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Designing an iris diaphragm, like most things, involves compromises. Usually there are three critical dimensions: The minimum aperture, maximum aperture and the outside dimension. But another important factor is the number of blades, as this determines the quality of the aperture – particularly noticeable at minimum aperture.
The Iris Calculator uses the ‘banana’ style blades. This type of blade are resilient to damage due to misuse and warping caused by heat, allowing the iris to continue to function under more extreme conditions. Of course the materials you choose to manufacture your parts in have a big impact on their reliability.
Because these blades span across the iris, a minimum aperture of 0 mm, or total closure, cannot be obtained using a single array. The blades become interlocked at very small apertures (around 1 mm, depending on blade material thickness) as there is no space for them to all squeeze in. You may find total closure achievable, but this is usually due to a lack of precision in manufacturing - whereby some blades close down slightly more than others - or using a low number of blades. Either way it is highly likely there will be light leakage.
A solution to ensuring the iris is ‘light tight’ is to design a ‘dual-plane’ iris, whereby half the array of blades are mounted on one side of the actuator plate, and the other half are on the other side.
To design a dual-plane iris, you’ll need to specify a negative minimum aperture value. This would then allow complete closure of the aperture with some slight over travel, ensuring no light leakages.
- A. Outside diameter
- This is the total size of the iris diaphragm. You don’t need to specify a dimension here, but you may want to if your iris is to fit a specific size.
- If you don’t specify a dimension, you may want to consider putting a value in ‘F. Outer wall thickness’ if you require some extra material - perhaps to allow for drilling mounting holes, or to aid gripping the iris when operating.
- B. Max. aperture
- Specify the maximum aperture here.
- This is the largest aperture your iris will open up to and where the inner edge of the housing will begin. This dimension is required to design your iris.
- C. Min. aperture
- How small do you want the aperture to close down to?
- Be careful of really small values. An aperture closing down to ~1 mm is unlikely to be achieved in reality, unless you’re using a low number of blades, or designing a dual-plane iris as explained earlier.
- D. Number of blades
- How many blades do you want in your iris?
- A low number allows smaller minimum apertures, but produces wider blades and a jagged minimum aperture. Using a low number also requires a larger housing outside diameter.
- A large number of blades produces a cleaner aperture, but at the cost of making the iris thicker in height, and an increased chance the blades will try and penetrate the housing wall when operating (thus preventing the iris from closing down to the minimum aperture specified).
- E. Blade overlap
- To prevent light leaks and ensure the blades glide easily over one-another, an overlap is required. The default 1 mm should be enough in the majority of designs, but you may want to adjust this depending on your needs (this measurement of overlap is with the iris at minimum aperture).
More overlap results in wider blades, and therefore a larger outside diameter.
- F. Outer wall thickness
- Use this to create a buffer of material between the blade/outer wall gap and the outside diameter. This allows for drilling mounting holes or to aid gripping the iris when operating. The default is 2 mm.
- G. Blade width
- This will be calculated using the other dimensions you specify.
- H. Blade/Outer wall gap
- Gap allowing the blades some slight over-travel beyond maximum aperture, and freedom to rotate. A larger gap may be necessary when specifying irises with small minimum apertures – dual-plane irises in particular - or with a high number of blades. This is to allow the blades room to fully close at minimum aperture, and not to penetrate the outer wall with the far end of the blade.
- I. Rotational movement
- The amount of rotation required to transition the iris from maximum aperture to minimum, and vice versa.
This value is calculated using the other dimensions you specify.
- J. Pin size
- Here you can specify the size of the drive and pivot pins you wish to use in your iris design.
- K. Pin clearance
- This is the gap between the drive pin and the slot it runs in. Too small and the iris will cease. Too big and the blades will move at slightly different rates causing an uneven aperture.
Clicking the ‘Calculate!’ button designs the iris using the dimensions you’ve specified. Any calculated dimensions will be shown, along with those you’ve specified. A scale drawing of the iris is also displayed to help visualise your iris design.
The iris is drawn at it’s minimum aperture position, with the aperture range shown as a grey circle or donut.
Pro users get two scale drawings. One at minimum aperture and one at maximum aperture.
On top of this, the blade array is overlaid. One of the blades is displayed darker to highlight the profile shape that has been calculated.
The housing is also displayed which (depending on scaling) shows the outer wall, blade/outer wall gap and blade pivot-point PCD. If you are using the Pro version, you will also see the pins, slot details and some dimensions on the drawing.
This feature is only available in the Pro version.
Once an iris has been calculated, the drawing can be downloaded as a DXF file for use in your CAD application of choice. DXF files are downloaded in zip format. Within the zip archive you’ll find the DXF drawing and a text file listing all the dimensions for reference.
The following elements are exported onto individual layers in the DXF file:
- Actuator Ring with pivot pin holes
- Blade profile with associated drive and pivot pin
- Maximum Aperture
- Minimum Aperture
- Pivot-point PCD (Pitch Circle Diameter)
- Slot Centre Lines
As only one blade is drawn, you may want to copy-rotate it to produce the rest of the blades in your array. You can do this easily using free CAD tools such as LibreCAD.
- A ‘leaf’ or lamella of material that pivots at one end.
- A group of blades laid out in a circular fashion to form an aperture.
- The body of the iris that contains the blade array and actuator plate.
- A hole or opening through which light travels.
- Pitch Circle Diameter.
- Outside Diameter.
- Drawing Exchange Format
- An iris diaphragm that is capable of closing down the aperture completely by use of two arrays of blades. Half mounted on one side of the actuator ring, and half on the other, thus achieving total blackout or closure.