Stray Light Suppression in Lens Housings
By David Alster
A typical lens consists of several refractive elements housed together in a housing.
The housing has several functions:
- Protection of the refractive elements
- Maintenance of the correct spacing between the elements
- Maintenance of centering between the elements
- Where required – generation of thermal correction
- Mechanical features for interface with a system
These functions are performed using spacers and internal steps in the housing. For thermal correction these spacers might be made of dissimilar materials
A typical lens will have a FOV of interest. The incident light in the FOV is imaged by the lens on a focal plane or transmitted to additional optics.
Unfortunately it is generally not possible to entirely prevent penetration into the lens of light from outside of the FOV of interest.
If this “stray light” is allowed to reach the focal plane the system signal-to-noise ratio will be lowered and the mission effectiveness will be impacted negatively.
The methodology used to minimize this problem to a negligible level is as follows:
- The internal surfaces of the housing and the spacers are coated with Acktar light-absorbing coatings
- Where possible – mechanical baffle elements coated with Acktar light-absorbing coatings are incorporated in the lens housing.
- Light entering from outside the FOV will suffer a series of reflections with the coated elements – 98% of the remaining light being absorbed in each reflection by the Acktar coating.
- In this way – the intensity of any residual stray light remaining after this series of reflections from Acktar coated elements which might reach the focal plane is negligible.