Schlieren Imaging - Revision history

Jinyi: /* Theory */

2022-04-30T12:06:07Z

Theory

← Older revision		Revision as of 12:06, 30 April 2022
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	There exist several kinds of schlieren setup. But the basic idea is to convert the refraction index gradient with a knife-edge or pinhole. Here we only give a representative analysis based on our setup shown below.		There exist several kinds of schlieren setup. But the basic idea is to convert the refraction index gradient with a knife-edge or pinhole. Here we only give a representative analysis based on our setup shown below.

	[[File:Setup V1.jpg\|~~x400px~~\|center\|thumb\|Fig.2 Schlieren Setup]]		[[File:Setup V1.jpg\|400px\|center\|thumb\|Fig.2 Schlieren Setup]]

	The light coming out from the pinhole is focused by the concave mirror. The position knife-edge is adjusted so that half of the focused light point is cut off. The light field is then recorded by a camera.		The light coming out from the pinhole is focused by the concave mirror. The position knife-edge is adjusted so that half of the focused light point is cut off. The light field is then recorded by a camera.

Jinyi: /* Result Analysis */

2022-04-30T11:50:31Z

Result Analysis

← Older revision		Revision as of 11:50, 30 April 2022
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	==Result Analysis==		==Result Analysis==
			We converted the pictures to the matrix and extract the backgrounds. After doing this, we can get a clear picture of the highest contract.

	[[File: picture00001.png\|800px\|center\|thumb\|Fig.After processing (blow)]]		[[File: picture00001.png\|800px\|center\|thumb\|Fig.After processing (blow)]]

	[[File: picture00002.png\|800px\|center\|thumb\|Fig.After processing (hand)]]		[[File: picture00002.png\|800px\|center\|thumb\|Fig.After processing (hand)]]

			We were trying to drive the LED by a function generator and power amplifier. However, the LED we have cannot support 40kHz blinking. So we changed our plan and use a laser diode with AOM to get the 40kHz light source. The only problem is that the laser beam is coherent light and cannot be fully cut by the knife edge like an ordinary focused beam. So the video shown before is half bright and half dark. And we can only see the air density fluctuation around the knife-edge.

	==Conclusion==		==Conclusion==

	A sensitive schlieren setup is established in this experiment. The schlieren is able to detect very small air density changes in daily life, such as the thermal convection around human skin, or a cup of coffee. We also tried to carry out a scale to calibrate the relationship between the brightness variation in the image and the refractive index gradient in space. However, the effect is highly limited by equipment. The size of the mirror bounds the choice of our sound wavelength. At the same time, usual light sources cannot offer a flashing frequency as high as 40kHz. So a laser source driven by the acousto-optical modulator has to be hired as the source. A short single-mode fiber is used to filter the distorted wavefront, and also works as a better point light source than the physical pinhole. But the coherence of the laser reduces the visibility of fringes badly.		A sensitive schlieren setup is established in this experiment. The schlieren is able to detect very small air density changes in daily life, such as the thermal convection around human skin, or a cup of coffee. We also tried to carry out a scale to calibrate the relationship between the brightness variation in the image and the refractive index gradient in space. However, the effect is highly limited by equipment. The size of the mirror bounds the choice of our sound wavelength. At the same time, usual light sources cannot offer a flashing frequency as high as 40kHz. So a laser source driven by the acousto-optical modulator has to be hired as the source. A short single-mode fiber is used to filter the distorted wavefront, and also works as a better point light source than the physical pinhole. But the coherence of the laser reduces the visibility of fringes badly.

