Laser Illumination

To achieve good quality images equal consideration should be given to the illumination source as to the camera resolution and sensitivity.

If you have an incorrectly lit subject the resulting image will always be of a poor standard irrespective of the quality of the camera system. There are a wide range of light sources suitable for the majority of high-speed imaging applications; however, some applications may require something utilising a laser.

Example 1: Excessive motion blur

Most high-speed cameras have an electronic global shutter offering exposure durations down to a few microseconds; however, where a given subject is moving at extreme velocities relative to the field of view, motion blur becomes more of an issue.

A high-repetition rate pulsed laser can provide effective shutter durations typically in the range of 30ns - 250ns at frequencies up to around 50kHz without the need for an image intensifier (you will need to match the laser frequency to that of your camera frame rate).

Typical applications requiring this level of shuttering could be ballistics, where velocities in excess of 700m/s are common, or working at high magnification where the effect of any movement is amplified. As an example, consider these two cases in turn.

Example 2: Cold light source

Subjects imaged using high-speed cameras require large amounts of light to illuminate them. With light also comes heat. Should your test subject be exposed to these high light levels for extended periods of time, the subject will get hot and its properties change.

For example if imaging the production of various types of fibre, it is likely that the heat from the lights will melt the fibres as they are being produced, or, if impact testing a plastic part, the light will warm this part and it will become more elastic, the validity of the test is then put into question.

To avoid this scenario, a pulsed laser could be used as a light source. Because the equivalent amount of light can be condensed into a very short period the heating effect is minimised.

NOTE: For small areas of interest there are now commercially available pulsed LED light sources that may also be suitable. These will be less expensive than a laser and safe to use in any environment.

Example 4: Light sheets

Only lasers have the ability to be focused to a thin sheet of light that can be projected over a useful distance.

Why use a light sheet?
Light sheets permit you to visualise a cross section of a flow or spray.

How does it work? Particles entrained in the flow scatter light from the laser that permits them to be imaged by the camera. If the particles only move a short distance before the next image is taken you can visualise the movement of this flow. Higher flow velocities will require higher frame rates to maintain a small particle displacement - typically no more than 50 pixels - larger than this will result in a less fluid view of the motion. By having a thin sheet of laser light you can define precisely the plane within your flow you are imaging.

You can build on this technique to map the instantaneous velocity of the entire flow field being imaged by using a technique known as PIV or Particle Image Velocimetry. Software is used to calculate the distance the particles travel and by including a linear calibration such as mm/pixel and the time interval between images you can obtain the velocity in m/s. Some flows such as sprays are self-seeded i.e. they already have particles present that can be imaged, other flows such as air or water flows may require seeding or tracer particles to be introduced. These particles are carefully selected based on their physical size, mass and light scattering properties so that they have no negative influence on the flow they are tracing whilst providing a bright reflection that the camera can image.

Who is using it?
Anyone wishing to study properties of sprays, liquid or air flows around objects, within rooms or through ducting pipes.