For a backlit scene, the light comes from behind the scene so that the object being viewed is shadowed into the camera. Backlighting the object, for maximum contrast will give the best repeatable results. Backlighting is recommended if the camera is used to measure the object or certain aspects of the object and/or for part recognition since the trip light level can move a large amount without degrading the results.
The backlit light source must be large enough so that the camera, without the object in the field of view will see a uniform amount of light. This is normally accomplished by using several flood lamps and shining the flood lamps onto a diffused surface (ground glass, or diffused white plastic, or frosted mylar), such that a uniform light source is created. Placing the object between the diffused surface and the camera will shadow the object into the camera with maximum contrast. Adjust the f-stop to the maximum value that the amount of light and integration time will allow. NOTE: For non- contact measurement of the objects' size, the magnification changes in relation to its distance from the camera to the object.
In selecting a lens, the magnification change as the object moves in the Z axis must be considered. The farther the lens is from the object the less the size changes as the object moves in the Z axis. The equation that relates the Z axis motion of the object to the change in lens-to-object distance is:
Z = change in object motion to/from the camera L= Lens to object distance;
% area change = 200 * ( Z/L + Z*Z/L*L)
For example, if the Z axis motion is 1/2 inch and the lens to object distance is 20 inches, then the change in size of the scene, as the computer sees it, is 5.25% in area. In comparing the MicronEye camera, lighting and processing, to other industrial systems that do gray scale processing, where lighting is not a dominant factor, there is usually a 300 to 1 cost trade-off. Placing more emphasis to correct the lighting so that a single threshold can be used produces a saving of 300 times.