P-Iris (Precise Iris)
Before we go into why the P-Iris is better lets get back basics of why you would want to open (or close) the Iris.
Yes, it controls the amount of light that that passes through, but did you know that this doesn't just adjust the brightness?
The Depth of Field (DOF) - which is the distance between the nearest and furthest objects that are in focus (without any lens adjustment) - is also affected.
Taking things a bit further, the aperture size affects also affects the depth of focus ...
How’s that different to the depth of field?
The depth of focus is the distance that the sensor can be moved, whilst maintaining an ‘acceptable’ level of focus (while the object stays in the same position).
The effect is similar to the depth of field whereby, a smaller aperture gives you a larger depth of focus…and vice versa.
With an Auto-Iris system, the aperture of the lens automatically adjusts itself in response to the light levels. The software initiates the change by controlling a drive that’s linked to the Iris, and it’s adjusted to open or close the aperture accordingly.
In a “DC Iris” system the circuitry, which converts the signal and controls the lens aperture, sits inside the camera. With the “Video Iris”, it sits in the lens itself. As these controls rely on the continuous signal level they do not operate correctly where the camera is intermittently triggered as in most machine vision systems.
This system does not respond to anything but light level changes. So although it may have adjusted the amount of light reaching the sensor, other image qualities, such as the depth of field, cannot be controlled.
Why constantly change the aperture? Set it to as small as possible, so you can have an amazing depth of field!
Yes, that would be great... but unfortunately it’s not possible. As well as having to have a ridiculously long shutter speed, diffraction starts to become a limiting factor.
Diffraction is the slight bending of light as it passes an object. It is caused by the interference of the light waves with each other as they travel past it.
Think of each point on the wavefront as being a constant new source for a circular wave. If there are two either side of it... the ‘outward’ directions cancel each other out and just travels forwards. If there isn't an interference on one side (due to an obstruction) it’s free to travel in that direction.
The light travelling right next to the edges of the diaphragm blades is diffracted. As the aperture reduces, the percentage of light that is diffracted, increases. Consequently, the sharpness of the image is reduced.
According to nyquists sampling theorem, the radius of the blur circle should be slightly larger than half the pixel size. So the size of the blur circle should slightly exceed the size of a pixel.
This is why, when the focal length and sensor size are both constants, cameras with small pixels are undesirable in applications where a large depth of field is required. The smaller they are, the less forgiving it is with the blur circle
At the other extreme, if the aperture is too big, aberrations such as chromatic and spherical aberration can start to have an effect on the image quality.
Spherical aberration happens when the light travelling through the centre of a lens undergoes a different magnification than the light that passes through the edges.
This happens because of the sphere shape that most lenses maintain throughout their body.
Aspherical lenses don’t do this (but they’re pretty expensive to make!)
Find out more about image aberration on our aberration of optics page.
The Precise Iris solution was designed to try overcome all of the drawbacks that we’ve just mentioned.
It doesn’t automatically adjust the aperture just to alter the amount of light that reaches the sensor...
It automatically adjusts itself to help improve the image quality.
Unlike the Auto Iris, the lenses used with the P-Iris solution contain a stepper motor to control the aperture. This enables much more precise control of it, moving to exact positions whilst feeding its location back to the software. The predetermined positions, named “f-numbers”, are points at which the lens performs at its best, with minimal optical distortion, in the current environment conditions. The combination of this precise control and the adjustment to some camera software settings is what results in the best image quality. The gain, exposure and iris adjustments need to be finely balanced as extremes of any of them can result in unfavourable effects.
The combination of all of these settings ...
Accurate Iris control
Accurate Iris control through the use of a stepper motor with constant confirmation feedback
Dynamic gain control
Dynamic gain control to ensure the image signal level is optimised, without compromising the image quality
Responsive exposure
Responsive exposure settings ensuring the right amount of light reaches the sensor, whilst maintaining image sharpness
... is what makes the P-Iris so much more precise than just opening or closing the aperture in response to variations in light.
On of the cameras in our portfolio that contains this level of control is the Spark series from JAI.
The automatic level control technology (ALC), designed by JAI, is an advanced auto exposure function that’s targeted for outdoor vision applications where the changes in light call for a challenging combination of settings. This works very well in traffic and security applications.
The ALC combines the gain and shutter control to achieve optimal exposure, whilst minimising noise and motion blur.
Under dark conditions, the exposure time is fixed to a preselected value, selected by the user, to avoid unacceptable motion blur. Simultaneously, the gain is increased to achieve an acceptable exposure level whilst being limited by the maximum amount of noise that is acceptable.
At the other extreme, as illumination increases, the gain reduces until it reaches zero, eliminating any noise that was introduced. And yes, the exposure time adjusts simultaneously, reducing itself until it reaches the acceptable minimum.
The ALC interface gives you the flexibility to adjust it all to accommodate the dynamic lighting conditions. The controls even allow you to specify which portions of the image to use when making its calculations. So, for example, in vehicle-mounted applications, the window can be set to not include the sky when it's making the calculations. You can even set the speed at which the camera reacts to changing conditions, and the type of algorithms used when making the calculations.
- Technical tip: P-Iris
- Aberrations of Optics
- Spherical Aberration
- Optics
- Optics technical overview
- Cameras
- Camera features and functionalities
- Camera features and functionalities - extended