Measurement Of Optical Path Length Research Articles

Angstrom-range optical path-length measurement with a high-speed scanning heterodyne optical interferometer.

A highly accurate method of optical path-length measurement is introduced by use of a scanning heterodyne optical interferometer with no moving parts. The instrument has demonstrated the potential to measure optical path length at angstrom resolution over continuous thickness in the micrometer range. This optical path length can be used to calculate the thickness of any material if the refractive index is known or to measure the refractive index of the material if the thickness is known. The instrument uses a single acousto-optic device in an in-line ultra-stable reflective geometry to implement rapid scanning in the microsecond domain for thickness measurements of the test medium.

Interferometer-less coherent optical range finder

A new coherent optical detection technique employing coherent frequency-domain reflectometry and a novel optical frequency sensor is demonstrated for high-precision optical path-length measurements. Using pulsed laser sources, an improvement of more than two orders of magnitude in spatial resolution over conventional optical coherent frequency domain reflectometry techniques is demonstrated. Varying degrees of spatial resolution ranging from several centimeters to a few hundred nanometers are achieved. High-precision distance measurement with long baseline is also presented.

Quantitative Phase Contrast Microscopy of Living Astrocytes By Numerical Reconstruction of Digital Holograms

We have developed a method of numerical holography which allows simultaneous intensity and quantitative phase contrast imaging. Measurement of optical path length provides precise information about the cell morphology. As presented in Fig. 1, the experimental set-up for the recording of digital holograms is basically a Mach-Zender interferometer. At the exit of the interferometer, a CCD camera records a hologram which is made by the interference between a reference wave R and the object wave O that is transmitted by the biological sample. In order to obtain a plane wave as reference, a beam expander (B.E.) including a pinhole is introduced in the reference arm and enlarges the beam diameter to about 2cm. A similar system which produces a smaller beam diameter (≈5mm) is inserted in the objet arm. To perform holography in the off-axis geometry, the mirror in the reference arm is oriented in such a way that the reference wave reaches the CCD camera with a small incidence angle.

Experimental and theoretical comparison of NIR spectroscopy measurements of cerebral hemoglobin changes.

Two near-infrared spectroscopy (NIRS) methods are available for measuring changes (Delta) in total cerebral hemoglobin concentration (CHC): 1) a continuous measurement of the changes in total hemoglobin concentration (Delta[Hb]tot) and 2) the difference between two absolute measurements of CHC, each derived from a small, controlled change in inspired O2 fraction. This paper investigates the internal consistency of these two methods by using an experimental and theoretical comparison. NIRS was used to measure [Hb]tot in five newborn piglets before and after a change in arterial PCO2. Delta[Hb]tot demonstrated a low coefficient of variation of 2.8 +/- 2.8 (SD) % which allowed changes in CO2-cerebral blood volume reactivity to be clearly discriminated. However, a high coefficient of variation of 22.8 +/- 3.5% on the DeltaCHC measurements obscured any CO2 reactivity changes. A theoretical analysis demonstrates the effects of optical pathlength, background absorption, scatter, and blood vessel diameter on both methods. For more accurate monitoring of CHC, individual measurements of optical pathlength and more accurate pulse oximetry are required.

Verification of Multichannel Liquid Dispenser Performance in the 4–30 μL Range by Using Optical Pathlength Measurements in Microplates

Pipettors and liquid handling systems should be checked regularly to verify their precision and accuracy. Professional organizations recommend performing 4-replicate tests monthly and 10-replicate tests at least quarterly (1)(2)(3)(4). Traditional gravimetric procedures are adequate to meet these testing requirements for single-channel pipettors but are tedious and impractical for multichannel devices. An alternative is to dispense a solution of colored dye into wells of a microplate and to measure the resulting absorbance values in a microplate reader. Volume calibration is performed with a calibration curve relating absorbance to volume (5). We recently reported a method for verifying multichannel pipettor performance by a spectrophotometric procedure that utilizes the near infrared absorbance of water and does not require addition of a dye (6). Water or other aqueous reagent is dispensed from the pipettor into microplate wells, and the optical pathlength in each well is determined in a microplate spectrophotometer. Water is essentially transparent from 200 to 900 nm but has a distinctive absorbance peak near 977 nm. As predicted by the Lambert law of light absorption, absorbance is proportional to the distance that light travels through the sample; thus the characteristic absorbance of water can be utilized to measure the pathlength of an aqueous sample. The maximum absorbance is affected by temperature; however, temperature dependency can be avoided by making the absorbance measurements at a temperature isosbestic point (near 1000 nm). Baseline absorbance is measured at a wavelength distant from the water absorbance peak, e.g., 900 …

