Opening and closing the iris adjusts the amount of light that reaches the camera sensor, and so it offers brightness control for the camera. The adjustable iris inside most lenses normally uses standard increments known as ‘F-numbers’ including 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16 and 22.
Each increment represents a reduction in the amount of light passing through the lens by 50%. However the size of the iris also affects both the depth of field and the depth of focus of the system. The depth of field is the distance between the nearest and furthest objects that are in focus. The smaller the aperture, the greater the depth of field. The depth of focus is the distance that the sensor can be moved, whilst maintaining focus for a fixed object. This determines how accurately that the camera needs to be positioned for a particular scene. Again, the smaller the aperture the greater the depth of focus.
Keeping the aperture as small as possible gives the best depth of field, but this could result in prohibitively long shutter speeds to allow enough light to reach the sensor. In addition, once an iris is reduced below F8.0, diffraction starts to become a limiting factor. Diffraction results from the slight bending of light as it passes the edges of the diaphragm blades of the aperture. The smaller the aperture, the larger the percentage of light that is diffracted.
Diffraction causes a point of light to spread out into a so-called ‘blur circle’ giving a reduction in the sharpness of the image. For best imaging results, this blur circle should be slightly larger than the size of each pixel in the image sensor. This means that care should be taken when choosing the most appropriate camera sensor for applications needing a small aperture. Sensors with small pixels should be avoided as they are less forgiving with the increasing blur circles resulting from smaller apertures.
Operating with a large iris also offers potential problems in addition to a reduced depth of field. Aberrations such as chromatic and spherical aberration can start to have an effect on the image quality. Chromatic aberration results from light of different wavelengths being focused at slightly different positions. Spherical aberration causes light from the edge of the lens to be focused at a different point to light through the centre. Clearly adjusting the size of the iris provides a trade-off between the amount of light reaching the sensor, shutter speed, depth of field and aberration effects.
Auto-Iris systems automatically open or close the aperture of the lens in response to the light levels. ‘DC Iris’ and ‘Video Iris’ systems are available. In a DC Iris system the circuitry sits inside the camera. With the Video Iris, the circuitry sits in the lens itself. While these systems adjust the amount of light reaching the sensor, by opening or closing the aperture too much they can introduce diffraction or other aberrations into the image. In addition, as these controls rely on a continuous level signal they do not operate correctly where the camera is intermittently triggered as happens in most machine vision systems.
The Precise-Iris (P-Iris) was introduced to address the trade-off between light output, depth of field and image quality by including control of camera software settings such as gain and exposure time. Unlike the Auto Iris, P-Iris lenses contain a stepper motor to enable the aperture to be moved to exact F-number positions giving much more precise control of light throughput. In this way, the gain, exposure and iris adjustments can be finely balanced for optimum performance.
Too much gain and noise starts to become an issue. Long exposure times (slow shutter speeds) introduce motion blur. Too small an aperture size introduces diffraction effects limiting image sharpness. The P-Iris aims to improve image quality by allowing the F-number for the iris and the exposure time to be set. These give the optimum depth of field and minimum diffraction while preventing motion blur.
The stepper motors enable the iris to stay frozen between image captures. This makes P-iris well suited whenever an external trigger is used with the camera to start image capture for single or multiple images. In situations when the preferred iris position and the camera’s electronic processing capabilities cannot adequately correct the exposure, a P-Iris camera will automatically instruct the iris to move to a different position. In this way P-Iris can make adjustments to deliver optimal image quality in all lighting conditions.
As the Europe’s largest independent supplier of machine vision technology, STEMMER IMAGING can offer a wide choice of P-Iris lenses and cameras. These include lenses from Computar, Schneider Kreuznach, Fujinon and Kowa. Cameras include the Allied Vision Prosilica series, JAI Spark series, and Teledyne DALSA Genie TS cameras.