Some
time ago the Home Office issued guidelines for the identification of
persons and vehicles. This is fine, but many system engineers stumble
when trying to find what camera and lens combination will satisfy these
guidelines. And what about end users who know even less about camera
and lens formats, how can they assess the merits of competing
specifications? This month all will be revealed for both groups.
Charts showing the horizontal and vertical fields of view for
many lenses and four formats are given in ‘The Principles and Practice
of CCTV’ and were published in the first issue of CCTV Today (Jan ‘94).
They also show the % of the screen height of a 1.7M person. The
Home Office guidelines had not been published for general use when I
produced the first draft of the book and so the 1.7M was my guess at
the average height. The current guidelines use 1.6M as the height.
The values for various degrees of identification are given as
the percentage the 1.6M figure would occupy of the monitor screen. I
call this the ‘screen height ratio’. The complete guidelines are
provided in several Home Office publications and so only the basic
ratios are given in this article. The publications are available free
from the Home Office and provide a lot more information as well.
These criteria are now becoming increasingly used as part of
the specification for many CCTV systems, particularly in Town Centre
schemes. Sometimes the specification will state the distance from the
camera for each criterion, sometimes the specification will ask the
question, ‘at what distances from the camera will the criteria apply’?
In either case it involves calculations that are not too difficult but
can be tedious to keep repeating for each lens and camera location.
Another problem that many people find difficulty in resolving
are the different fields of view obtained from various camera and
lenses formats, i.e. what is the result of fitting a 2/3" lens onto a
1/2" camera, and how does this affect the screen height ratio at
certain distances?
A word of caution, just about all lens manufacturers brochures
give the HORIZONTAL angle of view, whereas these calculations require
the VERTICAL angles of view. The vertical angle of view is the
horizontal angle times 3/4.
The field of view is the ratio of the sensor size to the focal
length and the distance to the subject. This is shown in diagram1. The
'width to height' ratio of the sensor is 4:3. The horizontal and
vertical angles and therefore fields of view are different and must be
considered separately.
Diagram 1 Field Of View
Note when using these ratios all the units must be the same, i.e. millimetres or Metres.
Sensor Sizes
Diagram 2 shows the sensor sizes to be used when calculating fields of view and angles of view.
Diagram 2 Sensor Dimensions
Example
Supposing it required to recognise a known person at 50M, using a 2/3" lens, the following is the calculation.
The scene height at 50M needs to be twice the standard height, 2 x 1.6=3.2M. Therefore:
The nearest standard would be a 10.5:105mm zoom lens to satisfy this requirement.
The formula can be worked backwards to find the scene height for
a given lens. It is a simple matter to put all these criteria into a
spreadsheet program and find the result for any combination. However,
this may not be very convenient for the many salespersons on the road.
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