MCS260B
CORNERSTONE 260B MONOCHROMATORS
72
18.3 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 monochromator, 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 Cornerstone 260 monochromators, 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 411: Sign Convention for the Angle of Incidence, Angle of Diffraction and Grating
Angle
