Back-illuminated sensors | Better, but only a bit

This is a new type of 'reversed' sensor design which aims to improve sensitivity or the quality which can be achieved at high ISOs. It's currently appearing in higher-quality compact cameras instead of conventional CCDs and their ever-increasing megapixel counts.

The problem with conventional 'frontside illuminated' (FSI) sensors is that between the microlenses and RGB filter on the surface and the photoreceptors themselves is a layer of circuitry which restricts the angle and amount of light passing through to the photoreceptors. This diagram, from sensor manufacturers VisEra Technologies, shows how the 'backside illuminated' (BSI) design gets round this.

The term 'backside' suggests that the sensor is illuminated in some way from the rear, but that's misleading. What's actually happening is that the microlenses and RGB filters are staying where they are, but the circuit and photoreceptor layers are reversed, so that the photoreceptors are now at the front.

In this position, they're not blocked by the layer of circuitry, which has two main advantages:

• The photoreceptors can be (a little) larger, which means higher sensitivity and reduced noise

• They can accept light at greater angles (chief ray angles - 'CRAs'), which means less vignetting and the potential for wider-angle lenses, better zooms and higher maximum apertures.

Backside illuminated sensors are already appearing in compact digital cameras, like Nikon's CoolPix P100, for example. But while they have a clear theoretical advantage, the results from the Nikon and other 'backside illuminated' cameras suggest that the improvements may be too small to see except in laboratory tests. This is because they tiny sensors in compact cameras have very clear performance 'ceilings' related not just to the photoreceptor size but to fundamental lens design issues and basic optical physics too.

Backside illuminated sensors could prove much more interesting if they reach the digital SLR market, though.