The Czerny-Turner configuration, uses a plane diffraction grating.
▲Figure 1-12:- Czerny-Turner configuration
In order to control the location of diffracted light, the grating should be illuminated by collimated light.
Incident light, diverging from an entrance slit is collimated by a first concave mirror. After diffraction from the grating, light is focused to an exit slit by a second concave mirror.
As a function of wavelength therefore, the grating is rotated to scan through a spectral range.
Stepping motor drives
It has been seen that in fixed angle monochromators, it is of question to move the diffraction grating through a range of angles in a repeatable manner. To this end, stepping motors are employed.
A stepper motor is a type of electric motor that moves in increments, or steps, rather than turning smoothly as a conventional motor does. The size of the increment is measured in degrees and can vary depending on the application. Typical increments are 0.9 or 1.8 degrees, with 400 or 200 increments thus representing a full circle. The speed of the motor is determined by the time delay between each incremental movement.
Inside the device, sets of coils produce magnetic fields that interact with the fields of permanent magnets. The coils are switched on and off in a specific sequence to cause the motor shaft to turn through the desired angle. The motor can operate in either direction.
When a current is passed through the coils of a stepper motor, the rotor shaft turns to a certain position and then stays there unless or until different coils are energized. Unlike a conventional motor, the stepper motor resists external torque applied to the shaft once the shaft has come to rest with current applied. This resistance is called holding torque.
Stepping motors, combined with gear systems or sine-bar mechanisms are used to provide high precision and highly repeatable monochromators.