Jinyi: /* Measurement results of Version 1 setup */

2022-04-30T10:57:37Z

Measurement results of Version 1 setup

← Older revision		Revision as of 10:57, 30 April 2022
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	[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]		[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]

	Also, when we set both AOM modulation and sound wave frequency as 40kHz, we can find that the patterns around the knife-edge don't move. And when we tune the AOM frequency to 40.001kHz, we can find that the pattern starts to move by 1Hz frequency because of the frequency mismatch between the light and the sound wave. The same thing happened for 2Hz, 3Hz, 4Hz, and 5Hz mismatches. We can see the pattern moves quicker when the frequency mismatch increases.		Also, when we set both AOM modulation and sound wave frequency as 40kHz, we can find that the patterns around the knife-edge don't move. And when we tune the AOM frequency to 40.001kHz, we can find that the pattern starts to move by 1Hz frequency because of the frequency mismatch between the light and the sound wave. The same thing happened for 2Hz, 3Hz, 4Hz, and 5Hz mismatches. We can see the pattern moves quicker when the frequency mismatch increases. Please watch the video attached below:

	[[File: Video_laser.mp4\|center]]		[[File: Video_laser.mp4\|center]]

Jinyi: /* Theory */

2022-04-30T10:55:50Z

Theory

← Older revision		Revision as of 10:55, 30 April 2022
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	In our experiment, the concave mirror is placed <math>2f</math> from both the light source and the knife-edge. The sample is placed right before the mirror. So the displacement of light on the knife-edge is <math>2f\epsilon_y</math>. This will cause a brighter or darker area at the camera, depending on the direction of the refractive index. The relative change of brightness can be directly written as <math>\delta=\frac{2f\epsilon_y}{r}</math>, compared to the input background. From the angle of Fourier Optics, the knife-edge acts as a spatial filtering component, while the concave mirror is a Fourier transformer. The filtered information in the frequency domain is reconstructed by the objective of the camera.		In our experiment, the concave mirror is placed <math>2f</math> from both the light source and the knife-edge. The sample is placed right before the mirror. So the displacement of light on the knife-edge is <math>2f\epsilon_y</math>. This will cause a brighter or darker area at the camera, depending on the direction of the refractive index. The relative change of brightness can be directly written as <math>\delta=\frac{2f\epsilon_y}{r}</math>, compared to the input background. From the angle of Fourier Optics, the knife-edge acts as a spatial filtering component, while the concave mirror is a Fourier transformer. The filtered information in the frequency domain is reconstructed by the objective of the camera.

			[[File:AOM.png\|x400px\|right\|thumb\|Fig.3 Acousto-optical Modulator]]

	A known density distribution is needed to scale a schlieren setup. We picked an ultrasonic wave for its regular density distribution. A one-dimensional acoustic wave function writes:		A known density distribution is needed to scale a schlieren setup. We picked an ultrasonic wave for its regular density distribution. A one-dimensional acoustic wave function writes:
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	The acousto-optic modulator is driven by an 80MHz sound wave, the amplitude of which is switched on and off by a function generator at a 40kHz frequency. The laser is diffracted into several orders according to the grating equation <math>Asin\phi_k=k\lambda</math>. In such a method, the submaximal fringes could be used as a fast-flashing light source. However, the AOM asks for a highly coherent wave, this makes the image of schlieren less clear since interference happens in the field of view.		The acousto-optic modulator is driven by an 80MHz sound wave, the amplitude of which is switched on and off by a function generator at a 40kHz frequency. The laser is diffracted into several orders according to the grating equation <math>Asin\phi_k=k\lambda</math>. In such a method, the submaximal fringes could be used as a fast-flashing light source. However, the AOM asks for a highly coherent wave, this makes the image of schlieren less clear since interference happens in the field of view.

