Following on from our previous post about improved CCTV image quality in fog and haze, this article will look at another challenging scenario for CCTV surveillance image quality: wide dynamic range lighting. To find out how to get high quality images, like the one above, in various difficult lighting scenarios, read on...
The problem: wide dynamic range lighting
It’s no secret that cameras rely on adequate and correct lighting to be able to take good quality photos or images. That said, however, lighting for CCTV surveillance is often far from perfect, and particularly challenging are scenes in which there are both bright and dark areas. In this situation, cameras are unable to obtain the optimum exposure to produce a high quality image with detail in both of these areas of contrasting light intensity. Surveillance cameras tend to adjust the brighter areas in the field of vision, making it difficult or impossible to see detail in the darker parts of the image. This can result in unsatisfactory footage and in some cases, footage that is partially or almost completely unusable.
Scenes with these high variations of light are known as wide dynamic range (WDR) images, due to the wide range of light in one image. Some of the common causes of WDR images include:
- Scenes with strong background lighting, i.e. where the subject is positioned directly in front of a bright source of light, such as a shop or office foyer with large windows and bright sunlight outside
- Areas with abrupt changes in lighting or where lighting conditions are reversed, i.e. where the camera looks from a well lit area to a dark area, or vice versa, such as a doorway or entrance to a garage or tunnel
- Scenes with rapid and constant change in light, like entry and exit points, or in which there is light from various angles, like a multi-window room
- Instances where there is a bright light directed towards the camera, such as car headlights on a highway
- Scenes that have intense light reflection off windows, such as in offices, shopping malls or areas with water features
The solution: WDR cameras
In the past, some CCTV cameras were equipped with Back Light Compensation (BLC) functions which essentially enable cameras to adjust the exposure in order to capture detail in a subject that is heavily silhouetted by a bright background. However, in most cases, BLC does not fully solve the problem as although the camera can show detail in the silhouetted subject, the bright background has no detail or limited visibility.
A foolproof and effective solution was therefore still required, which prompted the development of wide dynamic range cameras. Cameras with WDR ability specifically tackle scenes with varying light and harsh lighting conditions and offer near perfect exposure in both bright and dark areas simultaneously. This way, far more detail can be captured under these challenging circumstances than is otherwise possible with regular cameras.
WDR cameras function in either of the two following ways:
- Multi-frame imaging: different frames are taken of the same field of vision, each frame with a different dynamic range: long exposure for the dark areas and short exposure for the bright areas. These images are then seamlessly combined to create an image that retains detail in both the dark and bright areas of the image.
- Non-linear sensors: the WDR image is captured in one frame by using logarithmic sensors which vary the sensitivity of the sensor based on the different levels of light intensity in different areas of the image.
Multi-frame imaging WDR is more widely used as it is more suitable for capturing images in real time (the non-linear sensor method is slower) and can process fast moving objects better than non-linear solutions. Multi-frame WDR is also more cost effective and integration friendly, making it the preferred choice for most solution providers.
Other advantages of WDR cameras
As well as providing images that have accurate detail in areas of contrasting light intensity, WDR cameras also provide more natural colors, have the accurate white balance and tend to reduce or eliminate image noise. This not only means that the image quality is high, but by eliminating noise, the file size of the video footage or image is smaller which makes storage easier and more cost effective.
Offices, retail, businesses and banks where there is strong backlighting, such as at the entrance or in a heavily windowed foyer: detail needed for people (in some cases facial recognition) and visibility also needed for outdoor area.
Car parks, ports, airports and logistics centers where there are often scenes of reverse lighting: detail needed in dark indoor or shadowed area as well as bright outside areas.
Casinos and ATMs where lighting might be difficult or with high contrast: important to be able to see all detail to identify possible crimes or criminals.
Highways, gas stations and traffic monitoring where there is often bright lighting facing the camera (especially at night with headlights on, or at dusk with direct, low sunlight): details inside the car may be required as well as license plate recognition.
Schools, prisons and institutions where indoor and outdoor lighting, or dark shadows outdoors, may contrast dramatically: accurate monitoring of people and vehicles, as well as possible facial recognition or license plate recognition, is needed.
Due to their ability to produce high quality, useable images under challenging lighting, WDR cameras are becoming increasingly more mainstream and affordable, with various forms and functions to meet the demands of indoor and outdoor applications. It’s highly likely that your company or site will benefit from a WDR camera in one or multiple locations, but it’s important to first remember the following:
- WDR cameras should be used where appropriate: they have been developed specifically for scenarios with simultaneously light and dark areas in the field or vision, not for solely low light scenes or bright light scenes
- You should first consider what you are trying to achieve through each CCTV camera (where and what you want to monitor and to what level of detail) and consider the lighting conditions present, as this will effect where you should place the camera for the best results
- If the camera also needs to operate at night, it may be beneficial to disable the WDR function (if there is no high contrasting light) in order to to maximize the camera’s low light performance
For more tips, industry insights and news from ICD, follow us on LinkedIn or look out for more posts on our website.
*First 8 images in this article (excluding the BLC function picture) are courteous of Arecont Vision, from their extensive range of high quality WDR cameras.