth per pixel) x 30 images
per second = 2240 Mbit/second
USB 2.0 cameras with resolutions
of up to 10 megapixels only provide a data transfer rate of 480
Mbit/second.
|
Interface
|
Data
transfer rate
|
|
USB1
|
12
Mbit/second
|
|
USB2
|
480
Mbit/second
|
|
USB3
(not yet on the market)
|
5000
Mbit/second
|
|
HDMI
1.2
|
3960
Mbit/second
|
|
DVI
|
3700
Mbit/second
|
The greatly reduced image
reproduction rate under 10 images per second poses a problem when
focussing the microscopic images.
A strong time lag makes the choice of
the optimum focal plane more difficult. If a lower resolution is
selected, the focal plane is more difficult to find.
Conclusion: For professional use
in microscopy, we can only recommend USB cameras with reservations.
However, with a little patience and intuition, good results can be
achieved. The new digital HD SLR cameras, such as Canon’s EOS 5D II
with “Live View” and HDMI output, offer an alternative to expensive HD
video cameras. In video mode, these cameras deliver a real time preview
in HD (high definition) quality via the HDMI output. The large format
sensor chip of digital SLR cameras has an excellent signal to noise
ratio and a photo resolution of over 20 megapixels.
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