Solution (1)] Shomei problem

This is the answer to my previous post.

 

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Answer, 3

Large ring lighting (low angle placement)

 

It is.

Why does the rubbed area turn black?

Camera lens caps are,

Even though the normal surface is a little rough,

The rubbed area is smooth.

 

Going by "scattered light" and "direct light."

The coarse areas are light from low angles (low angles).

Hit the workpiece and scatter it,

Scattered light enters the camera.

Therefore, the overall image is bright.

If you look closely, it is speckled with black and white.

 

On the other hand, the areas that are worn and slippery,

Direct light from a low angle,

without reaching the camera positioned at 90 degrees to the workpiece.

It will be left out.

(If the angle of incidence is 30 degrees, the angle of reflection is also 30 degrees.)

 

So the rubbed area is black because no light reaches it.

By the way, it is easy to understand if you think of "slippery" as a kind of "mirror".