Filters are effects modules, often referred to as “plug-ins” which reside in the computer and can be accessed from the Photoshop’s Filter menu. They can be applied to the whole or to part of an image.

The use of filters within Photoshop is threefold. First, they imitate the effect of traditional photographic techniques - images can be blurred and sharpened in a digital simulation of depth of field and lens sharpness. They can also add painterly and impressionistic aspects to a picture and, finally, they are able, by all manner of digital metamorphoses, to turn the original image into a thing of new and startling beauty.

In evaluating these filters it is worth remembering the special requirements of schools and colleges. Graphics programs are memory-hungry beasts and Photoshop is no exception. Some of these plug-ins perform highly sophisticated calculations and, if used on a slow machine, can take an age to apply.

Adobe Gallery Effects 1-3 (Pounds 59 per volume: from Adobe. Tel: 0800 232223). Each of these volumes contains 16 filters providing unspectacular but rewarding effects such as Mosaic, Watercolour and Charcoal. The interface is well designed and uncomplicated but the preview window could be larger.

The Black Box (Pounds 160 direct from Alien Skin Software. Tel: 001 919 8324124) is a collection of six filters including Glow and Drop Shadow that can add depth and perspective to a selected image by the application of shadow and light. It can also produce a variety of embossing effects by placing a bevelled edge behind an object. Version 2.0 will feature previews, be able to work in Photoshop layers and include at least four new filters.

Xaos Tools (from Letraset, Tel: 0171 928 3411) produces two filter packages. Terrazzo (Pounds 159) is a superb kaleidoscopic tiling effects program but, at twice the price of its more versatile stablemate, something of a luxury in the classroom. Paint Alchemy 2.0 (Pounds 79) is one filter with a large number of adjustable parameters. Anyone who used the original program will find the speed and design of this new model a revelation. The application of different brushes, textures and colours makes this a subtle and extremely powerful filter.

The sophistication of the new generation of plug-ins has left some of their predecessors looking jaded and in need of an update. None more so than Andromeda Series One (Pounds 99 from Principal. Tel: 0181 813 5656) With the exception of the Design filter this collection now seems gimmicky and uninspired. Andromeda Series Two, however, is a different kettle of pixels (Pounds 99, also from Principal), enabling the user to wrap an image accurately around various three-dimensional shapes such as cylinders and cubes.

Kai’s Power Tools 2.1 (Pounds 150 from Principal 0181 813 5656) is a set of 37 filters which includes enhanced versions of some of the Photoshop suite such as Blur, Noise and Sharpen and two others, Gradient Explorer and Fractal Designer, which have been likened to a cross between your favourite toy and a bagful of electronic lollipops.

These can generate the most dazzling patterns, and the elements of serendipity and experimentation built into the interface ensure that the final effect is never entirely predictable. Alienskin Textureshop 1.0 (Pounds 69 from Roderick Manhattan. Tel: 0181 875 4400) generates the same sort of textures as the Texture Explorer in Kai’s Power Tools but it also creates unique 3D effects which can be further modified by the application of a controllable light source.

Finally, Flo 1.4 (Pounds 165 from Letraset) which, while not a plug-in, is a highly enjoyable image manipulationdistortion program whose results can be imported into Photoshop. The interface is clear and easy to use and there are few children who will not relish the opportunity to send their teachers or fellow pupils through a less than flattering digital hall of mirrors!