Backlight Shadowgraphy


Backlight / Shadowgraphy is a widely used imaging technique. Under the correct conditions it can provide high contrast images of objects. This is particularly important if you are interested in an objects shape or size.

The idea is a simple one - a light is shone towards the camera creating a white image, the subject is then passed between the light source and the camera thus blocking the light path and creating a dark area on the image. If the subject is stationary you can focus your camera onto the objects profile and obtain a sharp outline. If the subject is moving, in order to freeze the motion (remove blur) and obtain a crisp outline you will need to have a sufficiently fast shutter (short exposure time). In extreme cases a strobe light source such as a pulsed laser or LED can be used.

For more detail on when to use a laser please refer to Case Study: Laser Illumination. Using this technique you will not be able to obtain 3D information about the object shape. If you wish to resolve features on the object surface you can use a combination of front and back lighting. The technique is not limited to solids, it can equally be applied to fluids, multi-phase flows, in fact any subject that will completely or partially block the path of light to the imaging sensor.

Applications


- Spray Analysis
- Welding
- Bubble formation and growth
- Droplet impaction / coalescence
- Ballistics
- Particle size and shape