Question The CCTV/Surveillance Expert

Lens Application
Q:
What is IR Shift and what criteria will affect it?

A: IR Shift is the difference in focal points for a lens under white and IR lighting. Operators and end users are never slow to tell you that the more they zoom in on an object the more IR Shift becomes noticeable.

Locations requiring continuous day and night observation of objects that are 50 feet or more away are particularly fraught since the greater the zoom factor the more depth of focus goes down. At night when the iris is generally sitting wide open you are already at your worst depth of focus. This is why the IR Shift becomes more and more pronounced.

Day-night lenses correct IR Shift making both visible and IR rays focus on the same point, so eliminating the need for adjustment. It is critical to invest in high quality day-night lenses for applications using infrared illumination. If the application requires observation 24 hours a day and relies on IR rather than street lighting or floodlights, then a day-night lens becomes a must for performance.

In the past clients were often asked to specify whether day or night-time images were more important and were then expected to settle for an unfocused image for part of the day. This is no longer acceptable.

Of course, focus shift is not so important in a motorized zoom that is manned all the time since the operator can simply refocus. However, with the popularity of remote observation and automated intelligent scene analysis such usage is becoming rare. Similarly, an application that uses pre-sets will normally need a day-night lens since images during IR illumination employing a pre-set chosen in daylight are bound to be blurred.

Applications with a color camera set up for daytime viewing and a separate black and white unit with the lens focused under IR to compensate for the shift are cumbersome in the extreme and can be wasteful in their use of space. And of course there’s the expense and logistical consequences of switching circuitry. IR lenses can make more effective use of the client’s budget.

Additionally, you no longer have to find a reason for explaining why lenses go out of focus when IR lighting kicks in. While clients may initially balk at the extra cost of a day-night lens, the economies associated with being able to use a single camera soon win over.

Europe began exploiting IR illumination before the U.S. Use of day-night lenses to accommodate IR shift is thus far more widespread there but as the U.S. market moves to accommodate infrared lighting the issue of IR Shift is receiving more and more attention.

David Craig is Technical Manager of Rainbow CCTV.

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