Special Applications

ISSI provides custom made LEDs upon request and also several models for different experimental applications. We work with customers to provide solutions to their applications. Customization includes specific output wavelengths or modified mechanical design to fit specialized test cells.

PARTICLE SHADOW VELOCIMETRY

Particle shadow velocimetry (PSV) utilizes low-power pulsed light sources such as LEDs to measure the displacement of seed particles in a flow. PSV images can also be used to characterize particle parameters such as size and shape. Since this is a volumetric illumination technique, it relies on the receiver optics to optimize the depth of field for the measurement. Any pulsed light system can be used as the light source; however, LEDs are particularly well suited since they can be overdriven in a short-pulse mode to produce intense sub-microsecond light pulses. Furthermore, the use of LEDs combined with a high speed camera results in a high-speed system with bandwidth of 10’s of kHz. Because the technique does not rely upon weak-particle light scattering, lasers are neither necessary nor recommended for use with this approach.

PSV uses light extinction (particle shadows) rather than light scattering in a laser sheet, resulting in high-quality particle imaging at much lower illumination levels. These safe illumination levels can be generated by a pulsed-LED system at a fraction of the cost of a pulsed laser. Multi-color LED illumination permits the use of a low-speed camera, further reducing system cost. Different color LEDs are sequentially pulsed, at high speed, and the sequential particle images are captured in a single frame on the camera’s three color planes. Narrow depth-of-field optics are employed to image a two-dimensional plane within the flow volume, similar to what is achieved with a laser sheet. Measurement close to surfaces is possible because the system is not degraded by surface reflections which severely limit laser-based systems. The PSV approach is fully compatible with the seeding approaches and specifications used with conventional PIV systems.

PSV is not an alternative to Particle Image Velocimetry (PIV), but a complementary technique. PSV is advantageous over PIV close to structures or surfaces where laser scatter may cause issues with data collection.

PARTICLE SHADOW VELOCIMETRY

• Capable of increased measurement frequency since LEDs can be pulsed at higher rep rates than typical laser systems
• Due to the orientation and lower power of the LED, wall reflections are eliminated or significantly reduced compared to PIV
• Since PSV directly images the particle, information such as particle size and shape can be determined
• Optical alignment is typically simpler

Disadvantages of PSV compared to PIV

• LED pulse time is typically an order of magnitude longer than laser pulse time. This prevents the use of PSV in high speed applications.
• The depth of measurement in PSV is controlled by the camera optics, and is typically slightly thicker than the PIV light sheet.
• Higher magnifications or larger particles are required for PSV since the particle shadows are directly imaged instead of scattered light.

Cylinder in Cross Flow

PIV Acquisition: EF Lens with AF Bellows and LC-2 Lens Controller and PSP-CCD-C Camera

3-COLOR PARTICLE SHADOW VELOCIMETRY LED

Product ID: LM2X-DMHP-RGB
The LM2X-DMHP-RGB is a 2-inch, air-cooled LED light source that provides three-color (Red, Green, and Blue) outputs from its LED head. This LED is used for Particle Shadow Velocimetry (PSV) to illuminate small seed particles in a flow field to measure velocity and particle density. The LM2X-DMHP-RGB has three BNC inputs to control the timing and pulse width of each of the output colors from an external pulse generator. The pulsed LED operation is controlled by applying a TTL voltage to the external BNC(s) on the module. The rise time is less than 200-ns and the fall time is less than 100-ns and the duty cycle is 5%. The light distribution from the unit is approximately Gaussian for distance greater than 18 inches from the source.

Ordering information: LM2X-DMHP-RGB (LED head with short-pulse/high-power driver)

SPECIFICATIONS

Output Power	2.0 W
Input	48V 2.5A @ 120VAC
Stability	~0.1 % per hour after warm-up
Maximum duty cycle	5%
Rise time (10% - 90%)	< 200-ns
Fall time (90% - 10%)	< 100-ns
Operating temperature range	-10°C to 60°C
Wavelength	460-nm, 520-nm, 630-nm
FWHM	+/- 18-nm
Warranty	12-months
ECCN	EAR99

SCHLIEREN PHOTOGRAPHY

Schlieren photography is a technique utilized to image fluid density gradients. The density gradient of the fluid gives rise to refractive index changes which distort the collimated beam of light between two mirrors and thus the point of focus. Using a knife edge, variable density slide or color slides at the focus to exploit this effect allows high-contrast imaging of otherwise nearly invisible density gradients. At the focus, the light intensity is cut in half by the knife edge. Refractive index changes in one direction are brighter and in the other direction are darker. This type of imaging is widely used in wind tunnel, heating, ventilation and air conditioning (HVAC) research.

The ISSI product line includes a high-intensity LED point source with a variable slit attachment for use in schlieren photography. This LED is available in continuous/long-pulse and short-pulse/high-power mode. Output wavelength can be set to user specifications.

SCHLIEREN LED

Product ID: LMS-XXX
The LMS-XXX LED is a 2-inch, air-cooled LED light source used for schlieren and shadowgraph applications. The LMS-XXX has one BNC input to control the timing and pulse width of the output from an external pulse generator. The pulsed LED operation is controlled by applying a TTL voltage to the external BNC(s) on the module. The rise time is less than 200-ns and the fall time is less than 100-ns and the duty cycle is 5%. The light distribution from the unit is approximately Gaussian for distance greater than 18 inches from the source.

SPECIFICATIONS

Output Power	~2-3 W
Input	48V 2.5A @ 120VAC
Stability	~0.1 % per hour after warm-up
Maximum duty cycle	5%
Rise time (10% - 90%)	< 200-ns
Fall time (90% - 10%)	< 100-ns
Operating temperature range	-10°C to 60°C
Wavelength	User Specified
FWHM	+/- 18-nm
Warranty	12-months
ECCN	EAR99