Clearer vision: the case for aspherical lenses

May 2003 CCTV, Surveillance & Remote Monitoring

Aspherical lenses are nothing new. They have been available to professional security installers for many years, but have never really received the level of interest that they deserve.

When it comes to optics in general, there are two schools of thought. The first is that the lens affects the overall quality of the system, and therefore a quality item must be carefully selected for not only each application, but for each individual camera. Considerations must be paid to the field of view, ambient illumination, expected risk (both what the user wants to see and when they want to see it), camera type, etc. The second way of thinking is that lenses are, by and by, pretty much the same, and therefore paying more for a brand name unneccessarily eats into the budget. Sadly, those who subscribe to the latter way of thinking by far out-number those who understand the true value of optical devices and adhere to the first way of thinking.

Cameras are devices that change chrominance and luminance into electrical signals that are changed into images at a later stage in the surveillance process.

The lens is the device that focuses the chrominance and luminance onto the CCD. It is the quality of the light falling on the CCD that affects the final image. Lenses which do not correctly focus the light, or which create aberrations, will ultimately affect the overall performance of the system.

Aspherical options

The quality of lens determines many aspects of its performance. As well as the drives and the physical construction, the most important element of any lens is the quality of the ground optical surface. It is the surface of the lens that differentiates aspherical lenses from spherical lenses.

An aspherical lens is able to enhance a camera's performance because of its shape, and the way that shape defracts light so that it has a common focal point, no matter whether it is from the centre or the edge of the lens.

When cameras are tested for sensitivity, the manufacturers will usually define specific parameters which might not be achievable at all sites. In other words, some installers might find that getting a real improvement in sensitivity might be expensive if relying on a higher specification camera. Scene illumination is all well and good, but again there are issues involved such as long-term cost of ownership, light pollution (where white light is used), etc. However, improvements through the addition of an aspherical lens are just that - improvements!

Conclusion

The price differential between an aspherical lens and a spherical alternative is enough to raise a few eyebrows. It is not a case of a few rands more.

Aspherical lenses will not see in the dark; let us not think that they do away with the need for other considerations. However, every application has differing factors to consider, and where there is some form of incidental lighting, or where surveillance is required in difficult conditions but not in darkness, these lenses are definitely worth a look.

For more information contact C Video Concepts, 011 310 3625.





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