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RSOS-2 Retroreflective Shadowgraph Flow Visualization System

The RSOS-2 is a laboratory instrument consisting of a retroreflective screen and a tripod –mounted optical transmitting/receiving unit. A compact arc lamp in this unit projects a shadowgram upon the retroreflective screen, which is then imaged by a system camera.

First introduced in 2008 as the RSOS-1, this instrument has seen use primarily in the testing of kitchen ventilation hoods, where it enables the visualization of cooking-effluent spillage around the lip of a ventilation hood.  The application of optical methods like this to determine the threshold of cooking-effluent capture and containment for idle and heavy cooking conditions is detailed in
ASTM Standard F 1704-05.

The image below left depicts the RSOS-2 transmitter/receiver unit on its tripod mount with an auxiliary video monitor that provides a real-time shadowgraph image during operation of the system with its retroreflective screen (not shown).  Below right is shown a typical still-frame shadowgram captured by the system camera of a barbeque grill with turbulent convection from hot cooking units spilling into the face of the cook.

Picture of Retroreflective Shadowgraph System

The size of the field-of-view of the RSOS-2 is determined by the size of the retroreflective screen that is provided as a background for shadowgram imaging.  For example, a 10x10-foot screen affixed to the wall of a kitchen ventilation laboratory opposite the RSOS-2 transmitter/receiver unit provides a 5x5-foot field-of-view at the midpoint between the two, where the cooking and ventilation equipment are located.

The standard system camera provided with the RSOS-2, for applications such as kitchen ventilation testing, provides high-resolution still images like the one shown above as well as HD video at 30 frames/sec. . 

Portable Retroreflective Shadowgraph Flow Visualization System

Specification Sheet for PRess-1

Our Portable Retroreflective Shadowgraph Flow Visualization System, unlike the RSOS-2, is designed for simplicity and portability for field use in imaging thermal plumes from cooking and observing the capture and containment of these plumes by ventilation hoods.  It provides a real-time shadowgraph video on a hand-held monitor.  This image can be both observed and digitally stored as documentation.  In order to satisfy a need in the commercial kitchen ventilation industry, we have made this instrument field-portable and affordable, e.g. under $10,000 per unit, similar to related instruments such as flow hoods and anemometers.

The Portable Retroreflective Shadowgraph Flow Visualization System consists of a tripod-mounted optical transmitting/receiving module powered by batteries (shown in the left foreground in the image below), and a retro-reflective screen that can be hand-held or temporarily mounted to a wall or similar surface.  (The screen in the image below is 3x3 feet in size, yielding a shadowgraph field-of-view of about 1.5x1.5 feet centered about the lip of the ventilation hood as shown.)  The legs of the tripod and the extension arm are adjustable to allow the transmitting/receiving unit to sight along the lip of a ventilation hood at the retroreflective screen in the background.

Weighing less than about 40 pounds, this instrument can be carried by one person and set up and operated by one or two people in the field, including commercial kitchen environments. Anyone should be able to operate the device by following the instructions provided; and no scientific training is required.

To illustrate typical results from this system, a still-image shadowgram of a domestic electric range with the turbulent convection plume from a hot cooking unit being captured by an overhead ventilation hood is also shown below.  This image was extracted from a video record captured and archived by the instrument.


A Portable Retroreflective Shadowgraph System being used in a commercial kitchen
Shadowgram of Heat Rising from a cooking surface into a ventilation hood


Foucault Schlieren System

The Foucault test for telescope mirrors forms the basis for a simple but powerful schlieren optical system.  As shown in the diagram below, an LED, which is the light source for the schlieren system, illuminates a spherical first-surface mirror at its radius of curvature (equal to twice its focal length).  The mirror returns an image of the LED emitter to the radius of curvature, where an adjustable knife-edge cuts off some of the light beam.  The remaining light proceeds to a camera and is captured as a schlieren image.  If the mirror is perfectly spherical, then a uniform darkening of the schlieren image occurs as the knife-edge is advanced to cut off more light.

Practically, this schlieren instrument consists of two separate assemblies: the Foucault tester with LED, adjustable knife-edge, and provision to mount a camera, and a properly-mounted spherical first-surface mirror.  An image of our Foucault tester is shown below.

Foucault Tester Picture

The LED light source is masked to produce vertical slit illumination.  Adjustment knobs underneath the tester provide the knife-edge cutoff adjustment and also allow translating both knife-edge and LED along the optical axis in order to find the location of the mirror center of curvature.  A digital SLR camera and telephoto zoom lens are shown mounted on the tester platform, which is provide with a fixture for a tripod mount underneath.  The 3.7-4.0 DC volts required to power the LED can be provided by batteries or a DC power supply.

This Foucault tester works with virtually any precise first-surface spherical mirror to do schlieren imaging.  Below are shown images of a small 4.25-inch-diameter mirror and a larger 10-inch mirror in mounts appropriate for present purposes.  Smaller and larger schlieren mirrors than these are available.


4.25 inch spherical mirror picture
10 inch spherical mirror picture


Finally, typical schlieren images from these two mirrors are shown below in order to illustrate the capability of the Foucault Schlieren System.  A gray-scale image of a candle-flame plume is shown to illustrate results obtained by the 4.25-inch mirror shown above.  The two color schlieren images, depicting a candle-flame plume and a cup of hot coffee, were obtained with our Foucault tester and the 10-inch spherical mirror, both shown above.  (Note that color schlieren imaging requires a somewhat-more-complex version of the Foucault tester than the one shown above for gray-scale imaging.)

4.25 inch black and white schlieren image of a candle plume 10 inch color schlieren image of candle plume
10 Inch Schlieren Image Of Hot Coffee Mug