In our previous article we discussed how to read optical drawings. Here we will review one key parameter in detail - "Surface texture". In the example drawing below you can see three marks related to surface texture. Two identical marks for the front face and back face. And one for side face.
The ISO standard allows to define this parameter more exactly. In many cases this is level of detail may not be required, but in some applications surface texture is crucial. An optical engineering running a ray-tracing program like Zemax will determine the allowable tolerances to achieve the desired performance. But if these tolerances are not called out in production drawings problems can occur.
The meaning of these letters is as follows:
a is type of roughness measurement. Possible conventional signs are Rq (Root mean square (RMS) height variance) and PSD (Power spectral density (PSD) of surface height).
b is the value of Rq or PSD. Unit of measurement is micrometer (µm).
c is the method and level of the surface treatment. Possible conventional signs are G, P1, P2, P3, P4. Ground method is used if matte surface needed. P1, P2, P3, P4 methods are used for getting of polished surfaces. P level defines quality of polishing. These levels are described in the table.
|Level of the polishing||Number of the defects||Approximate roughness (in nm)|
|P1||80 < N < 400||< 8|
|P2||16 < N < 80||< 4|
|P3||3 < N < 16||< 2|
|P4||N < 3||< 1|
d\e parameter defines conditions for surface sampling. d is the value of minimum resolution for scanning of the profile surface. e is the value of the scan length. Here it is necessary be very careful with units of measurement. As opposed to Rq and PSD values which are measured in micrometer these values are measured in millimeters (mm).
As always a best practice is to ensure you and your lens manufacturing vendor are using the same definitions related to lens quality.