Domes... the undercover problem

I have watched with awe over the last five years as the number of makes, sizes and features of domes have proliferated at an enormous rate. Because of this, I read with interest a recent article on domes in CCTV Solutions.

The article by Kevin Smith of Philips CSI (September 99) was very informative, covering many aspects of dome development, from their higher speeds, built-in intelligence, ease of installation to aesthetic appearance ... much easier on the eye than the older pan/tilt camera units.

These domes have come from England, The States, Europe and from the many manufacturers in the Far East, with Taiwan and Japan being the main countries involved.

In the early days of domes, units were large and crude, merely covers over existing pan/tilt units, with normal size camera/lens combinations being fitted. Units were limited in their features, such as continuous rotation, due to the high cost of slip rings, and presets, due to lack of gap-less potentiometers.

MUSHROOMING SUB-SECTOR

But as we know, with the injection of camcorder technology into the CCTV industry, the dome variants have mushroomed so that a visit to IFSEC 99 found almost every manufacturer in the CCTV Village displaying a dome product of some kind or other.

Many domes still need their own dedicated control equipment to operate, which can make system design difficult and costly in certain cases. The introduction of domes such as the JVC TK-C675, which can be used from a large number of different control systems, was a welcome change.

However, a number of fundamental problems still seem to exist, as I see it, in dome design generally. There seems an obsessive urge to make domes faster, resulting in some that hurtle around at 360 degrees per second. This may be fine on an alarm input activating a dome to a preset position, but it can sometimes make slow tracking from a distance (i.e. zooming out) difficult, owing to the high gearing ratio involved.

Careful design and the use of quality gearing etc can help with this problem but, as often is the case, cost is a major factor in the manufacture of these units so quality can go out of the window.

HEATED ARGUMENT

As the size of the imager format gets smaller, so the size of the zoom lenses that are an integral part of the new dome cameras get bigger. Whereas a 10:1 was often used, we are now seeing 16:1 and 18:1 coming through.

One major drawback in the use of domes fitted with larger ratio zoom lenses is when they are used outdoors. Most of the small dome cameras were developed for inside use and as such were optically correct when used to zoom in on a subject at the other end of a warehouse or a superstore. The operator does not even have to focus on the distant subject as the dome units often have auto-focus features fitted.

To be used outdoors, these small domes have to be fitted inside a larger domed cover, with heaters and sometimes blowers to keep moisture in check.

As manufacturers have 'shoe horned' these essentially internal domes into increasingly smaller outer fixed covers, optical clarity has become a problem. The problem is optical refraction.

LEARNING CURVES

When light passes from one medium to another, i.e. air to water or, in the case of a dome outer cover, air to plastic then back to air, it is bent from its path by refraction. If the light goes through a flat glass, as in a normal camera housing, although it is refracted, it still enters the camera lens essentially unchanged.

If the light travels through a curved dome cover, however, which is bowed in two planes and also has a decreasing thickness towards its lower part, it has no chance of coming through unchanged and then on into the lens system.

The main problem then is when the zoom lens is looking at a distant subject through the upper part of the outer dome cover, where the plastic is somewhat thicker. Under these conditions a sharp focus is not easily achieved. The cover has to be of a reasonable thickness to offer some protection from the elements, so making a very thin cover is not feasible.

A dome some two metres in diameter would seem to be the answer, but of course, nobody wants something like that stuck outdoors on a pole!

The lesson is: To be forewarned is to be forearmed ... so before buying your small, high speed, all 'singing and dancing' dome get a demonstration from the manufacturer. Check out its worth under your working conditions:

• for focus at a distance with no distorted picture due to poor quality covers

• for control of panning speed

• and for its integration into other systems.