	~~[[File:AOM.png\|x400px\|right\|thumb\|Fig.3 Acousto-optical Modulator]]~~

	==Experiment==		==Experiment==

Jinyi: /* Theory */

2022-04-30T10:55:12Z

Theory

← Older revision		Revision as of 10:55, 30 April 2022
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	The acousto-optic modulator is driven by an 80MHz sound wave, the amplitude of which is switched on and off by a function generator at a 40kHz frequency. The laser is diffracted into several orders according to the grating equation <math>Asin\phi_k=k\lambda</math>. In such a method, the submaximal fringes could be used as a fast-flashing light source. However, the AOM asks for a highly coherent wave, this makes the image of schlieren less clear since interference happens in the field of view.		The acousto-optic modulator is driven by an 80MHz sound wave, the amplitude of which is switched on and off by a function generator at a 40kHz frequency. The laser is diffracted into several orders according to the grating equation <math>Asin\phi_k=k\lambda</math>. In such a method, the submaximal fringes could be used as a fast-flashing light source. However, the AOM asks for a highly coherent wave, this makes the image of schlieren less clear since interference happens in the field of view.

	[[File:AOM.png\|x400px\|~~center~~\|thumb\|Fig.3 Acousto-optical Modulator]]		[[File:AOM.png\|x400px\|right\|thumb\|Fig.3 Acousto-optical Modulator]]

	==Experiment==		==Experiment==

Jinyi: /* Measurement results of Version 1 setup */

2022-04-30T10:54:27Z

Measurement results of Version 1 setup

← Older revision		Revision as of 10:54, 30 April 2022
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	[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]		[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]

			Also, when we set both AOM modulation and sound wave frequency as 40kHz, we can find that the patterns around the knife-edge don't move. And when we tune the AOM frequency to 40.001kHz, we can find that the pattern starts to move by 1Hz frequency because of the frequency mismatch between the light and the sound wave. The same thing happened for 2Hz, 3Hz, 4Hz, and 5Hz mismatches. We can see the pattern moves quicker when the frequency mismatch increases.

	[[File: Video_laser.mp4\|center]]		[[File: Video_laser.mp4\|center]]

Jinyi: /* Measurement results of Version 1 setup */

2022-04-30T10:41:10Z

Measurement results of Version 1 setup

← Older revision		Revision as of 10:41, 30 April 2022
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	====Measurement results of Version 1 setup====		====Measurement results of Version 1 setup====
			During the experimental process, we notice that the output mode of the laser diode is very dirty and almost impossible to image. So we include a short single-mode fiber at 650nm to filter the higher-order modes. Below is the imaging results before adding the fiber and after spatial mode filtering. We can find the image quality is greatly improved.

	[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]		[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]

Jinyi: /* Measurement results of Version 1 setup */

2022-04-30T10:32:05Z

Measurement results of Version 1 setup

← Older revision		Revision as of 10:32, 30 April 2022
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	====Measurement results of Version 1 setup====		====Measurement results of Version 1 setup====

	[[File: picture00005.png\|~~800px~~\|center\|thumb\|Fig.Before filtering and after]]		[[File: picture00005.png\|600px\|center\|thumb\|Fig.Before filtering and after]]

	[[File: Video_laser.mp4\|center]]		[[File: Video_laser.mp4\|center]]

Jinyi: /* Measurement results of Version 1 setup */

2022-04-30T10:31:46Z

Measurement results of Version 1 setup

← Older revision		Revision as of 10:31, 30 April 2022
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	====Measurement results of Version 1 setup====		====Measurement results of Version 1 setup====

	[[File: picture00005.png\|~~400px~~\|center\|thumb\|Fig.Before filtering and after]]		[[File: picture00005.png\|800px\|center\|thumb\|Fig.Before filtering and after]]

	[[File: Video_laser.mp4\|center]]		[[File: Video_laser.mp4\|center]]

Jinyi: /* Measurement results of Version 1 setup */

2022-04-30T10:31:18Z

Measurement results of Version 1 setup

← Older revision		Revision as of 10:31, 30 April 2022
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	====Measurement results of Version 1 setup====		====Measurement results of Version 1 setup====

			[[File: picture00005.png\|400px\|center\|thumb\|Fig.Before filtering and after]]

	[[File: Video_laser.mp4\|center]]		[[File: Video_laser.mp4\|center]]