​A Johann curved optic is a flat plate prepared to be curved by pressing it onto a holder presenting a cylindrical surface machined at a radius that is the double of the radius of the Rowland circle.

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The Rowland circle inscribes the triangle including :

• the X ray source,

• the tangent point in the center of the crystal,

• and the detector.

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Crystal curved as Johann produces a semi focusing on the Rowland circle along the direction of the X-Ray beam, so called meridional direction.

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Only a small area from the center of the optics produces a focusing on the Rowland circle.

Other areas are focusing outside the Rowland circle.

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The X Ray beam is diverging in the perpendicular direction, so called sagittal direction.

 

The surface of a Johann curved doesn’t diffract at the Bragg’s angle on all the surface.

 

Its reflectivity doesn’t reach the intensity of a Johansson curved plate.

 

Miscut orientations are also available.​​​​​​​

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