Main principle behind imaging
The main idea behind spectral imaging of objects may be described as follows: Firstly, light from the object must be focused by a lens or mirror to form an image at the spectrographs entrance slit plane. The resulting spectrogram is the intensity distribution as a function of wavelength and position along the slit. The diffracted slit image contains both spectral and spatial information along a thin track of the object.
Secondly, in order to obtain the object's full spatial extent, it is necessary to sample the whole object. This requires the use of a high resolution rotary element. The whole idea is to record spectrograms for each track of the object as the image at the entrance plane is moved across the slit. A front surface mirror in front of the focusing element may be used in connection to a high resolution stepper motor. Another approach would simply be to rotate/move the whole instrument itself or the object on a conveyor belt etc. In fact, the use of a mirror enables us to sample target objects that are static or moving relatively to the instrument by rotating or keeping the mirror fixed, respectively.
Figure 2 shows our choice of experimental design. Please note that the use of lenses instead of mirrors is no limitation in implementation of the above idea. The use of a transmitting grating combined with a prism increases the resolution especially in the blue part of the spectrum. In addition, each optical element is aligned along the optical axis.
Figure 2. A 3 dimensional optical diagram illustrating the main principle. L1 is front lens, S1 entrance slit, L2 collector lens, G grating, P prism, L3 camera lens and CCD imaging detector. The optical axis is parallel with the X-axis of the XYZ-coordinate system. The slit is located parallel to the Y-axis. q is the angle between the mirror's normal and the optical axis. The detector is located in the lS - plane which is parallel to the ZY- plane.