reflecting gratings

Hello, can someone tell me how to model a reflecting grating? If you choose surface type as diffraction grating, a transmission grating will appear.

Ok. I have figured out.
In the sequential mode, both reflection and  transmission grating is represented by the surface type of "diffraction grating". The position of next surface will tell the computer that the grating is a reflection or a transmission. If the thickness of the grating is positive (+), the next surface will be in the right to the grating. The grating will be a transmission type. If the thickness of the grating is negative (-), the next surface will be in the left to the grating. The grating will be a reflection type.

Please speak in Chinese!