The Calibration and Validation of a Phase-modulated Near-infrared Cerebral Oximeter

Objective. This study was undertaken to compare the cerebral oxygenation measured by an experimental phase-modulated near-infrared (NIR) spectroscopy system with capillary saturation estimated from jugular venous oxygen saturation. Methods. Jugular venous catheters were placed in 30 patients undergoing carotid endarterectomy and 194 measurements of venous oxygen saturation were obtained intra operatively. Simultaneous measurement of optical path length at 754, 785, and 816 nm was performed using a phase-modulated near-infrared spectroscopy system. Optical calibration was performed using both an optical bench and a scattering mold. Hemoglobin saturation was calculated from NIR measurements using equations derived from diffusion theory. Capillary saturation was calculated from the arterial and venous saturations. Results. Jugular venous saturations ranged from 41 to 92%. When calibrated using the optical bench, the NIR estimates of hemoglobin saturation deviated from estimated capillary values by an average of 2.6% bias and 4.3% deviation. No systematic bias was noted. NIR values derived from mold calibration were less accurate and precise (4.6% bias and 6.9% deviation.) Use of the initial venous sample as an in vivocalibration improved the accuracy of the mold calibration but did not alter the performance of the bench calibration. Conclusions. Under the conditions tested, an experimental phase-modulated near-infrared spectroscopy system calibrated using an optical bench agreed with capillary saturation estimated from jugular venous samples. Further work is necessary to demonstrate valid performance of the system under other conditions.

Cerebral oxygenation and haemodynamics in the foetus and newborn infant.

Quantitative techniques have been derived for the measurement of global cerebral blood flow, cerebral blood volume, its response to changing arterial carbon dioxide tension and mixed cerebral venous saturation in the human newborn undergoing intensive care. Normal ranges have been established and significant disturbances of cerebral oxygenation and perfusion have been demonstrated in a variety of pathological conditions. Recently, absolute cerebral deoxyhaemoglobin concentration has been obtained in the newborn using second differential spectroscopy. When combined with the measurement of total cerebral haemoglobin concentration, the mean saturation of cerebral blood (SmcO2) may be obtained, allowing global cerebral oxygenation to be determined continuously in the intensive care unit. Marked changes in the concentrations of cerebral oxy- and deoxyhaemoglobin have been observed in foetuses undergoing labour. Measurements of SmcO2 from the foetal brain prior to delivery have shown the expected close correlation with acid-base status at birth. Although movement artefact remains a theoretical risk during uterine contractions, preliminary measurements of optical path length by intensity-modulated spectroscopy have demonstrated only small fluctuations. In future the clinical application of time, phase and spatially resolved spectroscopy is likely to improve both the quantitative accuracy and the regional specificity of physiological measurements in the foetal and neonatal brain.

Precise optical path length measurement through an optical fiber: Application to seafloor strain monitoring

An optical fiber strainmeter intended for measuring tectonic strains on the seafloor is under development. In this instrument, an optical fiber is stretched between two points fixed to the ocean bottom; relative displacement of these points causes a change in the elongation of the fiber. This associated change in optical path length is monitored by an electronic distance meter. The dominant sources of noise in determining the optical path length of the fiber stem from the dependence of the fiber's index of refraction on both wavelength and temperature. In a 50 day long experiment performed in the shallow ocean, a test fiber was installed along a 210 m long baseline on the bottom. The RMS variation in length was 5 mm except for two displacements of order 10 cm caused by known effects.

