WDR cameras
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WDR cameras can be used for solving the most
difficult situations in surveillance such as the foyer scenario.
In recent years advances in picture sensors and picture processing have led to video cameras with significantly higher performance levels. High-performance wide dynamic range cameras which can be of versatile use in this area of application have been available for a number of years. The Wide Dynamic Range (WDR) property means that the brightest and darkest areas of an image can be displayed simultaneously. We know from other branches of technology (for example, audio systems) that the performance level of an information-processing system chain is determined by the performance of its weakest element. The performance of cameras has increased enormously -- so this article is dealing with the sharpness of the lens. | |||
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Is there anyone who
hasn’t tried a strange pair of glasses on, just out of curiosity or for
fun? Some people become glass wearers
themselves. The world looks different to
them and the familiar becomes new and exciting.
Habits can change; they start reading again and move more confidently in
crowds. Surprised by this chain of
events, they ask themselves, how come they previously regarded their limited
perception as absolute. Someone who has
their eyes open has a new perspective.
I want to use this analogy to point to
the key role of optical systems in CCTV.
In recent years advances in picture sensors and picture processing have
led to video cameras with significantly higher performance levels. High-performance wide dynamic range cameras
which can be of versatile use in this area of application have been available
for a number of years. The Wide Dynamic
Range (WDR) property means that the brightest and darkest areas of an image can
be displayed simultaneously. We know from
other branches of technology (for example, audio systems) that the performance
level of an information-processing system chain is determined by the performance
of its weakest element. The performance
of cameras has increased enormously -- so this article is dealing with the
sharpness of the lens.
MONITORING FOYERS
There are three characteristics of
modern video cameras which are especially closely linked to lens
characteristics: Wide Dynamic Range (WDR), natural color reproduction with
increased color differentiation, and no glare as well as the use of cameras with
IR lighting. Cameras with these
characteristics are preferred for solving the most difficult situations in
surveillance.
The foyer scenario demonstrates the
necessity of the WDR capability. The
surveillance of foyers -- usually rooms with large window areas in the
background of the picture -- means that continuous monitoring needs to be
ensured during the day and, very often, during the night as well. By day, a variety of backlight situations can
occur, whereas at night, inexpertly installed light fixings can be a significant
problem. The observer is primarily
interested in identifying the people in the room, whose faces are usually poorly
lit. With the blazing light in the
background, it was impossible to produce usable image material over a longer
period of time using conventional technology.
Compromises often had to be made so as to get images on which people
could be identified at all. In this case
one would have tried to bypass the problem of the disruptive backlight by
repositioning and reorienting the camera.
Within modern glass buildings and at many exhibitions of high-value art
objects or jewellery such compromises can often not be made. A reliable remedy can only be provided by
cameras that are capable of compensating backlight situations in a reliable
automatic mode of operation. This
requirement by far exceeds a simple switching from normal operation to backlight
compensation because often the backlight situation will improve after a few
hours as the sunset and weather conditions change. Simply increasing the brightness of the image
by a few dB, an option that can be found in the menu of many cameras under ‘BLC
On’ will not suffice. Rather, an
application-oriented control of the dynamic range, the range of contrast as well
as the Gamma correction is required, which can only be realized with advanced
technology. With good reason, a renowned
chain store is using state-of-the-art WDR cameras to monitor its products on
strongly reflective shelves and above shiny marble floors. Investing in a discrete, vandal-resistant
model, available even with individual housing designs, pays off not just because
of the very detailed image material.
For technological reasons, certain
cameras feature another characteristic which often comes in very handy when
monitoring foyers: non-glaring. Extremely
bright sources of light create halos, whose size can only be minimized by using
appropriate sensor technology. If the
morning sun was located besides the person in the picture, the face of the
person would certainly be outshone. Even
an LED lamp, directed straight at the camera, would only slightly irritate the
non-glaring WDR camera because high-performance WDR cameras produce detailed
reproductions of the light source as well as the shadow.
Additionally, the foyer scenario poses
high requirements to the camera’s exposure control. It is essential that individuals are rendered
with adequate image brightness. In order
to ensure this, Cam_inPIX technology offers a special preset which guarantees
that, no matter how dark it may appear in front of the window, the person is
always rendered in great detail.
 
LENS REQUIREMENTS
A camera that is particularly suited
for monitoring foyers features an individually selectable image detail, the
so-called Backlight Zone. This area can
be changed in size and appropriately positioned within the picture. The distribution of brightness in this image
detail is then used for the control of the exposure, the dynamic range, and the
Gamma correction. This functionality
offers further potential areas of application.
The number of day/night cameras on
offer has risen significantly over the past few years. That is why, increasingly, day/night lenses
are being used that are more or less well corrected regarding the visible as
well as the IR light.
Having touched on the various
performance characteristics of the cameras and a few examples of application,
the following list enumerates the corresponding lens requirements:
Usually, designs with aspherical lenses
are used in order to achieve high sharpness and contrast with a fully opened
aperture. Apart from the lens design
being an important factor, minimal stray light, low glaring, high contrast and,
partially, the reproduction of colors significantly depend on the quality of the
coating as well.
Day/night lenses need to be corrected
across a wider spectral range, which is difficult to realize with small lens
surfaces. Thus, the following problems
occur:
Bulging of image fields and coma
clearly show with some objects at fully opened aperture and especially with
images of punctate light sources at night.
The pair of images of the foyer
scenario demonstrates that particularly the high contrast and a low sensitivity
to stray light are necessary to clearly show the dark person in front of the
bright background.
Another phenomenon that can often be
seen is that in areas where ‘bright’ and ‘dark’ areas collide, a blue light
appears which spreads out across the area.
This effect shall be described as blue color fringing. It occurs with lenses that do not have
sufficient chromatic correction with visible light.
The origin of this effect can easily be
explained by the fact that the white light comprises elements of every
color. The types of glass used for lenses
have different refractive indices for different colors (dispersion), which needs
to be considered in selecting the suitable types of glass for the lens
systems. Lenses showing this color error
have a different focal point for blue than for the remainder of the visible
spectrum. This means that the blue image
of the line where the ‘dark’ and the ‘blue’ areas meet cannot be focussed
properly.
Due to the high levels of performance
provided by advanced camera technologies, those cameras are increasingly being
used as ‘problem solvers’ in particular lighting situations. With the Cam_inPIX technology the strengths
and weaknesses of the lenses become more obvious as has been the case with
previous conventional technologies.
Correspondingly, the camera
manufacturer recommends suitable high-performance lenses.
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