By Dr. Kai Pahlke

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:

• High sharpness, even in the image corners (already with fully opened aperture)
• High contrast
• Minor stray light
• Low glaring
•  High chromatic saturation and differentiation of colors
•  Very good color correction with visible and IR light (no color fringing i.e., distortions of color on the edges of people or objects)
•  Minimal shift of focus when switching from visible light to IR light
•  Minimal bulge of image fields and coma

The images above demonstrate the effect of a special camera preset with a Cam_inPIX technology (right) in comparison to a high-value CCD camera (left).  (Photo by Dallmeier electronic)

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:

• Blurring during night-time operation
• Shift of focus when switching from visible light to IR light
• Color fringing within the visible light

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.

Dr. Kai Pahlke is with Dallmeier electronic GmbH & Co.KG (www.dallmeier-electronic.com).