Low-light performance of a surveillance camera is critical in both dark indoor environments and nighttime outdoor environments. There are four low-light features to consider when looking at IP surveillance solutions:
The amount of light that reaches a camera’s image sensor is determined by the lens in front of it. The lens is the optical path through which light travels and the lower the f-stop value, the more light reaches the sensor. The f-stop is used to measure the aperture or iris setting of the lens. The higher the f-stop, the smaller the iris (f16), and the lower the f-stop, the larger the iris (f1.2).
The trade-off with a large iris is the depth of field becomes shallow, especially on a telephoto lens. Depth of field is the range of distance where subjects are in focus. We’ve all seen great looking portrait photos where a close-up of a face fills the frame and the background is intentionally soft and out of focus. In the world of fashion, photography and art, this is quite often a desirable effect. In the world of surveillance, it’s the last thing you want. Luckily with wide angle lenses, the depth of field increases and usually everything in the scene is in focus. But even with a wide angle lens, the larger the iris setting (i.e. the lower the f-stop), the more shallow the depth of field becomes. So there is a trade-off to using a low light, large iris lens — a potential reduction in focus/depth of field.
The amount of light present in a scene is measured in lux. By definition, one unit of lux is equal to one lumen per square meter. But for the sake of this conversation, we’re more interested in how light sensitive a camera is rather than presenting a primer on lux and lumens. To provide some perspective on lux readings, here are some real world examples:
Most surveillance cameras perform well in adequate light. The challenge lies in low-light performance. In general, the lower the lux rating, the more light-sensitive the camera is. Oftentimes, the lux rating will be different when the camera is in color mode vs. black-and-white mode and this should be taken into consideration.
Unfortunately, there is no standard when it comes to specifying the lux rating for surveillance cameras, and manufacturers each have their own methods of testing. So although lux ratings are critical to low light performance, they should not simply be taken as truth — real-world testing is a must.
In the past few years, many manufacturers have started offering super low-light cameras. These cameras use a combination of improved optics, improved image sensors and improved processing to allow the camera to see in color in low light situations without the need for the camera to go into black and white night mode.
A day/night camera can mechanically switch between a color day mode and a black-and-white night mode. All color cameras, including the one in your phone, have an infrared (IR) filter permanently in place between the lens and the sensor. This IR filter blocks infrared light from reaching the sensor and only allows visible light to pass through. If both visible light and IR light hit the sensor, it is not possible to render the colors in the scene properly, so for day or adequate light use, an IR filter is critical.
When a scene becomes darker, a mechanical device in a day/night camera slides the IR filter over and a clear filter moves into the optical path, allowing both visible and IR light to pass through. At the same time, the camera’s processing switches into black-and-white mode to eliminate faulty color processing. The combination of available visible light and infrared light makes the camera much more light sensitive than just visible light alone, especially with light sources that put out a lot of infrared illumination, such as halogen lights.
No matter how light sensitive a low lux day/night camera with large iris lens is, there is no magic. At the end of the day (literally), illumination is the key. Illumination can be the ambient light that is already present in the scene or it can be visible or infrared (IR) illumination that gets added to the scene. If an application requires color video at night and the ambient light present is not bright enough, the only choice is to provide more visible light illumination (a camera in color day mode will not see IR light).
Since adding lighting to a scene can be costly or impractical, the most logical solution is to deploy a day/night camera that has built-in IR illumination. Integrated IR illumination is most commonly accomplished with an array or circular pattern of IR LEDs around the lens .When the camera goes into black and white night mode, the IR LEDs turn on and the scene becomes illuminated. The human eye merely sees a dull orange glow on the camera, but to the camera, most of the area in front of the lens is brightly illuminated.
Camera manufacturers specify the illumination distance, usually falling between 15 and 100 feet. External IR illuminators are also available, some with illumination distances of hundreds of feet. But as with lux ratings, real world testing is recommended until you become comfortable with a certain brand or model of camera and what its true low light capabilities are.