The bench test (2): Day and night

What our experts say...
Colour cameras are by their nature a compromise but where lighting conditions can vary from pitch blackness to intense sunlight there is no alternative. What separates the wheat from the chaff is a camera's ability to handle very low lighting levels, which has proved an enduring challenge for designers. One answer is to amplify the signal from the image sensor chip and this can help increase the amount of information in the picture but at the expense of significantly raised noise levels, which degrades the image. A more successful technique is frame integration or 'slow shutter' but the trade-off is a reduced image refresh rate and blurred movement.

A third method is to switch to mono operation at a predetermined lighting level. For this to work the infrared filter in front of the image sensor has to be dispensed with to increase the sensitivity. But IR filters have a purpose: to ensure colour fidelity in normal light, though the effects of doing away with the filter can be reduced by the video processing circuitry. The Samsung SDN-520 camera uses the latter method, with a twist, which it calls 'Day & Night'. In normal light it functions like any other colour camera, with an IR filter in front of the image sensor.

But as soon as the light levels falls the filter automatically swings out of the way and the camera switches to mono operation. Incidentally, this system was pioneered on consumer camcorders, particularly Sony models featuring a 'NightShot' recording mode. This enables recordings in near and total dark conditions (using a built-in IR illuminator). It is clearly no coincidence that the camera uses a Sony image sensor and a fair number of Sony video processing chips.

The general specification puts the 520 into the most populous sector of the market. Two models are available, with slightly different low-light characteristics; the SDN-510P uses a 1/3-inch Sony CCD with low light sensitivities of 0.5 lux (F/1.2) for a colour image and 0.03 lux in monochrome operation.

The SDN-520, looked at here, has a top of the line 1/3-inch Sony Ex-View Hyper Had CCD rated at 0.5 lux in colour and 0.01 lux for B&W operation. Both sensors have an operational 752 x 582 pixel array, yielding claimed resolutions of 480 lines (colour) and 520 lines (B&W). Key features include a simple motion detector facility, electronic shutter operating in manual or auto mode over a range of 1/50 – 1/10,000 sec, internal or external sync, backlight compensation, a positionable user-set on-screen ident of up to 14 characters and an RS-232 remote control. It can be used with video or DC controlled auto iris lenses and all functions are controlled from a simple to use set of on-screen displays. The all-metal case measures 55 x 57 x 135mm. At the front, embedded in a tough alloy casting, is a standard CS mount lens collar with a simple back-focus adjustment. A C-mount ring adaptor is included. On the back the familiar points of reference include a BNC socket for the composite video output and a pair of screw terminals for the 12-volt DC/24v AC power supply. Additionally there is a miniature slide switch for selecting auto iris mode, a small four-pin socket marked 'Remote', a green power on LED and a cluster of five buttons arranged in a standard cross-shaped 'cursor' formation. Removing the steel shells that form the case reveals a simple chassis design with the video processing and power supply circuitry plugged into a motherboard fixed to a steel base plate, bolted between the end caps. The tiny servo motor that moves the IR filter is mounted inside the front cap. Internal wiring is kept to a minimum and the whole assembly appears to be very rigid. It passed the SI rubber mallet test without missing a beat.

Set-up & operation: There are no nasty surprises. A mounting plate with standard 1/4-inch threads bolts to the top or bottom of the case and a microscopic Allen key is included for locking and unlocking the back-focus adjustment.

The only point to watch for when locating the camera is to allow for the video output plug and lead, which adds a good 20 to 30mm to the depth, so care should be taken when mounting close to walls. The case is not very well protected against ingress of dust or moisture, so appropriate measures need to be taken if used in a hostile environment. Pressing the centre button on the cursor control brings up the main OSD menu page. Item one is for setting the camera ID and positioning it on the screen.

Item two is the backlight compensation: it can be switched on and off and there is a simple weighting adjustment which, by default, focuses control on the centre 20 per cent of the screen area. The motion detector setup is next, the size and position of the activity can be adjusted from the cursor buttons; the area of concern is shown on the screen as an opaque rectangle.

There is also a sensitivity adjustment for the motion detector, on a scale of 0 to 10. Item four is for selecting and adjusting the auto iris lens and item five is for manually setting the shutter speed. Menu six is on gain control, the options being off or AGC with manual adjustments for brightness and gain limit. White Balance is next, there are three modes: auto, auto tracing and manual. Menu item eight is the colour control and there are four selectable modes: On – colour operation only, Off – monochrome only, Auto 1 — for daylight/ fluorescent operation and Auto 2 – when the scene is mostly lit by incandescent light. There are menu switches for selecting internal or external (line-lock) synchronisation, and Reset returns the camera to its factory default condition.

Performance: In good natural daylight picture quality is excellent with crisp natural colours, evenly balanced contrast, negligible amounts of picture noise and lots of fine detail, putting it within a whisker of the manufacturer's resolution figure and marking it out as one of the top performing cameras of its class. The auto exposure controls are fast and effective, but a touch jerky. If you watch closely you may become aware of the brightness 'stepping' in response to changes in lighting level.

Backlight compensation, despite being dubbed 'Super BLC' works well but doesn't appear to be more effective than most of its rivals. The motion detector is a very useful extra with a good range of adjustment, to help eliminate false activation, and it can be used to trigger an external alarm via the RS232 'Remote' connection on the back of the unit. So far so good, but the real test lies in the camera's low light abilities.

The motor that moves the IR filter in response to a drop in lighting level is surprisingly noisy but it only takes a fraction of a second and picture disruption is negligible. It would have been useful to have control over the point at which the change-over occurs. The change from colour to mono brings about a marked increase in picture sharpness and a drop in noise, though this does start to creep up again as the light level drops further. At the point when most rival cameras are starting to give up the ghost the SDN-520 is still going strong, delivering a clear and highly detailed image. And although there is a steady increase in noise the picture remains bright and stable with good contrast down to near dark conditions.

What the manufacturer says ...
Earlier this year, Samsung launched its new day/night camera, the SDN-510. This design uses a mechanically removed IR cut filter as opposed to a permanently removed filter. The principle of this design is simple and very effective. During hours of adequate lighting the camera is a high-resolution colour camera with the IR cut filter in place in front of the CCD. Colours are true because in effect the camera works as a normal colour camera.

The difference between this camera and most day/night cameras of this price is that when the light level falls below a set limit an electric motor rolls the IR filter away from the CCD and the camera switches to the mono mode, giving a high-resolution IR sensitive camera. When the light level increases the motor reverses and the camera reverts to a high quality colour camera with accurate colours.

Samsung has now gone one stage further and fitted the design with a Sony EX-View CCD which gives incredible low light performance in the mono mode while still giving superb colour reproduction during daylight hours.

Although this type of camera is not new the low price certainly is, and it is now comparable with some of the permanently removed IR filter types. Both cameras are fully functional with on-screen display and remote control facility using an RS-232C port.

The camera tested is the SDN-520 with the Sony EX-View CCD.