Low Light Surveillance
Most sites are at a higher degree of risk when light levels are poor. Darkness conceals the identities of intruders, and gives cover for criminals to work. For many, it is during such periods that surveillance takes on an increased level of importance. Benchmark looks at the best ways to maximise the performance of such systems in applications with poor or minimal illumination.
Light is an essential element of any video image. If there is no light, then there is no video image. There is no way of getting around that fact; a ‘no light’ camera simply doesn’t exist. If there is no light, there is nothing falling onto the chipset of the camera, which means that no signal can be produced. Cameras need light, at a certain level, if they are to produce anything in terms of an image.
When considering the performance of surveillance systems, people tend to consider extremes. There is daylight, and there is darkness. Interestingly, both conditions are relatively easily to cope with. In daylight, there is more than enough light to ensure correct operation of a typical video camera. In darkness, the answer is equally simple: add some degree of illumination.
Often it is the murky low light world between the two extremes that causes problems for video surveillance. Many feel that the answer lies with the camera. Pick a decent day/night camera, and it should be able to cope. Of course, it’s never that simple.
Day/night cameras and the various levels of image processing that they offer are one part of the equation. However, even the best day/night camera will struggle if it is fitted with a poor quality lens, or if there simply isn’t enough light to generate a video signal.
Light on demand
Often the simplest and quickest way of improving low light performance is to introduce additional illumination. Video surveillance can make use of two very different types of illumination. These are white light and infrared light.
White light exists in the spectrum between 380 and 750 nanometres, and is visible to the human eye. The biggest benefits of white light are that it allows colour images to be resolved, and that the illumination also serves to deliver general illumination, as well as making any out-of-hours activities obvious to passers-by! The downside is that some sites might require careful planning to ensure that any nuisance element is avoided.
Many fear that white light is ‘banned’ because of light pollution, but it’s not. In many applications, careful design can ensure that white light is used without any negative impact.
Infrared light can also be used by video surveillance. Infrared light is typically in the 750 nanometres and upwards frequency range. IR-capable cameras use near infrared, which is classed as light with a frequency of 700-1400 nanometres. Infrared light cannot be seen with the human eye, but many camera chipsets can use the illumination it provides.
Because infrared light is outside of the visible response spectrum, it has given rise to the notion of ‘no light’ cameras. This is incorrect, as there is light added to the viewed scene; it just is not within the visible spectrum! As infrared is measured in nanometres rather than lux, quotes of ‘zero lux’ minimum illumination can also give the impression that no light is required.
The benefits of infrared illumination are that it can be applied at any site, and it is covert. The downside is that when in use, colour detail cannot be rendered.
In recent times, advances in both types of illumination have seen an increase in high quality cost-effective options for all needs!
Understanding low light
An often-quoted but usually misunderstood specification is sensitivity. This figure should give a guide as to how well the camera will perform in low light conditions, and is quoted as a lux rating. Lower figures indicate better performance in poorly lit environments.
Many years ago, good colour cameras often boasted sensitivity figures of 8 or 6 lux. Nowadays, it is not uncommon to see figures of 0.0001 lux or lower! Sadly, this does not mean that cameras are much better. They still require a certain amount of light to fall onto the chip to produce a charge, which can then be converted into an image. The amount of light required hasn’t changed that much, but the specifications have!
Manufacturers measure sensitivity in different ways. Traditionally, sensitivity was measured with all processing switched off. Figures quoted were for a 1 volt peak-to-peak real-time video signal.
Today, the situation is very different. You will see figures quoted for 50 IRE, 30 IRE and even 20 IRE. Some will be with AGC on, and some with AGC off. Other processing functions, such as Frame Integration, Sens-Up or Slow Shutter might be used.
The need for light hasn’t changed. What has changed for the better, however, is the quality of image processing!
Low light can be considered to be the transitional period between daylight and darkness. Often there is not enough light to produce a high quality colour image, but the darkness is not significant enough to warrant the use of additional illumination. In such cases, processing comes to the fore.
The main processing option is gain: amplification of a weak video signal. When insufficient light falls onto the chip, AGC (automatic gain control) amplifies the video signal. AGC can be very effective, but the amplification of a weak signal also results in the amplification of noise. As a result, many cameras now include some form of noise reduction. A delicate balance is required between gain and noise reduction to ensure the image quality doesn’t suffer.
Another process often used goes by many names: frame integration, slow shutter, sens-up, etc.. The aim is to try and maximise the amount of light falling onto the chip. If the shutter speed is reduced an increased amount of light will reach the chip. However, when used the camera might no longer deliver 25 frames per second. The other issue is blur.
While processing does play a role in the delivery of the final image, any excessive use to try and force a camera to work in conditions that do not provide enough illumination will compromise the quality.
An optical issue
One of the more efficient tools for enhancing low light performance is often forgotten. Decent lenses, and aspherical lenses, can have a significant impact. Remember that the goal is get the maximum amount of light to fall onto the chip. High quality lenses can help. Aspherical lenses are ground with more complex shapes (hence the slightly higher cost). This delivers better focussing of light across the whole lens surface, meaning more light falling onto the chip.
In Benchmark tests, quality lenses have consistently achieved a higher degree of low light performance than standard lenses. This is increasingly important where HD and megapixel cameras are used, as the chip contains significantly more pixels (HD has 2,000,000 compared with 400,000 for 4CIF video), all demanding light!
Where HD and megapixel cameras are used, appropriate lenses should be employed. The high resolution requires more precise optics, and lenses of the required rating should be used. This means 3 megapixel lenses for HD1080p video, as the ratings are typically based upon 4:3 images!
Also, where infrared light is used, lenses must be IR Corrected to stop focus shift when the illuminators are used!
Video Surveillance Illuminators
Artifical illumination for video surveillance was once the almost exclusive preserve of high risk sites. This was because of high capital costs and maintenance requirements. However, the introduction of SMT LEDs and advanced manufacturing techniques has seen the capital cost of devices significantly reduced, with the requirement for maintenance virtually eliminated.
The benefits extend to both infrared and white light illuminators, and ensure that all applications can enjoy the right levels of illumination.
Benchmark has tested a number of white light and infrared illuminators, including units from Bosch, Iluminar, Raytec and Vista, all of which achieved Recommended Status.
The Vario i8 from Raytec is an infrared illuminator with a frequency of 850 nanometres. A 940 nanometre variant is available. The illuminator has a maximum range of 220 metres, and is capable of delivering fields of view of 10, 35 and 60 degrees. These coverage angles are field selectable!
The illuminator uses SMT LEDs, and employs light intensification miniature optics and thermal management to enhance the lifespan of the LEDs. Another technology added is HRT â€“ hot-spot reduction technology.
The WL436-60-24 from Iluminar is a white light video surveillance illuminator. The unit has a range of 53 metres, with an angle of illumination of 60 degrees. Brightness is quoted as 2784 lumens, and colour temperature is 6500K, which is similar to daylight.
The long range unit uses 24 energy efficient high powered LEDs., and the current is controlled using integral circuitry.
Achieving low light performance calls for a balance of quality lenses, cameras with appropriate functionality, and additional appropriate illumination. Use basic lenses, low quality devices or generic illumination, and the final image will be poor!