Newport Oriel MS260i Series, Руководство пользователя

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M74050
1/4M IMAGING SPECTROGRAPHS
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19.3 GRATING ORDER
It is important to note the sign of “m” is given by either form of the grating equation and can be positive
or negative. In a spectrograph, the angles I and D are determined by the rotational position of the
grating. We use the sign convention that all angles that are counter clockwise from the grating normal
are positive, and all angles that are clockwise to the grating are negative. See Fig. 4. The incident
light, diffracted light and grating rotation can be at positive or negative angles depending on which
side of the grating normal they are. The half angle is always regarded as positive.
If the angle D is equal to I and of opposite sign, then the grating angle and order are zero, and the
light is simply being reflected. If the grating angle is positive then the order is positive (m=1), if the
grating angle is negative, then the order is negative (m = -1). The half angle of the Cornerstone 130
is 5.1 and the orders are positive. With Oriel’s MS260i spectrographs, the half angle is 11.83 and
the orders are positive.
The grating equation is also satisfied for wavelengths in higher orders, when | m | is >1. Therefore λ2
= λ1/2 for m = ±2, λ3 = λ1/3 for m = ±3, etc. The wavelength λ2 is in the second order and λ3 is in
the third order, etc. Again, this concept is illustrated on the previous page.
Usually only the first order, positive or negative, is desired. The other wavelengths in higher orders
may need to be blocked. The input spectrum and detector sensitivity determine whether order sorting
or blocking filters are needed.
Figure 41: Sign Convention for the Angle of Incidence, Angle of Diffraction and Grating Angle