The power required for illumination is at least an order of magnitude less than that for other methods

The bubble size required for a given size image is smaller because the entire bubble throws a shadow. Other methods produce spots of light smaller than the bubbles. The image of a bubble does not change in brightness with depth in the chamber as is the case with side illumination

The uniformity of appearance of the images makes it possible to overexpose uniformly so that the images on the film are smaller than the resolving power of the lens would indicate by about a factor of two

The background-to-noise ratio in automatic track following equipment is much lower because the image on the film is clear against a blackened background

The lights can be located outside the chamber where they are readily accessible when there are light failures. An objection to Scotchlite arises from the fact that the front element of a wide angle lens must be made small so that the ring light around the lens may have a sufficiently small radius to satisfy the angularity requirements of the Scotchlite. This restriction on the front element of the lens makes the problems of distortion in the image more difficult. VIII-2 An objection raised by others has been heard which goes as follows. Suppose that due to thermal turbulence the image of a bubble is badly out of focus. Then the contrast of the image is sufficiently low as to fail to leave perceptible lightening on the black background of the negative. However, if sufficient illumination is available for dark field illumination, a blur will appear as a darkening of the negative. The difficulty with the argument arises from the fact that clear tracks will then be greatly overexposed, will spread by halation on the film, thereby losing much detail. This is a characteristic of the variation of exposure with depth in the chamber when side lighting is used. This objection to bright field illumination can be stated in another way. Suppose the bubbles are very much smaller than the circle of confusion of the lens. Then with bright field illumination the bubbles will not show. However, with dark field illumination sufficient light can be powered in to make them visible. The same difficulty with this argument appears again. Bubbles close to the lens will be badly overexposed, thereby losing detail as before

Another argument in favor of bright field illumination concerns the ability to scan visually very fine tracks. A very fine white line on a black background is easy to see. A similarly fine black line on a white background takes much greater visual acuity. Since the widths of tracks in these large chambers vary by at least a factor of five depending upon the distance from the lens, this greater ability to see very narrow images on the film becomes very important. Photographs of Tracks