Frequently Asked Questions

What is PSP?

Pressure Sensitive Paint is an optical sensor for pressure measurements. A paint-like coating is applied to a surface and the fluorescence of the paint is monitored with a camera. The paint fluorescence is a function of the local barometric pressure, and therefore, each pixel on the camera acts as a pressure transducer. 

What is TSP?

Temperature Sensitive Paint is an optical sensor for temperature measurements. A paint-like coating is applied to a surface and the fluorescence of the paint is monitored with a camera. The paint fluorescence is a function of the local temperature, and therefore, each pixel on the camera acts as a thermocouple. 

Can PSP be used in liquids?

Yes, but not to measure pressure. PSP is based on oxygen quenching and will respond to dissolved oxygen in liquids. Sensors for monitoring dissolved oxygen in liquids based on PSP are available. However, since liquids are not compressible, the paint will not respond to changes in static pressure in the liquid. 

Can TSP be used in liquids?

Yes, but you need a temperature gradient to see anything.

Will PSP detect a “force impact” with a memory?

No. PSP is does not respond to contact forces. We do have another sensor, Surface Stress Sensitive Films, that does respond to contact forces. For an explanation of the photo-physics of PSP, see Pressure Sensitive Paint.

How long will a PSP/TSP coating last?

The lifetime of the pressure and temperature sensitivity varies among paint formulations and is dependent upon the environment. The decay mechanism is generally photo-degradation. The paint or painted model should be stored in a dark environment and this will extend the life of the coating. It is not unusual for a painted model to be used for testing in production wind tunnels for 5-10 days.

Is PSP/TSP difficult to apply?

PSP/TSP is applied using common spray-painting techniques.

What is PSV?

Particle Shadow Velocimetry (PSV) is an optical technique for measurement of two components of velocity in a plane. PSV shares many of the attributes of microPIV and Forward Scatter PIV (fsPIV). It utilizes low-power pulsed light sources such as LEDs rather than lasers to illuminate and track the displacement of seed particles in a flow. The particle marking is based on extinction rather than scattering, and therefore, LEDs provide sufficient power. The major advantages of PSV are 1) price (LEDs cost much less than Lasers), 2) safety (No potential for eye damage), 3) surface scatter (PSV uses volumetric illumination so measurements close to surfaces is possible), and 4) speed (measurements have been made at over 15-kHz). For more details about PSV see our PSV page.

What is the temperature range of PSP and TSP?

For PSP the temperature range is generally 0 to 50 degrees C. Several paints will operate up to about 100 degrees C and formulations for testing at Cryogenic temperatures are available. TSP formulations generally operate between 0 and 100 degrees C. Again, formulations for Cryogenic testing are available and commonly used for detection of boundary layer transition.

What is a background image?

The background image is an image of the background of what the camera sees, with the LED off and no paint illumination. The background image is taken to compensate for any background lighting present in the area of the experiment. This image is then subtracted pixel by pixel from your wind-off and wind-on images.

What is wind-off/wind-on?

Wind-off and wind-on refers to the two images captured during data acquisition and used to compute a pressure field. Wind-off is the image taken without the load applied to the test object. If you were in a wind tunnel, this would be the image taken without the tunnel running. Wind-on is the image taken with a load applied to the test object. In a wind tunnel, this would be the image taken when the tunnel is running.

What is the dark threshold?

The dark threshold is a set intensity value that will exclude data with a signal level below that of the dark threshold. This dark threshold has reference to a minimum signal level that yields meaningful data.

Why do you take a ratio of two images?

The luminescent intensity from a pressure-sensitive coating can be a function of several parameters such as; spatial variations in excitation illumination, pressure-sensitive dye concentration, paint layer thickness, and camera sensitivity. These spatial variations are eliminated by taking a ratio of the fluorescence of the paint at the test or wind-on condition with the fluorescence of the paint at a known reference or wind-off condition. The process is described in detail in Pressure Sensitive Paint.

Why do you average images?

Image averaging is done to reduce noise in the final data by improving the shot noise. Assuming that the major source of noise in the data acquisition system is camera shot noise, averaging a larger number of images will improve the signal-to-noise ratio by the square root of the number of images. For a complete analysis of noise sources in PSP, see Pressure and Temperature Sensitive Paint by Liu and Sullivan.

Why do you have four images each time you capture an image with the Color camera?

The four images seen with the color camera correspond to the red, green, and blue pixels on the CCD array. The color camera separates these colors from the true color image and saves all of them for each capture. The filter pattern is 50% green, 25% blue, and 25% red, meaning there are two green, and one each of red and blue for each image. The emission of the pressure probe in the paint is recorded by the red channel, and the reference probe is recorded by green. The wavelength of light emitted from the pressure probe is in the range of the wavelengths of red light (630-740nm) and is therefore seen as red by the camera. The wavelength of light emitted by the reference probe is in the range of the wavelengths of green light (520-570nm). No probe in the paint corresponds to the blue light wavelengths (440-490nm), but the blue channel can be used as a background to correspond to any illumination effects on background objects.

What is the reference probe used for?

Binary paint is composed of two probes, a pressure sensor and a reference probe. The reference probe is used to compensate for errors caused by model displacement/deformation. It can also be used to compensate for temperature. 

What is the long-pass filter?

The long pass filter is used isolate blue-UV excitation light from the LED and pass the red shifted fluoresce from the PSP or TSP.

What is the LED used for?

The LED is used to excite the dye in the paint and cause it to fluoresce.

How is the paint calibrated?

An aluminum coupon is painted with PSP and the coupon is placed into a calibration chamber where the pressure and temperature can be controlled. The sample is illuminated using an LED and the luminescence from the sample is collected through a long-pass filter onto a CCD camera. The temperature and pressure within the chamber are varied over a range of temperatures and pressures while the luminescence from the sample is recorded at each condition. 

What is the paint calibration file?

Once the paint is calibrated, a calibration equation is created by performing a curve fit to the data.  This is accomplished by fitting a first or second order least squares curve into the data.  The variables of interest are Temperature, Pressure, and Intensity Ratio.  The resulting equation, along with specific calibration information, is stored to the calibration file. This file is then used to convert processed data back to pressure.

What kind of spatial resolution can be achieved?

This is limited only by the CCD array and the optics.  For example, use of a PSP-CCD camera with 1 to 1 optics would result in a very small region of interest where the array was imaging about 10-micrometers per pixel.  This would represent a limit on the spatial resolution.  In reality, the paint thickness is closer to 20-30 micrometers so this would represent a limit on the spatial resolution.  However, in principal the spatial resolution is limited by the optics, not the paint.

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