This article talks about Fresnel lens basics and cutting-edge measurement techniques for Fresnel optics - conventional & advanced measurement techniques: LUPHOScan & the PGI Optics.
What is a Fresnel lens?
A fresnel lens an optical lens with a structured surface. It is composed of several concentric zones and constructed from its corresponding a sphere. Each group is an annual piece of the aspheric surface
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Difference between Asphero Diffractive lens & Fresnel lens
People are often confused about the Fresnel lens and aspherical diffraction lens. Aspheric diffractive also has several concentric zones and constructed from the corresponding asphere. So how to distinguish between them and what is difference.
1. The first difference is the underlying surface for the typical Asphero diffracted lens. Typically, it is a spherical shape. For the Fresnel lens it is often just a flat and a line surface.
Fig1: Asphero Diffractive lens Fig2: Fresnel lens
2. The main difference is the steps. So, for the Asphero diffracted lens, it has small steps typically only few micrometres but for the Fresnel lens, it has a relatively is quite large steps. A step height can be up to about several hundred micrometer and very easily visible.
3. For Asphero diffractive lenses, the steps are often a constant height, but for Fresnel lens it has different type of designs. One type is the constant step height with various pitch, another is a constant pitch with various step heights and the third one is a combination of these two.
Fig3: Asphero Diffractive lens
Fig4: Fresnel lens
4. For Asphero diffractive lens, the accuracy is higher than the Fresnel lens and the Fresnel lens are usually made of glass or plastic
Fresnel Lens Applications
Fresnel lenses have a wide range of applications. Its size is different and can be varied in size from very large to very small - from meter to millimetre. We can be separated into three different type of applications.
- Light collimation
- Light collection
- Magnification
Fresnel lenses are used for the lighthouse and for the different type of ultramobile lights such as headlights, traffic lights and the other type of lights, camera lenses, solar PV cells in the solar field, projection televisions, magnifiers and overhead projectors.
Why Fresnel lens? (Advantages of Fresnel Lens)
- Fresnel lenses have a lot of benefits. They can make the optical system much smaller, more compact, much thinner, and much less weight and it is often made of glass or plastic.
- Lower cost
- Improved optical performance or efficiency (Ex: In solar field its transmitted solar radiation is improved compared to thick ordinary lenses.)
Measurement of Diffractive/Fresnel lens – Taylor Hobson’s measurement techniques
Taylor Hobson has two distinct types of techniques for measuring the Fresnel lens.
- LUPHOScan (non-contact 3D)
- PGI Optics (contact)
LUPHOScan instruments measure the whole surface form, the
Form Talysurf PGI instruments measure the 2d form error, geometric parameters.
How does the LUPHOScan Measures Fresnel lenses?
LUPHOScan uses a multi wavelength interferometry technique therefore it has a quite large absolute minimum range. So, this is very beneficial for measuring this kind of optical lens with large steps and high expression high aspect ratio of steps.
Probe movement
The LUPHOScan probe moves from the center to the edge horizontally, the probe changes its tilt angle as it moves from one zone to another zone and makes sure the probe is vertical to the optical surface. As a result, it can collect the surface data for each zone and therefore measure the whole surface form in 3D.
Probe tilt angle
The probe is always perpendicular to the surface and follows the slope in each zone and can measure the required optic active areas, the shadow areas not measured.
Max slope (Geometric limits)
To increase the surface slope measurement range, we developed the Fresnel probe and use this probe to avoid touching the surface when measuring the Fresnel lens, its maximum slope can increase to 55 degrees.
Characteristics
- Measure any types of Fresnel design, including constant step heights, constant pitch and mixed types, because it has a large absolute environment range.
- It can measure step heights up to ± 600 µm
- Measure any types of underlying shapes flat, spheric, aspheric, or freeform
- Various shapes of Fresnel Optics like segmented, D-cut or annular Fresnel
Measurement and Analysis of Fresnel lenses - PGI Optics, AAU and Talymap Measurements
- 2D measurements, multiple traces
- Stylus selection – material, Fresnel lens design
- 10° chisel stylus and diamond stylus
- Measurability
- Form error (2D)
- Geometric parameter measurements
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Analysis of Fresnel Lens
- Form error - AAU (Aspheric Analysis Utility)
- Geometric parameters (Talymap, TalyContour and AAU): Step heights, Zone width, Draft angle, Facet slope, Shape of edges and pockets (blue), Others.
Conclusion: LUPHOScan - fast, non-contact, accurate 3d surface form error measurements for various fresnel optics. PGI Optics - user define multiple trace 2D form error, geometric parameters. For more details about the Taylor hobson measurement techniques, luphoscan and PGI Optics products