If you deal with vintage lenses long enough, you’ll come across the need to address this issue. Sometimes you may not be aware that something is degrading within your lens, but an experienced technician can spot this during the assessment processes. Let’s start by explaining what a doublet is and why it may begin to separate.

A doublet, simply put, is two elements that are cemented together using optical adhesive. The advantage of this when designing an optical system is to allow more corrections to be made for performance, such as chromatic aberrations and/or spherical aberrations. This can be done by using two different glass types which would have different refractive indices. They may physically look like one element, especially if the edge blacking hides the join. The two surfaces that are to be cemented together also would not have any coating on them. This is because once the cement has bonded the two surfaces, there is no need for an anti-reflective coating.

As the cement ages, it can begin to degrade and break down. Early signs of this often look like a rainbow effect around the edge of the doublet (similar to an oil leak on a wet surface). It may also appear as tiny white/silver specs that one might think are in the coating (similar in appearance to condensation marks). On rare occasions it can appear that the doublet has cracked or shattered, but this is just the stress from the cement releasing.

This image of a 24mm Canon K-35 internal doublet, displays the rainbow effect that can often appear. 

Once a doublet begins to separate, there is no way to reverse this effect. The best option is to fully separate and recement the doublet using modern-day optical cement. Before beginning the separation process, it is good practice to mark the elements against each other for rotational alignment as well as measuring and checking the alignment of the doublet.

One method to separate the doublet is to heat it up until the cement melts away and allows the two elements to be parted. However, this method carries a high risk of damage. Due to the two elements being manufactured out of different glass types, they have a different coefficient rate of expansion. This means that one element is likely to expand quicker than the other when heated. As the elements are still held together with a strong cement, this can cause damage to one or both elements. Most vintage lenses are not readily available to get spare parts, therefore this is not the best option.

Another, more preferred option, is to bathe the doublet in a chemical solution at room temperature. While this method is far safer, it still carries a low risk that one element may become damaged during the process. This is why it is best to start with weaker chemicals and wait before progressing to more aggressive chemicals. The main downside to this method is the time it can take to fully separate the two elements. Depending on the degradation of the cement, this process can take up to 12 weeks (and sometimes more) for the chemicals to work their way between the two elements and fully release the cement. This requires frequent monitoring to check on the progress, though at no point should one try to force the elements apart.

Once the doublet is separated, all the remaining cement needs to be removed from both surfaces. It is at this point the elements can be inspected and checked for any defects before the re-cementing process begins. The re-cementing process needs to happen in a clean environment by a skilled technician. Preparation is key to make sure the process is a success. If any mistakes are made during the re-cementing process, it can cause weeks of delays as well as additional risk to the elements.

This image displays the cement being cured using UV light while brass blocks provide the centring.

Both elements are cleaned and prepared for the recementing. A few drops (depending on size of doublet) of optical cement are added, generally to the concave surface if possible. Then the other element is placed on top. At this point the element can be checked for alignment. Once happy with the alignment of the doublet, one final check is done to make sure there is no contamination or bubbles in the cement. Once the technician is satisfied, the cement is cured using a UV chamber. At this point, the process is complete, and the technician can continue with the service of the optical unit.

Generally, once the doublet has been recemented together and checked for alignment it would need to be re-edge blacked to prevent any internal reflections from stray light rays.

If you’d like a quote or more information on separating and re-cementing a doublet, please get in touch with us at sales@truelens.co.uk or calling our office on +44 (0)1455 848411.