Single-beam diode-laser technique for optical path-length measurements

A simple single-beam technique employing radio-frequency modulation of a tunable diode laser with homodyne demodulation is demonstrated as a means of measuring optical path lengths. This technique offers a straightforward method for determining path lengths traversed through optical multipass cells or performing optical range-finding over short (i.e., tens of meters) standoff distances. The radio-frequency phase-sensitive nature of the technique permits narrow-band detection and high signal-to-noise ratios, even when range-finding measurements are made with range resolutions of «1 m. This compares favorably with traditional short-pulse, wide-bandwidth optical range finders.

Using a moving diffraction grating to simulate the function of an acousto-optic modulator

We present an instructional undergraduate laboratory that introduces the student to an acousto-optic modulator (AOM) in the context of an optical heterodyning experiment. A moving diffraction grating is used to illustrate the internal functioning of an acousto-optic modulator and to make optical heterodyning experiments accessible to any undergraduate laboratory. The concepts and techniques presented can be used from the introductory through advanced level, in that, students gain direct laboratory experience with diffraction, the Doppler shift of light, construction of a Mach–Zehnder interferometer, optical path length measurements using a heterodyne technique, and sophisticated data analysis techniques.

Measurement of cranial optical path length as a function of age using phase resolved near infrared spectroscopy.

Near infrared spectroscopy (NIRS) has been used to measure concentration changes of cerebral hemoglobin and cytochrome in neonates, children, and adults, to study cerebral oxygenation and hemodynamics. To derive quantitative concentration changes from measurements of light attenuation, the optical path length must be known. This is obtained by multiplying the source/ detector separation by a laboratory measured differential path length factor (DPF) which accounts for the increased distance traveled by light due to scattering. DPF has been measured by time of flight techniques on small populations of adults and postmortem infants. The values for adults are greater than those for newborns, and it is not clear how to interpolate the present data for studies on children. Recent developments in instrumentation using phase resolved spectroscopy techniques have produced a bedside unit which can measure optical path length on any subject. We have developed an intensity modulated optical spectrometer which measures path length at four wavelengths. Two hundred and eighty three subjects from 1 d of age to 50 y were studied. Measurements were made at a fixed frequency of 200 MHz and a source detector separation of 4.5 cm. Results suggest a slowly varying age dependence of DPF, following the relation DPF690 = 5.38 + 0.049A0.877, DPF744 = 5.11 + 0.106A0.723, DPF807 = 4.99 + 0.067A0.814, and DPF832 = 4.67 + 0.062A0.819, where DPF690 is the DPF measured at 690 nm and A is age is expressed in years from full term. There was a wide scatter of values, however, implying that ideally DPF should be measured at the time of each study.

The noninvasive measurement of absolute cerebral deoxyhemoglobin concentration and mean optical path length in the neonatal brain by second derivative near infrared spectroscopy.

We have used second differential near infrared spectroscopy of water to determine the mean optical path length of the neonatal brain. By obtaining the ratio of the second differential features of deoxyhemoglobin to those of water, the absolute cerebral concentration of deoxyhemoglobin can be monitored continuously and noninvasively. Nineteen neonates were studied; the gestational age at birth varied from 23 to 38 wk, and the postconceptual age, when the spectra were recorded, ranged from 35 to 48 wk. The calculated mean deoxyhemoglobin concentration was 14.6 +/- 4.0 microM; the differential path length factor (mean optical path length/optode separation) calculated from the water peak at 730 nm was 4.66 +/- 1.01, and that calculated at the 830-nm peak was 3.91 +/- 0.75. These values are consistent with path length measurements using laser time-of-flight spectroscopy on postmortem neonates and phase-resolved spectroscopy on live neonates. Induced arterial oxygen saturation decreases from 98 to 93% showed no significant change in the mean optical path length, despite significant cerebral desaturation. Changes in the deoxyhemoglobin concentration after this procedure were identical, whether measured by second differential analysis at 760 nm or by multilinear regression over the wavelength range 740-900 nm. When combined with existing methods of measuring total cerebral hemoglobin concentration, second differential near infrared spectroscopy can be used to derive the mean cerebral oxygen saturation. A preliminary experiment outlined the feasibility of this approach and yielded a saturation value of 63%, consistent with near infrared sampling of a predominantly venous pool in the brain.

