The first one developed incorporates a prism, using the property of refraction, to disperse the light into the constituent colors. The second type has a grating, a series of very finely ruled, evenly spaced, lines that uses the optical property of diffraction to disperse the colors. The primary advantage of the diffraction grating is its linearity, which is of particular importance when trying to measure the wavelengths of lines. Hence, diffraction gratings are used more extensively in research today. However, the broadening of the blue end of the spectrum, and the blue sensitivity of the early photographic emulsions have produced comparison spectra that usually range from 3500 to 5000 angstroms, which is a problem when making comparisons visually.
All eyes receive slightly varying wavelength ranges, but usually detect from violet through red.
But the spectral sensitivity of the human eye is not evenly distributed over the full color range. The intensity of the object also comes into play. There are also two types of gratings, transmission and reflection. The former is made on a glass or plastic substrate where the light passes through the optic diffracting the light on the opposite side. The latter has its lines ruled on glass and is then aluminized, just as a telescope mirror is, and diffracts and reflects the light energy at the same time.