The state of fear or sexual arousal that a person experiences when looking at something will be measurable thanks to a technology which can gauge how pupiles dilate.

A Danish firm has developed an eye sensor which not only analyses where a person is looking but also more minute variations, such as the way pupil size changes and the way they blink. Both measures are related to a person's emotional response to what they are seeing, and are likely to be of use to advertisers keen to monitor the effectiveness of their campaigns.

A person's 'blink rate' relates directly to the heart rate, and in turn to instinctive reactions such as fear and sexual excitement. Pupils work in a similar way - for instance by dilating before an imminent, life-threatening event - making their behaviour an important aspect of the way someone responds to what they see.

To use the equipment, a person sits down in front of a computer, and is shown a series of images. While they watch the screen, a camera behind the screen tracks the movement of their eyes. A computer then rates various aspects of their gaze - including the speed of their blinks and their pupil dilation - on a scale of one to ten.

"A fast blink can indicate a a high state of arousal - there are many different kinds of blinks," Peter Hartzbech, chief executive of iMotions, the company which makes the technology, said.

Techniques for gauging emotional response, for instance by measuring the sweatiness of the skin, or heart rate - both indicators of fear - have been around for decades. But Mr Hartzbech said that his firm's method was unique because it was less intrusive. The subject did not have to be hooked up to any equipment using wires, and the technology also allowed for natural 'noise' in pupil dilation that was caused by variations in light, he said.

Future applications could involve placing a camera above a driver in the car to watch for variations in pupil dilation in the event of a looming accident, and in surveillance to gauge fear in suspects

The technology is initially being targeted at market research companies, which give feedback to advertisers about the effectiveness of campaigns. Executives at such firms were sceptical, however, saying it wasn't clear what could be learnt from the eye-tracker that couldn't be ascertained from traditional 'focus group' studies, during which people are asked about their responses to ads.

"People are quite conscious of their emotions - so it's not as if you couldn't ask them, for instance, whether they were afraid," Dan White, research director of Millward Brown, an arm of the agency WPP, said.

Les Binet, European director of DDB Matrix, a part of DDB London, the London-based agency, said: "You're also only going to be able to pick up certain types of emotions. These techniques might be good at gauging fear, but how are they going to go picking up, say, wry amusement?"

Mr Hartzbech said that "pupilometry" was concerned with more instinctive responses, which people could be dishonest about in a question and answer context. "What we're measuring is something more primitive - a lower-level response similar that has been around since man was in caves," he said.

Emma Kirk, strategic director at User Vision, a Edinburgh-based user experience research firm which is already using the technology, said: "Eye-tracking has been around for a while, but what's different about this is that it gives us an understanding of how engaged a person is. It shows how turned off, excited, and energised a person was."

Richard Storey, chief strategy officer at M&C Saatchi, the London-bsed ad agency, said that the advertising industry had for five years been looking for a "silver bullet" which would "put into a scientific quantitative format what usually resides in a person's gut. "The problem is advertising doesn't work in a straightforward way, and attempting to describe in a single process how ads work is absurd."

Dr Sam Hutton, senior lecturer in experimental psychology at the University of Sussex, said that the technology had potential to be used for advertising research, but that more work was needed first, for instance on how to control the "biggest influence" on pupil size, which was the luminance of an image.

"There are also issues as to whether changes in pupil size can really be interpreted as reflecting emotional changes in the viewer. They are more likely reflecting changes in the amount of cognitive processessing being done. These may correlate with changes in emotional state, but they're not the same thing," Dr Hutton said.

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