Near infra‐red spectroscopy

AnaesthesiaVolume 50, Issue 12 p. 1015-1016 Free Access Near infra-red spectroscopy D.N.F. Harris, D.N.F. Harris Department of Anaesthesia, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London W12 0HSSearch for more papers by this author D.N.F. Harris, D.N.F. Harris Department of Anaesthesia, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London W12 0HSSearch for more papers by this author First published: December 1995 https://doi.org/10.1111/j.1365-2044.1995.tb05941.xCitations: 7AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References 1 Edwards AD, Wyatt JS, Richardson C, Delpy DT, Cope M, Reynolds EO. Cotside measurement of cerebral blood flow in ill newborn infants by near infrared spectroscopy. Lancet 1988; 2: 770– 1. 2 Brazy JE, Lewis DV, Mitnick MH, vander Jobsis Vliet, FF. Noninvasive monitoring of cerebral oxygenation in preterm infants: preliminary observations. Pediatrics 1985; 75: 217– 25. 3 Hampson NB, Camporesi EM, Stolp BW, Moon RE, Shook JE, Griebel JA, Plantadosi CA. Cerebral oxygen availabilty by NIR spectroscopy during transient hypoxia in humans. Journal of Applied Physiology 1990; 69: 907– 13. 4 Germon TJ, Kane NM, Manara AR, Nelson RJ. Near-infrared spectroscopy in adults: effects of extracranial ischaemia and intracranial hypoxia on estimation of cerebral oxygenation. British Journal of Anaesthesia 1994; 73: 503– 6. 5 Williams IM, Picton AJ, Hardy SC, Mortimer AJ, McCollum CN. Cerebral hypoxia detected by near infrared spectroscopy. Anaesthesia 1994; 49: 762– 6. 6 Pilkington SN, Hett DA, Pierce JMT, Smith DC. Auditory evoked potentials and Near infra-red spectroscopy during cardiac arrest. British Journal of Anaesthesia 1995; 74: 717– 9. 7 McCormick PW, Stewart M, Goetting MG, Dujovny M, Lewis G, Ausman JI. Noninvasive cerebral optical spectroscopy for monitoring cerebral oxygen delivery and hemodynamics. Critical Care Medicine 1991; 19: 89– 97. 8 Brown R, Wright G, Royston D. A comparison of two systems for assessing cerebral venous oxyhaemoglobin saturation during cardiopulmonary bypass in humans. Anaesthesia 1993; 48: 697– 700. 9 Harris DN, Bailey SM. Near infrared spectroscopy in adults. Does the Invos 3100 really measure intracerebral oxygenation Anaesthesia 1993; 48: 694– 6. 10 Harris DN, Cowans FM, Wertheim DA, Hamid S. NIRS in adults—effects of increasing optode separation. Advances in Experimental Medicine and Biology 1994; 345: 837– 40. 11 Harris DNF, Bailey SM, Dob D. Effect of optode separation on brain penetration in adults. 22nd Annual Meeting ISOTT, Istanbul , August 1994. 12 Harris DNF. 9th International Cerebral Haemodynamics Symposium Charleston, S. Carolina Feb. 1995. 13 Duncan A, Meek JH, Clemence M, Elwell CE, Tyszczuk L, Cope M, Delpy DT. Optical pathlength measurement on adult head, calf and forearm and the head of the newborn infant using phase resolved optical spectroscopy. Physiology, Medicine and Biology 1995; 40: 295– 304. 14 Amory DW, Pan AL. Personal communication. 15 Harris DN, Cowans FM, Wertheim DA. NIRS in the temporal region—strong influence of external carotid artery. Advances in Experimental Medicine and Biology 1994; 345: 825– 8. 16 Olsen KS, Henriksen L, Owen-Falkenberg A, Dige-Petersen H, Rosenorn J, Chraemmer Jorgensen B. Effect of 1 or 2 MAC isoflurane with or without ketanserin on cerebral blood flow autoregulation in man. British Journal of Anaesthesia 1994; 72: 66– 71. 17 Harris DN, Bailey SM, Smith PL, Taylor KM, Oatridge A, Bydder GM. Brain swelling in first hour after coronary artery bypass surgery. Lancet 1993; 342: 586– 7. 18 McCormick PW, Stewart M, Lewis G, Dujovny M, Ausman JL. Intracerebral penetration of infra-red light Journal of Neurosurgery 1992; 76: 315. 19 Kirkpatrick PJ, Smielewski P, Whitfield PC, Czosnyka M, Menon D, Pickard JD. An observational study of near-infrared spectroscopy during carotid endarterectomy. Journal of Neurosurgery 1995; 82: 756– 63. Citing Literature Volume50, Issue12December 1995Pages 1015-1016 ReferencesRelatedInformation

