Final Report: Fresnel Sapphire Split-Prism Spectrometer for Engine Emissions Diagnostics

EPA Contract Number: 68D01038
Title: Fresnel Sapphire Split-Prism Spectrometer for Engine Emissions Diagnostics
Investigators: Rentz, Julia
Small Business: OPTRA Inc.
EPA Contact: Manager, SBIR Program
Phase: I
Project Period: April 1, 2001 through September 1, 2001
Project Amount: $69,594
RFA: Small Business Innovation Research (SBIR) - Phase I (2001) RFA Text |  Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)


Optra, Inc., has proposed and executed a breadboard-scale demonstration of a novel, high-throughput prism spectrometer for vehicle emissions diagnostics. This system operates in the 3-5.5 micron spectral range, wherein lie molecular resonance bands for most of the constituents of interest for this application. The measurement principle is a passive molecular emission measurement of the relatively high-temperature emissions of a vehicle. Species that will be targeted by Phase II algorithm development include: CO2, CO, NO, and hydrocarbons. Other constituents that resonate in this spectral range include: CH4, nonmethane hydrocarbons, N2O, NH3, O3, and SO2; these also may be included in the algorithm set. The Phase II project also will investigate particulate matter quantification through a spectroscopic scattering measurement.

All of the technical objectives set for the Phase I effort were met. These included the design and fabrication of the prism as well as the design and construction of the breadboard-scale prism spectrometer incorporating an uncooled microbolometer detector array. The system was tested for noise-equivalent spectral radiance, spectral range, and spectral resolution. In addition, an ambient CO2 measurement was demonstrated. A spectral analysis expert who will be contributing to the Phase II development effort was identified. Finally, a system cost model for the final product was composed.

Summary/Accomplishments (Outputs/Outcomes):

As a result of the Phase I work, it has been established that this system is entirely feasible. The feasibility of the design and fabrication of the novel prism as well as the rest of the spectrometer has been demonstrated. To the best of Optra, Inc.'s knowledge, this is the first group to employ an uncooled microbolometer detector array in a dispersive spectrometer of any type.

Results of the performance evaluation agree quite well with the predictions of the analytical model. A noise-equivalent spectral radiance between 7'10-4 and 3'10-4 W/(cm2?ster? µm) over the entire 3-5.5 micron spectral range was measured; the predicted value from the radiometric calculation was 5.7'10-4 W/(cm2?ster?µm). The projected value for the final system improves to 4.5'10-6 W/(cm2?ster?µm) with the implementation of the microbolometer array, which has been optimized for this wavelength region, as well as an antireflective coating on the prism. This noise-equivalent spectral radiance measurement also inherently demonstrates the system's spectral range. A spectral resolution of just under 100 nm at 3 µm, located at the edge of the field, was measured; again, this is within close agreement with the optical model of the breadboard system. An ambient CO2 concentration of 0.2 percent was measured inside the laboratory, which is believed to be a realistic value.

The system cost model forecasts a reasonable selling price, especially when compared to bench-top sensor suites required to measure all of the molecular species mentioned above.


In conclusion, it is believed that the feasibility of this metrology concept clearly has been demonstrated. The measured performance strongly supports the analytical model. The Phase I breadboard design and fabrication support the concept of a small, portable, ruggedized gas sensor system that will be extremely useful in automotive emissions diagnostic applications. Phase II plans include a hundredfold increase in sensitivity, extending detection limits into the parts-per-million range. The system resolution will be on the order of 32 nm, allowing for adequate discrimination of molecular species. In light of the Phase I success and a cooperative effort with the spectral analysis expert, it is believed that the Fresnel sapphire prism spectrometer is a strong candidate for Phase II consideration. When coupled with Optra, Inc.'s experience in building compact, ruggedized spectrometer systems, it is anticipated that a marketable product will result from a Phase II effort.

In parallel with the technical effort, a considerable amount of commercial preparation has been accomplished for the Phase II and future manufacturing activities with regard to this system. At this point, Optra, Inc., is in communication with three separate large companies that are in the vehicle emissions monitoring systems industry and are interested in contributing to the commercial future of this technology through a partnership. Potential applications for this system, aside from the obvious automotive venue, include emissions monitoring of other nonregulated engines (lawn and construction equipment, diesel trucks, recreational vehicles, and aquatic vehicles), stack monitoring, process control, and combustion monitoring.

Supplemental Keywords:

prism spectrometer, stationary spectrometer, vehicle emissions monitor, on-board emissions monitor, remote emissions monitor., RFA, Scientific Discipline, Air, particulate matter, air toxics, Environmental Chemistry, Chemistry, Civil/Environmental Engineering, mobile sources, Engineering, Engineering, Chemistry, & Physics, Environmental Engineering, monitoring, particulates, Nitrogen Oxides, hydrocarbon, spectrometer, engines, fresnel sapphire, air pollution, emissions, split prism , diagnostics, carbon dioxide, carbon monoxide, hydrocarbons, split-prism spectrometer