microS

™

 

A Micro-Optical Sublayer Profilometer and Shear Stress Sensor 

 

The microS is the world’s first optical sensor for the direct 
measurement of the boundary sublayer velocity gradient and 
wall shear stress. Made possible by patented technology, it is 
capable of performing measurements just 75 

µ

m or 135 

µ

m 

from its face (in air) within a probe volume of only 30 by 30 by 
15 microns. Designed to be flush-mounted to the surface in 
question, it is absolutely nonintrusive, has no moving parts, 
and can measure flow direction along with the gradient. The 
probe itself (the face) is only 0.375” (9.5 mm) in diameter and 
1.18” (30 mm) long so it can be installed in even the tightest of 
spaces. Sensor electronics can be stowed safely away from 
the experiment, and since all the sensitive optics are fixed 
within the probe, no user alignment is necessary. 

The microS is based on technology developed at the California 
Institute of Technology and NASA’s Jet Propulsion Laborato-
ries. Today there are customers around the world in academic, 
industrial, and research settings enjoying the reliability and 
ease of use of MSE’s microS. 

The microS System 

The microS System consists of a transceiver probe, 
driver electronics, bandpass filter, and BP-microS Burst 
Processor acquisition hardware and software. Option-
ally, the system can be ordered with a computer with the 
hardware and software installed and tested by MSE 
technicians. 

The microS System is a standalone system – no other 
lasers or optics are required. Simply place the electron-
ics at a convenient distance to the acquisition computer 
and install your probe at the desired location. A 15-foot 
(4.5 m) cable is standard (longer lengths are optional). 
The Burst Processor software collects data and presents 
flow statistics. 

The microS System is at home either in the lab or at the 
field; its robust construction makes it perfect for harsh 
environments. 

 

 

 

 

 

Above, top: shear stress on the surface of a cylinder in a water channel flow as 
a function of angle along the cylinder (comparison of microS measurements 
and theory). Bottom: the RMS of the shear stress in the 10cm/sec case, show-
ing a rise at the stagnation and separation points. 

 

  

*

17” LCD monitor for scale

microS probe

15 foot (4.5 m)
standard cable

Driver electronics
& bandpass filter

Driver electronics
& bandpass filter

 

123 W. Bellevue Dr., Suite 1

Pasadena, CA 91105 

USA 

Info@MeasurementSci.com 

Phone: +1 (626) 577 0566 

Fax: +1 (626) 577 0565 

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microS

™

 

A Micro-Optical Sublayer Profilometer and Shear Stress Sensor 

 

The microS Concept 

MSE’s microS probe is similar to a standard LDV in that it uses the interference pattern of two coherent beams as the il-
lumination. Unlike an LDV, where aberrations are kept to a minimum to make the fringes as straight as possible, microS 
measurements are made possible by the interference pattern of two cylindrical waves which form a diverging fringe pat-
tern, first demonstrated in [1]. Since the probe volume is only 30 

µ

m tall, the velocity profile in the region can be assumed 

to be linear. If y is the distance from the wall, then u = Cy where C is the slope,

 ∂

u/

∂

y, of the [linear] velocity profile. The 

spacing between fringes at a particular height y can be written as 

δ

 = ky where k is the constant obtained from calibration 

of the probe. A particle passing through the probe volume at a particular height y will reflect the fringes such that the re-
ceiver sees a sinusoid intensity variation with frequency f = u(y)/d(y) = C/k. Since k is known from calibration of the probe, 
C can be determined, and since the probe volume is microscopically small and close to the wall, the microS directly 
measures the derivative of the velocity profile at the wall. Shear stress can then be calculated by multiplying the meas-
urement by the dynamic viscosity of the fluid. Additionally, the microS’s fringes are tilted relative to the normal so that flow 
direction can also be detected. 

1. Naqwi, A. A. & W. C. Reynolds (1987), "Dual cylindrical wave laser-Doppler method for measurement of skin friction in fluid flow", NASA STI/Recon Technical Report N 87 

 

 

Probe volume

Flow direction

Particle speed

Magnified 350 times

1/

f

V

t

As seen by the photodetector,
the light signal is a sinusoid
(corresponding to the regions
of constructive interference)
modified by a Gaussian
envelope.

The sinusoid is isolated
by a bandpass frequency
filter.

du/dy

= fringe divergence

´

f

The frequency of the sinusoid is measured
(usually by FFT). The fringe divergence is
known (from calibration), thus the slope of the
velocity profile at the wall can be calculated.

The interference pattern of
two coherent cylindrical
waves is used to create a set
of linearly diverging fringes.

The probe volume is defined by the field
of view of the receiver (shown in yellow).
As a particle travels through it, the
receiver records the reflection of each
fringe off the particle

 

123 W. Bellevue Dr., Suite 1

Pasadena, CA 91105 

USA 

Info@MeasurementSci.com 

Phone: +1 (626) 577 0566 

Fax: +1 (626) 577 0565 

U.S. Patents Nos. 6,717,172 and 6,956,230