A day/night lens may be used on any camera. However, a high quality day/night lens is absolutely required when using a day/night camera. Day/night lenses are designed with the criteria that they focus the infrared light to the same plane as the visible light. This means that the scene is in focus whether using visible light from the sun, moon, or street lights; IR light from these same sources or artificial IR illumination LEDs, or both. Without a day/night lens, the camera would give a soft focus effect at night when both IR and visible light are used at the same time. Integrators also need to understand IR corrected lenses. As mentioned earlier, many natural light sources contain infrared light. However, the amount of light changes as the light source changes over a 24-hour time period. Because wavelengths vary under different types of light (artificial and natural), a non-day/night lens may not be able to focus all the light to the same plane resulting in a condition known as focus shift. This occurs when the camera is focused during the day under natural or visible light (daytime).
But be aware of what happens with the IR blocking filter. This filter is in the optical path during the day to block IR light and maintain good color fidelity. At night, it is moved out of the optical path, allowing the IR light to reach the sensor chip, thus increasing the image brightness but causing colors to get altered. In night mode, when the IR blocking filter is removed, the additional IR light reaching the image sensor may be out of focus or blurred.
An IR corrected lens such as Theia Technologies' SL183 megapixel lens minimizes this light de-focus, which results in a continually focused image during the day and at night. This eliminates the need to re-focus a camera lens at night or when using IR illuminators to provide a light source.
Also be aware of back-focus. Many video cameras come with an automatic back-focus feature that can enable a camera to see a sharp image in IR light without the use of an IR correcting lens. When the camera is operating in night mode, the mechanism automatically moves the sensor to the focal plane for IR light. However, during twilight when there is a combination of visible and IR illumination, the image will not necessarily be in sharp focus. A quality day/night corrected lens is still required to keep the image in good focus with this type of camera.
New lenses on the market even provide day/night performance for megapixel cameras in applications such as parking lots, warehouses, power transformer lots, lobbies, ATMs and other places that need surveillance on a 24/7 basis. Achieving day/night correction with a megapixel quality lens is difficult due to both the increased light spectrum that must be focused at a single plane and the decreased focus spot size required for megapixel cameras.
Illuminate with the right sources
Artificial IR illumination can be provided by IR LEDs. These can enhance the naturally occurring light in the scene. Such lights can be mounted anywhere as long as the IR light can shine on the object that the camera is watching. There is no requirement that the IR lights be mounted at or near the camera. They can be scattered throughout a parking lot, thus bathing the entire area in IR light. However, many visible light sources such as street lights, warehouse lights, incandescent lights, not to mention the sun and moon, are also sources of IR light that can be recorded by the camera's sensor. If the scene has such a light source at night, a day/night camera can record the scene without the aid of additional IR illumination.
Before suggesting a day/night camera as a possible solution for your customer, know its capabilities and the possible applications it may be used in. Which lighting setting is it most efficient in? When is it necessary to use a day/night camera? What type of day/night lens must it complement with? Know what's on the market and be familiar with the solution you offer. Your end-user will thank you for it.
Integrator Tech Tips
Consider the following five points when deciding on a specific model camera for day-time surveillance and which works best for night-time visibility.
1. What is the target scene? What is the distance to the target, what zoom is required, what's the horizontal field, what's the depth of field and what lens type are you using?
2. What is the resolution? What is the number of pixels per foot required at the target distance to obtain usable evidence?
3. What are the lighting conditions? What type of low-light performance (lux rating, @fnn, IRE, AGC) is needed? Will illuminators be required? Are there challenging conditions that will require a wide dynamic range camera?
4. What's the needed frame rate? What frames per second (fps) are required to capture activity for evidence?
5. What's needed regarding the mechanical construction? What IP rating is required? For outdoor use? For indoor use? Vandal resistance? What's the operating temperature range? What form factor-fixed, minidome, Pan, Tilt, Zoom (PTZ)-is needed? What power supply is needed?