A Prism Delivers ~95% Light Throughput from ~400 to 1000 nm
Light throughput (efficiency) compared•Prism: close to 95% from ~400 to ~1000 nm. •Diffraction grating: Varies across the spectrum. Maximum efficiency is at a single “blaze wavelength”Polarization:•Prism. Polarization independent •Gratings: Light throughput can be highly polarization dependentHigh order diffraction•Prism: NONE: all wavelengths are refracted to the detector•Diffraction grating: can present many ordersBenefit: A prism is significantly more sensitive than a diffraction grating over an extended wavelength range - even when light is polarized.
Prism Efficiency: Of all spectrometer characteristics light throughput (efficiency) is easily the most most critical. Sensitivity is directly dependent on light throughput. If you cannot find a signal resolution is meaningless. In the case of a prism there are significant advantages:•Internal transmittance from ~400 to 1000 nm is close to 100%•Fresnel reflection off the prism faces can be reduced to near zero with anti-reflection coatings. See the profiles in the figure opposite.Diffraction Gratings Efficiency:Diffraction gratings efficiency is not so fortunate. Gratings are available either “ruled” or “holographic.” For maximum efficiency the groove profile has to resemble a right triangle. This correct triangular groove profile produces a "blazed" grating, where the angles are selected to produce maximum efficiency at just one wavelength At all other wavelengths efficiency drops-off, sometimes precipitously. Efficiency can also be highly dependent on polarizationLow groove density holographic gratings (<600 g/mm) are very difficult to “blaze” at visible wavelengths, consequently, their maximum efficiency is significantly less than that of ruled gratings. Efficiency is directly affected by the number of diffraction orders because a grating blazed in first order is also blazed in 2nd, 3rd, 4th... orders. The net result is that higher orders drain light away from the spectral detector. View the diffraction grating movie below to see examples