White light schlieren optics using bacteriorhodopsin as an adaptive image grid

Applied optics. JUL 01 1997 v 36 n 19 Page: 4446

R. E. Peale[1], A. B. Ruffin[2], and J. E. Donahue[1]

[1] Department of Physics, University of Central Florida, Orlando, FL 32816

[2] Department of Applied Physics, University of Michigan, Ann Arbor, MI 48109

Abstract



A schlieren apparatus using a bacteriorhodopsin film as an adaptive image grid with white light illumination is demonstrated for the first time. Relevant spectral properties of the film are characterized. Potential applications include a single-ended schlieren system for gas-leak detection.

Introduction

Remote imaging of leaks is needed at large industrial complexes including, for example, the space shuttle launch facilities at Kennedy Space Center. Backscatter/Absorption Gas Imaging (BAGI) [1] is useless for gases which lack strong IR absorption, such as O2, H2, He, and N2. BAGI and other laser techniques, e.g. Raman imaging, may be dangerous for personnel and equipment. A system using white light, ideally ambient light, is preferred.

The schlieren method is well established for imaging gas plumes[2]. The test region is sandwiched between a source grid and its negative, onto which the source grid is imaged using appropriate optics. Only rays deflected by a plume in the test region may pass the image grid. These rays are used to image the plume. By thus eliminating the uninteresting background, the signal-to-noise is greatly enhanced, and otherwise invisible plumes appear in high-contrast detail.

All usual schlieren set-ups sandwich the test region between optics, and this limits the field of view to industrially uninteresting scales. Peale and Summers[3] showed that the optics on one side can be replaced by a high contrast pattern on flexible reflecting cloth. Here, a zoom lens images the pattern onto its negative. This scheme can be scaled to large fields of view with only modest cost increases, in principle. An obvious extension of this idea is to substitute a naturally occurring high contrast scene for the pattern-cloth combination, thereby creating an essentially single-ended schlieren system. The negative can be photographic, but exposure, development, reinsertion, and realignment are time consuming operations, during which the scene and its illumination may change.

Erasable photochromic films offer an attractive alternative to photographic creation of the image grid. Films made using bacteriorhodopsin (BR) from the purple membrane of Halobacterium Halobium are promising because larger absorbance changes and more optical cycles (106) can be realized than with man-made photochromic chemicals[4]. The BR ground state (bR state) has a strong absorption in the yellow-green. In the simplest model, absorption by this band pumps BR into its long-lived excited M-state, which absorbs in the blue. Thus, a negative of a scene in blue light can be created in a BR film using yellow-green light.

Downie demonstrated a BR schlieren apparatus using blue and green laser light, but an attempt with white light and color filters was unsuccessful[5]. We demonstrate in this paper successful white-light BR schlieren with performance a factor of only 2 times less than a traditional schlieren apparatus. The BR system is still sufficiently sensitive to observe heat waves generated by the human body. This result is a first step toward realizing a truly single-ended remote imaging system for leaks using ambient light. A variety of laboratory applications can also be envisioned.

Though the general spectral properties of bacteriorhodopsin films are well established, a presentation of relevant optical behavior using the common low-cost sources and filters used here is of value, since a previous effort using different optics reported failure[5]. A brief summary of the apparatus used is given in the experiment section. The results section gives the relevant spectral properties of the BR film and finally schlieren results.

Experiment



The bacteriorhodopsin film was obtained from Bend Research and had a nominal optical density of 2.8 at 570 nm (absorbance of 6.5). The wild-type BR was incased in polyvinyl alcohol to form a ~100 um film. The nominal M-state lifetime was 1 to 5 s; a half life of about 10 s was observed for the recovery of the ground state absorption in the dark.

Transmission spectra were collected using a diverging incandescent source, a 400 to 700 nm variable interference filter having 20 nm bandwidth, and a large area photovoltaic detector. Low intensity measurements were performed with this set-up by placing the BR film just in front of the detector where the beam spot size is about 2 cm. Here, no time dependence is observed in the transmission. Higher intensity measurements were performed by placing the film just after the interference filter where the spot size is still just a few mm and strong time dependence is observed. Data were recorded on a strip chart recorder.

Time dependent transmission data were also recorded using a variety of long pass filters, blue-violet band pass filters, and a common flood lamp. This source and the optimum filter pair were used subsequently for schlieren experiments. A large-area Si detector was used in photovoltaic mode with a variety of low impedance load resistors to maintain linear response. The output was recorded on a strip chart. Neutral density filters were moved from front to back of the BR film to provide a range of incident intensities at the BR film while keeping the average intensity at the detector constant.

