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8. Allowable Flaws
What is a flaw in our context?

It is an imperfection in optical parts (e.g. windows, mirrors, prisms, lenses).

At a first glance, optical surfaces are intended to be ideally smooth and clean, and optical glass is intended to be ideally homogeneous. Thoroughly manufactured parts indeed don't have visible flaws; simply beautiful.

But free-of-failure does always mean maximum cost ... undesireable in industrial processes.

So we will first discuss the kinds of possible flaws, second the position in imaging ray bundles where they show maximum or minimum interference.

8.1 Types of Flaws
Looking at origin and location of flaws, we notice  fingerprints - dust and dirt - scratch and dig - and glass enclosures.
Though enclosures are extremely rare in optical grade glass, I got a fine image which I'd like to show:

fig.8: flaws that can be enclosed ... (111 kByte)

In real life, all kinds of dirt and mechanical damage will prevail.
But no matter which reason a flaw might have, in our context the optical properties are important:

a) Amplitude Flaws

These flaws absorb and/or reflect light   and/or they cause stray light.

Examples are spots of paint, grains of dust, unmolten grains enclosed in glass.

b) Phase Flaws

These flaws refract light.

Examples are schlieren   and curvature- or planity failures like bulges.
You know schlieren? - They are caused by local changes in density (i.e. refractive index). Schlieren get visible in critical dark-field illumination.

c) Intermediate Flaws

Classifying into "amplitude" and "phase" flaws is difficult. The very most flaws show amplitude- as well as phase effects. So the above classification is related to the prevailing effect only.

But typically both effects are shown by bubble, scratch and dig, fingerprint.

8.2 Influence of Flaws
Looking at imaging ray bundles, we note that the same flaws act extremly different ... depending on where they are positioned in the optical path (para. 1, fig. 1).

A) Object Plane   O   and Image Plane   O' :

Amplitude flaws show maximum effect. They are fully visible, i.e. reproduced in the image.

Phase Flaws have nearly no effect.

B) Principal Plane   H :

Phase flaws show their maximum effect. They destroy image definition (MTF). This is used in "fog" or "soft focus" filters.
And they introduce distortions of any kind.

Amplitude flaws have nearly no effect; depending on area fill factor, they just diminish image luminous incidance.
Though stray light can occur which would reduce image contrast and hence, MTF.

C) In-between Positions:

You'll get intermediate effects, mixing the influences described in A) and B).
Proceeding from   O   or from   O'   to   H ,
- amplitude flaws get more and more blurred, while
- phase flaws gain more and more influence.

For quantitative results you will need to conduct a series of tests. Or you must ask your lens designer to perform a non-sequential ray tracing session with statistical analysis.

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Last modified Sept 12th, 2003 00:25