116 | TRAFFIC MANAGEMENT
Precision iris lenses
P-Iris lenses optimize resolution and depth of field, and are especially useful for outdoor ITS applications Words | Lumenera Corporation, Canada
When selecting a lens for outdoor applications where the lighting can be variable (e.g. ITS applications such as tolling and speed enforcement), there are typically three types of lenses to choose from: a fixed iris lens, a manually configurable iris lens, or a motorized iris lens using DC-Iris technology. Traditionally, if a camera-configurable iris lens were required for a specific application, the only available option was a lens with DC-Iris. However, this technology has limitations. With these lenses, selecting a precise iris value is not repeatedly attainable and the lens often strays from its selected iris value over time. DC-Iris lenses have mainly been used to adjust the level of light that reaches the sensor and are only reliable when the iris is fully opened or set to its smallest aperture. Consequently, attaining accurate mid-range values has been a challenge for this lens technology.
A step up in precision More recently, Precision Iris (P-Iris) technology has entered the ITS market as an alternative to DC-Iris lenses. The P-Iris lens can reliably maintain and return to a precise iris value, and the iris can adjust to bright or dark lighting conditions. The stepper motor that is used in P-Iris lenses makes it possible to set and maintain a precise iris opening, allowing the lens to be set to an aperture that optimizes the depth of field and minimizes diffraction, for extended periods of time. Typically, the stepper motors have roughly 72-74 positions, allowing for an extremely fine granularity when setting the aperture value. This translates to enhanced precision when selecting the f-number to be used, enabling the user to set the aperture to the exact point where the depth of field is greatest and where diffraction has no impact on the sensor based on its pixel size, thus rendering a sharp and blur-free image.
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Because the resolution of the lens increases as the aperture opens and the depth of field increases as the aperture closes, an optimum position of the iris exists between these extremes. It is here that the lens can achieve a high depth of field without being impacted by the reduction in resolution caused by diffraction at small aperture values.
ITS applications When it comes to P-Iris lenses for ITS applications, it is important to ensure that the camera supports P-Iris and comes equipped with a port to control and power the lens. Currently, the only Sony Pregius-equipped USB 3.1 camera on the market that supports P-Iris is the Lumenera Lt545R. This camera uses the 5.1MP IMX250 Pregius sensor from Sony and since this sensor becomes diffraction limited at roughly f/5.06 (assuming 550nm light and a circle of confusion of 6.8µm – double the pixel diameter), a traditional lens would be diffraction limited at f/5.6 and would lose significant depth of field at the next full f-stop – f/4.0. Using the Lt545R with a P-Iris lens enables the user to set the aperture much closer, to f/5.06, enabling a greater depth of field without surpassing the sensor’s diffraction limit. Additionally, if lighting conditions were to change and the limits on exposure time and gain have been reached, the iris can still be used to help control the amount of light that reaches the sensor. Lumenera’s Pregius-based cameras with P-Iris support provide for easier system integration and improved versatility of applications. Again, the precision benefits that come with P-Iris when paired with a Pregius sensor provide a solution for those looking to implement global shutter CMOS cameras in outdoor applications where the lighting can be variable. n
Main: Lumenera’s Lt545R camera with a P-Iris lens Inset: The impacts of aperture
