The Optics Passed Inspection. So Why Did the System Still Fail?
Optical
9/28/20262 min read
The optical components arrived.
The dimensions were within tolerance.
The surface quality passed inspection.
The coating was tested.
Everything looked correct.
But after assembly, the optical system still didn't perform as expected.
So what went wrong?
This situation is more common than it may seem.
Passing an incoming inspection does not always mean that an optical component will perform perfectly in the final system.
Here are several things worth checking.
1. The Specification May Be Correct — But Not Complete
A drawing may specify:
Diameter
Thickness
Material
Surface quality
Coating
But what about centring, wedge, clear aperture, or actual operating wavelength?
A component can meet the listed requirements while still having characteristics that affect system performance.
This is why the optical specification should be connected to the actual application.
2. The Coating May Be “Correct” — But Not for the Actual Conditions
An optic may be described simply as:
AR coated
But AR coating performance depends on wavelength, bandwidth, angle of incidence, polarization, and environmental requirements.
If the production system operates differently from the original test conditions, the coating may not perform as expected.
3. Mounting Can Change Optical Performance
Sometimes the problem isn't the optic itself.
A lens can be manufactured correctly but become distorted when it is mounted with excessive mechanical stress.
This can affect:
Wavefront
Centration
Image quality
Alignment
For precision optical systems, the interaction between the optic and its mount matters.
4. “Within Tolerance” Does Not Mean “Perfectly Identical”
Suppose a drawing allows a certain tolerance range.
Two components can both pass inspection while having slightly different actual values.
One may be near the lower limit.
Another may be near the upper limit.
Both are technically acceptable.
But when several tolerances accumulate across an optical assembly, the final system performance can still be affected.
This is where tolerance stack-up becomes important.
5. The Problem May Appear Only After Assembly
This is one of the most difficult situations for both engineers and purchasing teams.
The supplier says:
“The parts passed inspection.”
The assembly team says:
“But the system doesn't work.”
Both statements can be true.
The real question is not simply:
“Is the component qualified?”
It is:
“Is the component qualified for this optical system?”
A Better Way to Investigate the Problem
When an optical system underperforms, don't immediately assume the component is defective.
Check the entire chain:
Drawing → Material → Manufacturing → Coating → Inspection → Mounting → Alignment → Operating Conditions
A problem at any stage can affect the final result.
The Takeaway
Good optical manufacturing is not only about making a component that passes inspection.
It is about understanding which specifications actually matter to the application.
The best optical specification is not necessarily the tightest one.
It is the one that controls the factors that can actually affect system performance.
At Positive Optics, we support custom optical components based on drawings and application requirements, including lenses, windows, filters, prisms, and mirrors.
When a component is difficult to specify, the application details can be just as important as the drawing itself.