Simply, megapixel count is meaningless. Good interferometer performance is a balance of image resolution (not pixel count), distortion, field flatness, fringe resolution and retrace errors (See blog post).
There is a myth that more pixels yield more resolution. Consider a pinhole camera. You could place a 11 Megapixel camera at the pinhole camera image plane, it is easy to see the resolution is limited by the pinhole size.
In an interferometer the situation is the similar. The image resolution is determined by the system stop size, the quality of the optics and if the camera has enough pixels, “enough” means more pixels do not increase resolution. Further the higher the resolution, the higher the imaging system numerical aperture, requiring tighter manufacturing and assembly tolerances. This means costs increase nonlinearly for little potential gain and potential degradation of distortion, field flatness, fringe resolution and retrace errors.
At ÄPRE we depend upon the theoretically proven parameter of Optical Transfer Function (amplitude and phase) to design our imaging systems and match the coherent optical imaging resolution to the camera, while balancing the important parameters which are rarely specified in commercial interferometers.
Case 1: Form and MSF Surface Measurement
Consider rough figuring and final figuring. The quicker you can start to converge to the final figure the more money you will save. This means you must accurately measure a far from perfect surface and determine the CNC polishing program to achieve final figure. This situation requires:
- High fringe resolution to measure as far from perfect as possible
- Low retrace errors , which occur at high fringe densities, to accurately measure an imperfect surface
- Low image distortion to tell the CNC polisher where to polish
So the surface correction requires three rarely specified parameters, not image resolution.
Now when documenting final figure and achieving MSF control you need:
- Image resolution to see the fine MSF detail
- Field Flatness to achieve the same MSF resolution across the part
- Fine adjustment focus control to optimize resolution. (see “ How to Focus an Interferometer ”)
Case 2: Optical System Alignment
Optical systems are aligned to low order Zernike components. Image resolution is not critical. Critical are:
- Fringe resolution to measure as far from perfect alignment as possible to begin adjustment
- Low retrace errors , which occur at high fringe densities, to accurately measure the farthest from alignment condition and accurately predict how much to correct to ultimately converge rapidly.
These examples demonstrate how balanced interferometer performance is best.
Not necessarily. The zoom system interferometers actually have their best resolution at ~1.2X with a 1-megapixel camera, resolution is set by the interferometer fixed-size field stop. This means as you zoom the imaged area is smaller but the resolution does not improve, and can get worse! Further most 4 inch aperture zoom system interferometers, no matter the date manufactured, achieve about 400 μm image resolution at best. A diffraction limited fixed magnification with a 1-megapixel camera, like ÄPRE’s General Purpose S100|SR, has 200 μm resolution ÄPRE’s High Performance S100|HR has 100 μm resolution image. So the well designed fixed imaging system always has better resolution, plus a balanced design of low distortion, flat field and low retrace errors; parameters not even conceived as important when zoom systems were designed.
The fixed imaging system offers more opportunity to control interferometer performance. But just because it has fixed imaging does not mean performance is optimized! Important performance parameters are:
- Distortion : So errors are imaged where they actually are
- Retrace Errors : So areas with lots of fringes don’t produce hidden errors
- Field Flatness : So the the focus is sharp across the image field of view
- Resolution : Optimizing the imaging to be diffraction limited
- Fringe Resolution : The maximum number of fringes measureable (high slopes)
The main differences are image and fringe resolution.
The HR is intended for the added measurement of mid-spatial frequencies and resolves more fringes (higher slopes) making it able to measure parts that deviate further from spherical or flat.
The SR is a high performance general purpose interferometer with 5X better performance in all aspects compared to other general purpose interferometers, but at the same price point.
ITF stands for Instrument Transfer Function and is a historically new term that is targeted to be a cousin to MTF of an optical imaging system. Being a new term there is little theoretical rigor to the parameter (i.e. refereed journal papers laying out the theoretical underpinning and uncertainties of measurement). Interferometers have a nonlinear response function. Even if out of focus the interferometer will report a surface height, whereas a camera (where MTF applies) just blurs out and no data seen if far out of focus. What is the measurement uncertainty of the reported ITF at a certain spatial frequency? What is the theoretical ITF and how does an instrument deviate from that value? ÄPRE simply optimizes our designs around the well characterized OTF or optical transfer function that has a strong theoretically underpinning.
Focus minimizes diffraction at edges and maximizes image resolution. An out of focus image will display diffraction at the edge of a part. Also artifacts like dust and scratches display diffraction ringing if out of focus degrading surface data. Just like a camera the higher the image resolution system the more critical focus becomes to achieve all the resolution possible.
Focus control is important. In ÄPRE REVEAL focus control provides both continuous motion and minute jogging steps. In many interferometers focus is achieved with a remote control providing only the ability to switch a motor on for continuous drive and direction control. Making it nearly impossible to find the critical focus position. With a high resolution interferometer visual focusing is not adequate and minimizing the edge diffraction by measuring it is the only way to sharply focus the image. See our application note “How to Focus an Interferometer” to achieve the best results.