Standard schlieren optics in the configuration shown in Fig. 1 collected images of a variety of phase objects for comparison with BR schlieren. The optics were located at the center of curvature of the mirror on either side of its symmetry axis. The 19-inch, six-foot focal length spherical mirror imaged the source grid onto the image grid. The grids were Ronchi rulings having 50 lines per inch from Edmund Scientific. An 5 V incandescent lamp diffused by thin teflon illuminated the source grid. A Javelin JE2062IR black-and-white video camera monitored the test region, which was located just in front of the spherical mirror. A zoom lens optimally filled the camera image plane with the test region.

The BR schlieren set up is shown schematically in Fig. 2. A ground glass plate diffused the flood lamp illumination and reduced UV emissions. The heat absorbing filter was Schott KG2. The long pass filter was Schott OG550. The blue-violet band pass was Schott BG12, which was combined with a neutral density filter having an optical density of 1.

Results



The basic spectral properties of the BR film were measured first using monochromatic light in order to help select band pass filters for schlieren experiments. Fig. 3 presents absorbance spectra. The open circles were taken at an intensity sufficiently low that no time dependence of the transmission was observed. The cross symbols in Fig. 3 give the absorbance spectrum immediately after completion of a higher-intensity bleach transient using 525 nm light. Since BR has no absorption at 1 um, the experimental absorbance at 1 um was subtracted to eliminate contributions of reflection and scattering. After the bleach, the absorbance is everywhere lower except near 400 nm. The solid diamond symbols in Fig. 3 represent the difference in the two curves, which reveal the decreased absorbance in the yellow-green region and the increased absorption in the blue-violet. These changes result from the light induced population of the BR metastable M-state.

A suitable long-pass filter for writing the image grid in the BR film was determined as follows. Fig. 4 plots the absorbance change at 400 nm vs. write intensity for different long-pass filters. A 400 nm band-pass filter was used for reading. The 550 nm long pass (Schott OG550) produced a larger absorbance change than the 475 nm long-pass filter.

Next, the optimum band-pass read filter was determined. Fig. 3 suggests that the largest absorbance increase occurs at or below 400 nm. Published results indicate that wavelengths below 400 nm should give only smaller changes[4]. This was tested using the 550 nm long-pass filter to bleach the ground state and a number of blue-violet band-pass filters to read the effect. Fig. 5 presents the results, where the center wavelength of the read filter is given for each solid symbol. The largest absorbance increases are confirmed to occur near 400 nm, where the bandpass filter used was Schott BG12.

Figs. 4 and 5 reveal that the best filter combination for the schlieren experiments is the 550 nm long-pass for writing the image grid and the 400 nm band pass for reading it. The largest expected absorbance change at the read wavelength is about 1.3. An additional observation is the appearance of an optimum write intensity well below the damage threshold of the film. This phenomenon is unexplained by the simple two state model but is consistently observed. Illumination with wavelengths coincident with M-state absorption is known to accelerate ground-state recovery [4]. To allow for adequately long schlieren read times, attenuation of the 400 nm read light by a factor of 10 was found to be adequate.

Next, schlieren images of two different gas plumes are presented. For reprographic purposes, the images have been processed with a ramp filter to enhance the contrast. This procedure consisted of multiplying the image histogram by 0 for pixels in the 1-64 grey scale range, by a linear 0 to 1 ramp for pixels in the 65-192 grey scale range, and by 1 for pixels in the 193 - 256 grey scale range. Raw digital data can be obtained by contacting the authors. In these experiments, a 1:1 image of the source grid was bleached into the BR film at an intensity sufficient to cause an absorbance change of 0.8 at 400 nm. The bleaching time was 10 sin each case. The gas plumes were absent from the test region while writing the image grid. Then the read filter was inserted, the gas-plumes activated, and an image of the test region recorded. Real time images of the test region were observed on a video monitor. After switching to the 400 nm read light, the picture gradually brightened, and the contrast of the schlieren images gradually faded to invisibility after several minutes. An image was then collected of this scene as a reference to obtain information about the fading of the schlieren effect with time.

Fig. 6 is a BR-schlieren image of a high-velocity jet from an inert-gas duster. The gas is 1,1,1,2-Tetrafluoroethane. Each distinct section of the jet appears darker on its left side and lighter on its right. This is because schlieren detects index gradients[2], which change sign when crossing a phase object. (The dark crescents are scratches in the mirror's coating.)

Fig. 7 presents BR schlieren results for a low velocity flow of He gas. Turbulence is observed. Again, identifiable regions are darker on the left side and lighter on the right.

In Figs. 8 and 9 we present intensity profiles across portions of the unprocessed images. The heavy curves are BR schlieren data. The lower heavy curve, taken immediately after writing the image grid, shows intensity variations of ~10% with spatial frequencies of a few tens of profile points. The upper heavy curve, taken after several minutes of read-light illumination (reference image), shows an overall transmission increase and a near disappearance of the intensity variations.

