Consider a reflective grating so that mirror faces replace the slits. The incident light (plane wave) is now on the same side as the reflected light All of the previous discussion applies to the grating, but now we add one more degree of control to the light. As before, the order locations are determined by the grating pitch, the wavelength and the incident angle. The 0^th order simply follows the specular reflection relative to the normal of the grating surface $\left({\mathrm{\theta }}_{\mathrm{r}}=\mathrm{ }{-\mathrm{\theta }}_{\mathrm{i}}\right)$ .

Figure 2-1 Location of Diffraction Orders

Now, however, the groove faces can be made with a normal that is not collinear with the grating normal. The result being that the center of the Sinc² envelope is now decoupled from the 0^th order. Rather than being locked to the 0^th order the center of the envelope now points in the direction of the specular reflection from the individual groove faces. [i.e. $\left({\mathrm{\varphi }}_{\mathrm{r}}=\mathrm{ }{-\mathrm{\varphi }}_{\mathrm{i}}\right)$ relative to the groove face normal]

Figure 2-2 Location of Sinc Envelope Center

When the incident angle and the groove tilt are arranged so that the Sinc² envelope center (peak) lines up with an order, then the order is said to be “blazed”. A majority of the energy is directed into the “blazed” order.⁽¹⁾ When the envelope center falls between orders then energy is distributed into multiple orders. The consequence is that no one order receives a majority of the light. A condition often referred to as “off-blaze”.

Figure 2-3 Blaze Condition (2^nd Order)

The location of the 0^th order depends only on the incident angle but all other orders depend on incident angle, grating pitch and wavelength. Similarly the location of the Sinc envelope peak is only dependent on the incident angle and the tilt of the groove faces, but the nulls of the envelope are dependent on the width of the slit and wavelength. The consequence is that for all $\mathrm{m}\mathrm{ }\ne 0$ a given order is only blazed for one wavelength.

Notice that only the 0^th order and the envelope peak moves geometrically with changes in incident angle. All other orders are equally spaced from 0th order in the mapped space. This means that as we change the incident angle, if the groove tilt is not 0 then the non-zero orders move relative to the Sinc envelope peak. Therefore, in a “non-blaze” arrangement, changing the incident angle such that. when possible, one of the orders lines up with the Sinc envelope peak produces a “blaze” condition.