Spectrophotometer analysis
The method of scientific measurement, that by it can measure the intensity of passed, absorbed, and reflected light, by different materials, is called spectroscopy. And a device that uses this technique to measure the intensity of light, called a spectrophotometer.
In other words, spectrophotometer is a quantitative method, which is used to analysis the interaction of light with the atoms of the target material. Since the passed or absorbed spectrum of a material (such as fingerprints) is limited to only one material (or combination), we can use this technique to study and identify the required materials.
When light radiates to material, the specific energy (or colors) of light may be absorbed by the material, and other colors may pass or reflect. Like prism, that as shown in figure 1, incident light to it, be converted to different lights. It is for that reason that the refractive index of prism is different for different wavelengths (different colors).


Figure 1: White incident light to the prism convert to the different colored lights.

Similarly, for the total range of electromagnetic waves can be used from different prisms or diffraction gratings.
Figure 2 shows schematically one of the optical arrangement, using the prism. Light source L, after colliding with the lens reaches to the slit, which is located on the focal plane of collimator lens. Collimated passing light, then hit to the prism, and is distributed under an angle, which is dependent on to the wavelength. Lens forms ultimately the obtained image of the passing light through the slit on the lens focal plane.
The location x (λ), related to this image, is a function of wavelength. Linear scattering depends on spectrum scattering (which is related to the type of the used prism) and lens focal length.


Figure 2: separating the spectral lines using the prism

Today, as shown in Figure 3, instead of the prism is used commonly from diffraction grating to separating the spectral lines. In these devices, lenses give their place to two mirrors.



Figure 3: separating the spectral lines with a grating

Should be gave in mind that the range of light is visible more wider than the range of colored light. This means that, in addition to colored light, light has a wide range of wavelengths, including gamma rays, X-rays, ultraviolet region, infrared region, micro waves and radio waves. As a result, the light that we see is a combination of different colors, which are not absorbed by the sample. If the material there is no absorption, it may be white or colorless. In other words, the type of samples paint is due to a special light, which sample absorb or pass.
Figure 4 shows, one of the spectrophotometric device, which is used for UV, VISIBLE and IR regions. In figures 5 and 6, it can be seen, some of measured spectra by this device, for silver and gold metallic layer.


Figure 4: a sample of spectrophotometer device, manufactured by shimadzu company.


Figure 5: The reflective spectrum of silver metallic layer, measured by a spectrophotometer

Figure 6: The reflective spectrum of gold metallic layer, measured by a spectrophotometer