Lens test is a crucial part of selecting the right lens manufacturing vendor. The process is a multi-step journey. To start, you will need detailed production drawings to ensure the vendor understands what you require and the tolerances involved. As you get quotes, you may find qualified suppliers with good pricing and communication. This, however, can only get you so far. Some vendors will quote a price for your lens without ever discussing how they plan to test it, or whether they even have the ability to perform a proper lens test.
In our work, we’ve encountered suppliers unable to make a simple focal length test for a lens they produce. Moreover, they were unable to provide a photo of a USAF test target. Without testing, knowing the lens you designed is the same lens that was delivered is impossible.
So before you agree to a production contract, make sure you also agree on how the lens will be tested.
The testing you will need will depend on the type of lens you are sourcing, and the precision required in your application.
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In most cases, you don’t need all the tests listed. For example, in many applications a photo of a USAF target is good enough. The more expensive and time consuming MTF test may not be necessary.
As a starting point for a conversation, here are some important and testable lens parameters.
Lens element documentation and lens test data
- Glass quality (Nd, Vd)
- Measured Diameter
- Measured Thickness
- Measured Radius
- Measured Surface Sag
- Surface Irregularity – Interferometer (Waves, PV)
- Surface Irregularity – Profilometer (Microns, PV)
- Measured clear aperture
- Roughness (RMS)
- Coating confirmation and specification
For aspherical elements, additional details are important including:
- Vertex radius
- Asphere figure error (PV)
- Typical slope error
- Centering (Beam deviation)
This level of detail is likely ‘over kill’ for many applications. With a multi elements lens, you probably care more about ‘does the lens work?’ than ‘ did the surface quality on element 2 match the drawing?’
Below is a list of common and less common tests to measure the optical quality of completed lens assemblies. As with the list above, they range from easy to test to expensive and complex to test.
For a laser collimation lens
- Focal length
F Theta scan lens
- Focal length
- Back focal length
- Blur spot size
- Distortion
- Field shape
- MTF across entire field ( more expensive equipment required
- Resolution
An objective for imaging
- Focal length
- Back focal length
- Blur spot size
- Distortion
- Chromatic aberration
- Field curvature
- Stray light
- Aerial image resolution
- MTF across field of view
Double Gaussian lens
- Magnification
- Distortion
- Thickness
- Conjugate distance
- On-axis MTF
- Off-axis MTF
- Resolution measurements (USAF test)*
There is no ‘right’ test or set of tests that will apply to every lens type and application. It is essential to review your supplier’s capabilities for lens testing- the tests they do in house, the tests they need to outsource, and to discuss their suggestions for the best tests for your lens. This conversation will teach you as much about your vendor as the pictures on their website or the price they quote in their proposal. Read more about our lens testing services here.
