M77781A
MS257
™ USB/RS232
MONOCHROMATOR AND SPECTROGRAPH
65
Blaze
The blaze wavelength is the wavelength at which the most energy is dispersed. Gratings can be blazed
by making the groove faces at a specific angle to the grating surface. The blaze wavelength is achieved
by cutting a groove with an angle such that the facet normal bisects the angle between the incident and
diffracted rays (see Figure 23). Blaze is important because it allows a grating to be chosen for optimal
efficiency in a given wavelength range. As a general rule, one should select a grating with a blaze
wavelength at the extreme short wavelength end of the region of interest. This is because the majority of
sources and detectors are more efficient at longer wavelengths. The selection of a grating blazed at a
short wavelength compensates for the lack of radiation and detector sensitivity in that wavelength region.
Efficiency
The greatest efficiency one can expect for a grating at the blaze wavelength is about 80%. Thus 80% of
the incoming radiation at that wavelength is diffracted into the 1st order. The rest is lost to the remaining
orders (both positive and negative), absorption and scatter. Light of half the blaze wavelength, which is
diffracted with a low efficiency in the first order, will often have a greater efficiency in the second order
than in the first. For example, a grating with a blaze wavelength at 500 nm has an efficiency of only 10%
for first order 250 nm light. In the second order, the efficiency of the 250 nm light is 30%.
Sinusoidally grooved gratings have very flat efficiency curves across the operating wavelength range,
usually substantially below 50%. Such gratings are regarded as unblazed.
