High Contrast Imaging Through Scattering Media Using Structured Illumination Fourier Filtering

Abstract

In imaging, obtaining high contrast is of great importance. When imaging through scattering media, optical filtering techniques are used to filter out multiple light scattering which causes loss of contrast. Imaging through scattering media is used in Bio-Medicine when looking through human skin; in characterization of spray systems in Combustion Engineering as well as in other applications where visualization within fog, smoke or turbid water is necessary. This project focuses on the development of a novel optical filtering method called Structured Illumination Fourier filtering, or SIF. It is shown here that SIF can achieve high contrast imaging using visible light through a scattering medium of optical depth up to OD = 15. Measurements are performed through a water dispersion of polystyrene spheres of D = 0.13 μm. The performance of SIF is compared to the individual performances of Fourier filtering and Structured Illumination. SIF is shown to outperform these techniques in optical depths above OD = 10. A common way to increase contrast through a given scattering medium with small particles is to use a longer wavelength. It is shown here that SIF can achieve equally high contrast through a medium using blue, green and red light. Also, a comparison between the recorded images and simulations from the newly developed online Monte Carlo software Multi-Scat shows great promise for the validation of the software.

Popular Abstract

New technique gives vision through thick fog
Everyone who’s been in a car driving through a thick fog knows that it’s impossible to see ahead. Turning on the high beam doesn’t help, in fact it seems to make you see less. But what if the key to seeing through the fog wasn’t just more light, but to use the light smarter. A new optical filtering technique forms the light into stripes and filters it through a pinhole, making it possible to take high contrast images through thick fog.
For a driver to see a road sign through a fog, light rays must hit the sign and travel in a straight line to the driver’s eyes. Unfortunately, fog is a medium which scatters light. It is composed of millions of small droplets of water that the light rays can get reflected by. The reflected rays bounce between the droplets so that when they reach the driver, they seem to originate from the fog itself. But there is usually a small part of the light that doesn’t scatter in the fog, but travels straight through it. Unfortunately for drivers, this small part is hidden in the large amount of light that scatter around, making it difficult to see.
While they cannot help the driver see, optical filtering techniques exists that makes it possible to record images through thick fogs. They work by separating light that has traveled straight through from all the scattered light. Structured Illumination Fourier filtering, or SIF, is a powerful new such technique that is being developed at Lund University. With SIF, images with very high contrast can be recorded through scattering media that can’t be seen through at all.
Seeing, or taking images, through scattering media is not only desirable when driving a car. Human tissue is another material which scatters light in much the same way as fog. This makes imaging through scattering media very important in bio-medicine. Other media which could be imaged through using SIF are turbid water, thick smoke or milk.
Imagine that you want to take an image of toy figure inside a glass of milk. Now, instead of using the flash of the camera, you illuminate the glass of milk by putting a laser on the other side of the glass, directed back towards the camera. By doing this, you can take an image of the toy figures shadow. To be able to take an image of the figure, SIF uses two tricks:
The first trick is to put a striped mask on the laser. Instead of a normal laser beam, the laser light becomes stripes of light. When this light enters the milk, it starts bouncing around just like the high beam in the fog. This scattered light no longer shows any sign of the stripes. But the small part of the light that doesn’t scatter, travels straight through the milk and onto the camera. This light can now be recognized by its stripes and all other light can be removed. By recording three such images a complete image of the toy figure with very high contrast can be constructed.
The second trick is to put a blocking disc with pinhole behind the lens inside the camera objective. A lens refracts incoming light differently depending on the incoming angle. The striped light traveling straight ahead is allowed through the pinhole, but light that has been scattered in the milk arrives in an angle, misses the pinhole and is blocked off.
By combining these two tricks, SIF can be used to take better images through thicker scattering media than before. The SIF technique is very new and is still being developed. A new step in the development of SIF is taken in this master’s thesis.

• author: Frantz, David
• supervisor: Edouard Berrocal
• organization: Combustion Physics; Department of Physics
• course: PHYM01 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: scattering media, Structured Illumination, optical Fourier filtering, Modulation Transfer Function, Monte Carlo simulation
• language: English
• id: 8919846
• date added to LUP: 2017-07-04 15:47:42
• date last changed: 2017-07-04 15:47:42