Cold shields are used to protect image sensors from stray radiation particularly in the 8-13 micron wavelength range. They are commonly used at cryogenic temperatures which increases the signal to noise ratio in thermal imaging systems. To enhance the system performance still further cold shields are often gold coated and can reflect 99% of infrared energy. They were originally used by the military in the late seventies.
Since diamond machined aluminium cold shields can be machined to a highly reflective surface with integral mounting and location points they are a cost effective and lightweight solution to enhance the performance of infrared systems.
These are components that have two different radii at 90 degrees to each other and are often used to correct astigmatism. They can be machined by rotating the component the required distance from the axis of rotation. The second radius is then machined at right angles to it.
If these radii are too large it becomes impractical to swing the component at the required distance from the centre of rotation. It is now possible to use a technique called free-form machining using the slow tool server.
This technique utilises the machine’s capability of controlling the angular position of the C axis. A program is written that generates an angular position of C and a corresponding position in Z. This is repeated for every position of X. This technique enables the manufacture of optics that previously would have been impossible.
This group of components are machined by rotating the diamond tool at a precise radius at high speed. The component is then moved at an angle through the rotating tool path. This is known as R-theta fly cutting.
The resulting surface produces two foci along the optical path. This is often used to produce a “virtual image” or move the position of a focus.
Aspheric optics are rotationally symmetrical. This group of optics include ellipsoids, parabola, hyperbola etc. The radius of curvature varies radially from the centre of rotation. Diamond machined aspheric optics do not suffer the spherical aberrations found in traditionally polished spherical optics. This allows optical designers to reduce the number of elements needed in the optical system. They are increasingly being used in camera phones, contact lenses and, in fact, all facets of the photonics industry.
Aspheric surfaces are historically defined by the equation:
Z = sag of surface parallel to the optical axis s = radial distance from the optical axis C = curvature, inverse of radius k = conic constant A4, A6, A8 = 4th, 6th, 8th… order aspheric terms
The use of single point diamond turning for aspheric lenses, mirrors and moulds is ideally suited to this rotationally symmetrical surface form. SMT have been diamond turning these for many years. We use the best single crystal diamonds. These tools are manufactured and inspected to ensure that they have a cutting edge roundness of less than 30nm.
There are many advantages in diamond machined aspheric optical elements.
The performance of optical systems is greatly increased.
The number of elements can be reduced within an optical system.
Surface finishes <2nm
Integral optical alignment surfaces are possible.
These geometric shapes are ideally suited to single point diamond turning.
The machining configuration for these types of components is similar to a standard lathe.
However, our diamond machines are able to machine to unparalleled accuracies.
Our slideways have a straightness of better than 200nm over
the full travel.
The air bearing spindles have an axial and radial accuracy of better
The CNC feedback resolution is 0.034nm.
These ultra accurate machines, along with the many years of diamond machining experience our engineers have, enable SMT to supply some of the most accurate components available.
In today’s digital world the need for high speed rotating and oscillating scanning optics is ever increasing. The use of lightweight materials in these optical systems is fundamental.
SMT have been involved in manufacturing these systems for 20 years. Surface figure errors of less than 30nm rms are achievable. Optical designers are always looking to reduce “scatter”. If the correct substrate material is chosen surface finishes of as little as 20nm rms can be maintained. This results in minimal surface scatter.
The optical performance of these systems also relies on the mechanical dimensional tolerances. When diamond machined reference faces, diameters and bores are incorporated into the optical design the full potential of diamond machined optical components can be realised.
The optical engineers at Symons Mirror Technology have been diamond machining high precision flat surfaces for more than 20 years, after developing the process for diamond machining 14” aluminium hard disk drives.
We can machine plano mirror surfaces from Ø1mm to Ø500mm. Our expertise and experience has enabled us to develop processes to machine thin (0.30mm thick) mirrors flat to a few wavelengths. We have machined 2mm thick mirrors flat to λ/10 each side with a surface finish of 2.5nm rms.
Not all of these precise flat surfaces are used as optical components. We regularly machine panels 700mm x 150mm x 2mm thick from 99.999% pure aluminium to a surface finish of 10nm rms. These surfaces are used to grow an inorganic membrane used in the medical industry.
Because of our ability to machine up to Ø500mm we are able to provide our customers with lightweight precision plates on which to mount their optical assemblies and test fixtures.