The light curves in Figs. 8 and 9 are profiles of images taken with the standard schlieren set-up. An attempt was made to locate high contrast regions in these images with spatial frequencies similar to those plotted for BR schlieren. Since these regions can occur at different locations in front of the non-uniformly illuminated mirror, the slope of the baseline can differ from their BR schlieren counterpart (heavy curves). Additionally, a smaller range of profile points happened to be collected from the standard schlieren images than from the BR schlieren images. Despite these unimportant differences, it is clearly observed that the intensity variations having spatial frequencies of tens of points are only about 2 times larger than the BR results. This suggests that BR schlieren can be as sensitive as standard schlieren but with the advantages of adaptability and automatic image-grid alignment.

A variety of other phase objects were also imaged using both standard and BR schlieren set-ups. Heat waves were clearly observed from a soldering iron and even weakly from hands waved in the test region. Static phase objects such as glass plates revealed streaks or wood-grain patterns. Such static phase objects needed to be inserted in the BR schlieren test region immediately after writing the image grid. If they rested in the test region during the writing, their index gradients were invisible during the read cycle. This is because distortions in the image of the source grid cause a similarly distorted BR image grid, which remains distorted for the read cycle. In contrast, we observe turbulent jets and heat-waves during the read cycle even when they are present during the write step.

Summary

An adaptive schlieren apparatus using a bacteriorhodopsin film as a photochromic medium for writing image grids has been demonstrated using white light as the illumination source for the first time. Images of phase objects reveal only 2 times less sensitivity than found using a standard schlieren apparatus. The potential for laboratory applications, where intensity and scene contrast can be controlled at will, seems clear. It also seems feasible to the authors, from a qualitative inspection of the intensities used in the present experiments, that enough reflected sunlight from a suitably high contrast scene could be collected and concentrated on a BR film to realize an adaptive single ended schlieren system for industrial leak detection. This remains to be tested, however, and it is impossible to predict the limits of sensitivity of such a system without further experiments .

Acknowledgments

This work was performed in the Special Projects Instrumentation Laboratory at the Kennedy Space Center in Florida. REP was supported a NASA/ASEE Summer Faculty Fellowship administered by the University of Central Florida. JED was supported on a student grant under the same program. ABR was supported by a National Physical Science Consortium Fellowship with NASA/KSC corporate sponsorship. The authors thank Bob Youngquist, Stuart Gleman, and Carl Hallberg for equipment, assistance, and cooperation.

References

[1] Herbert Kaplan, "A Big Advance in Gas Imaging," Photonics Spectra, Feb. 1996, p 44-46.

[2] Selected Papers on Schlieren Optics, SPIE Milestone Series, vol. MS61, J. R. Meyer-Arendt, ed., (The International Society for Optical Engineering, Belingham, Wash., 1992).

[3] R. E. Peale and P. L. Summers, "Zebra schlieren optics for leak detection," Appl. Optics 35, pp. 4518-4521 (1996).

[4] Robert R. Birge, "Protein-based optical computing and memories,"Computer, November pp. 56-67 (1992).

[5] John D. Downie, "Application of bacteriorhodopsin films in an adaptive-focusing schlieren system," Applied Optics 34, pp. 6021-6028 (1995).

Figure Captions

Fig. 1. Schematic of standard schlieren optics. a) Incandescent lamp; b) teflon diffuser; c) source grid; d) spherical mirror; e) phase object; f) image grid; g) camera with zoom lens.

Fig. 2. Schematic of the bacteriorhodopsin schlieren set-up. a) Flood lamp; b) ground glass diffuser; c) heat-absorbing filter; d) source grid; e) spherical mirror; f) filter holder with two interchangeable filters; g) bacteriorhodopsin film; h) zoom lens; i) camera.

Fig. 3. Absorbance spectra before and after creation of M-state population in the bacteriorhodopsin film using 525 nm light and their difference.

Fig. 4. Absorbance change at 400 nm as a function of write intensity for different long-pass write filters. The wavelengths given are the pass edges.

Fig. 5. Absorbance changes measured with various band pass filters as a function of write intensity using a 550 nm long pass as the write filter. Wavelengths given are band-pass centers.
Fig. 6. Bacteriorhodopsin schlieren image of the spray from a compressed gas duster. The vertically oriented light and dark bands comprise the image of the jet. The black crescents are scratches on the mirror.

Fig. 7. Bacteriorhodopsin schlieren image of a low velocity He gas plume.

Fig. 8. Image profiles for the compressed-gas duster spray. The thin curve is from standard schlieren results. Thick curves are from bacteriorhodopsin schlieren immediately after the write procedure (lower) and several minutes later (upper) .

Fig. 9. Image profiles for the low velocity He plume. The thin curve is from standard schlieren results. Thick curves are from bacteriorhodopsin schlieren immediately after the write procedure (lower) and several minutes later (upper).