Axis Communications Q2901-E Thermal network cameras Performance consideration - Page 5

Nomographs

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>> At least 1.5 pixels are needed for detection, that is, the observer can see that an object is present. >> At least 6 pixels are needed for recognition, that is, the observer can distinguish the object, for example, a person in front of a fence. >> At least 12 pixels are needed for identification, that is, the observer can distinguish an object and object characteristics, for example, a person holding a crowbar in his hand. Johnson's criteria were developed under the assumption that visible information was processed by a human observer. If the information instead is processed by an application algorithm there will be specific requirements on the number of pixels needed on the target for reliable operation. All intelligent video software algorithms need to work with a certain number of pixels. The exact number may vary but as a as a rule of thumb at least 6 pixels across the object are required, which is the same as recognition according to Johnson's criteria. Even if a human observer would be able to detect the object, the application algorithm often needs a larger amount of pixels at a given detection range to work properly. To find out the available number of pixels at a given range, a nomograph is used. 4. Nomographs A nomograph is a two-dimensional diagram explaining the relation between the focal length of the lens, the number of pixels across the object, and the range. For example, if the number of pixels required and the distance at which an object needs to be recognized are known, it is possible to calculate which lens or camera to use. Equally, if the camera and the number of pixels required are known, the distance at which the camera can detect an object is indicated by the nomograph. Figure 4: Example of a nomograph. Nomograph - AXIS Q1921/-e Long distance 10,0 (A) Focal length 10 mm 19 mm 35 mm 60 mm Pixels across 0.75 m target 1,0 100 (B) 1000 Distance to target (m) For example, take an AXIS Q1921/-e Thermal Network Camera with a 60 mm lens pointed at a person with a critical dimension of 0.75 m (2.46 ft.). The nomograph in Figure 4 shows that the object will be recognizable at 300 m (328 yd.) and 6 pixels across the object (A). If only detection is required, the range will instead be 1 200 m (1 312 yd.) and 1.5 pixels across the object (B). 5

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At least 1.5 pixels are needed for detection, that is, the observer can see that an object is present.
>
At least 6 pixels are needed for recognition, that is, the observer can distinguish the object, for
example, a person in front of a fence.
>
At least 12 pixels are needed for identification, that is, the observer can distinguish an object and
object characteristics, for example, a person holding a crowbar in his hand.
Johnson’s criteria were developed under the assumption that visible information was processed by a
human observer. If the information instead is processed by an application algorithm there will be
specific requirements on the number of pixels needed on the target for reliable operation. All intelligent
video software algorithms need to work with a certain number of pixels. The exact number may vary but
as a as a rule of thumb at least 6 pixels across the object are required, which is the same as recognition
according to Johnson’s criteria. Even if a human observer would be able to detect the object, the
application algorithm often needs a larger amount of pixels at a given detection range to work properly.
To find out the available number of pixels at a given range, a nomograph is used.
4. Nomographs
A nomograph is a two-dimensional diagram explaining the relation between the focal length of the lens,
the number of pixels across the object, and the range. For example, if the number of pixels required and
the distance at which an object needs to be recognized are known, it is possible to calculate which lens
or camera to use. Equally, if the camera and the number of pixels required are known, the distance at
which the camera can detect an object is indicated by the nomograph.
For example, take an AXIS Q1921/-e Thermal Network Camera with a 60 mm lens pointed at a person
with a critical dimension of 0.75 m (2.46 ft.). The nomograph in Figure 4 shows that the object will be
recognizable at 300 m (328 yd.) and 6 pixels across the object (A). If only detection is required, the range
will instead be 1 200 m (1 312 yd.) and 1.5 pixels across the object (B).
Figure 4:
Example
of a
nomograph.
1,0
10,0
100
1000
10 mm
19 mm
35 mm
60 mm
Nomograph - AXIS Q1921/-e
Long distance
Focal length
Distance to target (m)
Pixels across 0.75 m target
(A)
(B)