Measurement of changes in optical pathlength through human muscle during cuff occlusion on the arm

Concentration changes of haemoglobin and cytochromes indicative of tissue perfusion and oxygenation can be quantitatively evaluated from near-infrared (NIR) measurements of absorption and optical pathlength through tissue. The accuracy of such measurements is limited as current bedside instrumentation cannot measure optical pathlength. Using a recently developed phase-resolved spectroscopic technique we can produce continuous and simultaneous measurements of changes in both attenuation and optical pathlength at four wavelengths in the NIR in a bedside instrument. The change in optical pathlength through arm muscle during a cuff occlusion is compared with the absolute pathlength estimated from time resolved spectroscopy measurements in the laboratory. These show that the pathlength varies by 4–10% during the occlusion, demonstrating the need for continuous measurements of optical pathlength.

Customised optrode holder for clinical near-infra-red spectroscopy measurements.

NON-INVASIVE near-infra-red spectroscopy has been found to be a useful means of monitoring intracerebml oxygenation and haemodynamies in the fetus, newborn infants and adults (WICKRAM~INGHE e ta l . , 1985; WYATT et aL, 1986), and oxygen transport in muscle (DE BLASl et al., 1993). To quantify oxyand deoxyhaemoglobin concentrations, attenuation measurements are combined with path-length data that can be measured by time-resolved spectroscopy (DELPY et al., 1988). An alternative and cheaper approach for the measurement of optical path length and haemoglobin saturation can be obtained by the recent development of frequency-domain spectrophotometers (GRATrON etal . , 1994). Clinical studies involve the attachment of optrodes to the skin surface and the assumption that, during the study, the optrode spacing should remain constant. Optrode movements can occur in all clinical applications, in particular during labour and muscle exercise. Changes in optrode position (distance and angle between optrodes) and optical contact cause variations in the total detected intensity. Recently, a method has been described of separately fixing the two optrodes inserted in a covered plastic shell against the skin by collodion (LIEM et aL, 1992). A silicone rubber moulding was designed for fetal head monitoring (PEEBLES et aL, 1992). However, neither device prevents the artifacts caused by changes of inter-optrode distance and angle. For mare than a century, the healthcare industry has generated a growing need for materials with the properties and ftmetional performance of vuleanJsed rubber. For a long time, natural rubber was the principal elastomer, followed by butyl and silicone rubber. Recently, lightweight low-temperature thermoplastic materials have found different applications in orthopaedics and dentistry. These materials, combining rigidity and mouldability, can be moulded to fit the optrodes and adapt the holder to the monitored tissue area. 2 Method

Transcranial optical path length in infants by near-infrared phase-shift spectroscopy.

