The PARISS® Hyperspectral Imaging System A Comparison of Prism Versus Diffraction Grating Efficiency Light throughput (efficiency) compared • Prism: close to 100% from ~390 to ~1000 nm.  • Diffraction grating: Varies across the spectrum. Maximum efficiency is at a single “blaze wavelength” High order diffraction • Prism: NONE: all wavelengths are refracted to the detector • Diffraction grating: can present many orders Benefit: A prism is significantly more sensitive than a diffraction grating over an extended wavelength range. 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 ~390 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 as “ruled” and “holographic.” For maximum efficiency the groove profile has to resemble a right triangle. This 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. Low groove density holographic gratings (<600 g/mm) are very difficult to “blaze”, 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 efficiency in first order where the detector is located. View this movie to see how diffraction gratings present multiple orders.  All light that is sent into higher orders is lost to the detector Copyright © LightForm Inc, 2011