Preventing Lens Separations with
Norland Optical Adhesives


Lens separations or bond failures can be divided into two types, adhesive and cohesive. Identifying which of these two types of failure is occurring will determine what steps to take to prevent them.

1. Adhesive failure occurs when the cement separates from the glass surface. It is seen as a shiny area in reflected light. It can be any shape and can be identified by the presence of Newton rings, or a color fringe.

2. Cohesive failure occurs when the cement pulls away from itself. This can be identified by the fern-like voids, called feathering, appearing along the edge of the lens or sometimes in the interior.


When adhesive failure is identified:

Because Norland Optical Adhesive has excellent adhesion to glass, adhesive failure is not a serious problem. When this type of failure does occur it is usually caused by a lack of cleanliness of the glass surfaces. If there is a trace of oil, fingerprints, dirt or any contamination between the cement and the glass there is a potential for adhesive failure. The solution for this is to look closely at the cleaning process. Things like contaminated solvents or improper handling could be the cause.


When cohesive failure is identified:

Cohesive failure is due to internal stress in the cement and is caused by uneven curing. By uneven curing we mean there is a great change in UV light intensity from one portion of the cement to the next causing one portion to cure slower than the rest. Simple causes of uneven curing are shadows cast by such things as the ground edge of the lens or a part of a jig or clamp used to align the lens.

The danger of curing unevenly is greatest during the precure. This is when the cement changes from a liquid to a solid and shrinkage occurs. Normally this slight amount of shrinkage will not form stress but will pull the two glass surfaces closer together.

After the lens is precured the two glass surfaces cannot move. Any cement in a shadow will not have been precured yet and when it does cure it cannot pull the surfaces closer together. This area now has internal stress which will eventually lead to cohesive failure.

Simple causes of cohesive failure can be corrected by eliminating any shadows from the bondline. Ways to do this include changing the position of the light source, adding more lights or clamping the lens in a different way.

A less obvious cause for uneven curing is due to the configuration of some lenses. Certain lenses will send the light entering towards the center of the lens so that it doesn't reach the edge. These lenses can be identified by the fact that the bondline edge is not visible to the eye when observed through the top lens. See Fig. 1. Because the edge is not cured at the same time as the rest of the lens it will have internal stress which will lead to cohesive failure.

There are a number of ways to overcome this. An even cure can be had if the lens is exposed to UV light through the bottom lens. If it is not possible to invert the lens for the precure, the light can be placed at the same height as the edge of the lens and only cure the bond joint during the precure. Fig. 2. This is done as the lens is rotated during alignment. The lens is then inverted and the whole optical bondline is given an even cure. Another method for precuring is to have the light at a very low angle to the top lens surface. The low angle of light is able to reach the edges in many cases. Fig. 3.

In summary, Norland Optical Adhesives will do an excellent job of bonding precision optical components when properly handled. If separations do occur, first identify if it is adhesive or cohesive failure and then take the appropriate steps to correct them.

Bondline edge receives no light. Precure only bond joint. Low angle of light reaches bondline edge.

 

 

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