Near-infrared spectroscopy (NIRS) is an emerging technique for noninvasive, bedside monitoring of cerebral oxygenation and blood flow. Traditionally, it has relied on the Beer's Law relationship in which the concentration of light-absorbing oxygen-carrying pigments is proportional to their light absorbance, and inversely proportional to an optical path length (a measure of the distance traveled by photons passing through the tissue). In practice, NIRS has been based upon assumptions that mean transcranial optical path length, the average optical path length for a given patient, is constant among patients and independent of the wavelength of light used. The objective of our study was to measure mean optical transcranial path length in infants as a step in allowing quantitation of cerebral oxygenation. We measured mean transcranial optical path length in 34 infants, aged 1 day to 3 years, using amplitude-modulated phase-shift spectroscopy at 754 nm and 816 nm. Optical transcranial path lengths (mean +/- SEM) were 8.6 +/- 0.9 cm, 11.1 +/- 0.9 cm, and 11.3 +/- 0.9 cm at 754 nm, and 8.8 +/- 0.9 cm, 11.2 +/- 0.8 cm, and 11.1 +/- 0.9 cm at 816 nm, using emitter-detector separations of 1.8, 2.5, and 3.0 cm, respectively. Optical path length increased as emitter-detector separation, head circumference, or age increased. Variance in the ratio of mean optical path lengths at the two different wavelengths exceeded that accounted for by variation in repeated measures alone (p < 0.001), suggesting that optical path length is also not independent of wavelength. NIRS instrument emitter-detector geometry, subject age, head size, and wavelength used each influence optical path length. Quantitative NIRS measurements in clinical use may require concurrent measurement of both absorbance and optical path length at each wavelength, or use of newer measures that are not based upon Beer's Law assumptions.

Optical pathlength measurements on adult head, calf and forearm and the head of the newborn infant using phase resolved optical spectroscopy

The authors have used an intensity modulated optical spectrometer, which measures the phase shift across tissue experienced by intensity modulated near-infrared light, to determine the absolute optical pathlength through tissue. The instrument is portable and takes only 5 s to record pathlength at four wavelengths (690 nm, 744 nm, 807 nm and 832 nm). The absolute pathlength divided by the known spacing between the light source and detector on the skin is the differential pathlength factor (DPF) which previous studies have shown is approximately constant for spacings greater than 2.5 cm. DPF results are presented for measurements on 100 adults and 35 newborn infants to determine the statistical variation on the DPF. All measurements were made at a frequency of 200 MHz with source-detector spacings of >4 cm. Results at 807 nm show a DPF of 4.16(+or-18.8%) for adult arm, 5.51(+or-18%) for adult leg, 6.26(+or-14.1%) for adult head and 4.99(+or-9%) for the head of a newborn infant. A wavelength dependence was obtained for DPF on all tissues and a difference in DPF between male and female was observed for both the adult arm and leg. The results can be used to improve the quantitation of chromophore concentration changes in adults and newborn infants.

Non-invasive measurements of optical pathlength in human infant brain : F. Faris, Y. Wickramasinghe, R. Houston, M. Thorniley, P. Rolfe, K. Palmer and A. Spencer, Department of Biomedical Engineering and Medical Physics, University of Keele, Hospital Centre, Thornburrow Drive, Hartshill, Stoke-on-Trent ST4 7QB, U.K.

Non-invasive measurements of optical pathlength in human infant brain : F. Faris, Y. Wickramasinghe, R. Houston, M. Thorniley, P. Rolfe, K. Palmer and A. Spencer, Department of Biomedical Engineering and Medical Physics, University of Keele, Hospital Centre, Thornburrow Drive, Hartshill, Stoke-on-Trent ST4 7QB, U.K.

Latent image measurements in electron beam exposed polymethylmethacrylate

Undeveloped and unbaked paterns or latent images (LI) in polymethylmethacrylate (PMMA) have been observed and measured using different measurement techniques. Results from two common-path baseband optical techniques have been compared with the results of profiling LI using an atomic force probe. These results indicate that 5 Å detection limits in optical path length measurements are easily achievable at near video rates using simple optical systems.

Measurement of Optical Path Length for Cerebral Near-Infrared Spectroscopy in Newborn Infants

The time taken for an extremely short pulse of near-infrared laser light to traverse the heads of 6 preterm infants was measured after death. The values obtained were used to calculate a differential path length factor (DPF), defined as the mean distance travelled by the photons divided by the distance between the points where light entered and left the head. The DPF was found to be 4.39 +/- 0.28. Knowledge of this factor will permit accurate quantitative measurements to be made by near-infrared spectroscopy of a range of indices of cerebral oxygenation and haemodynamics in live infants.