OF THE big names in the Green on Red gallery's stable of artists, Bridget Riley is without doubt one of the biggest. Now 76, she is often considered the most significant artist of the 1960s Op Art (optical art) movement and is one of Britain's best-known living artists.

But when it comes to Bridget Riley, nothing is ever as straightforward as it seems. For starters, she distanced herself from the Op Art movement and, despite her enduring use of geometrical forms and precisely rendered designs, professes to have no interest in geometry or mathematics. Her kaleidoscopic, black-and-white abstract paintings, with their optical illusions and mind-bending effects, are iconic works of the '60s that permeated pop and fashion culture and became visual expressions of the era. And yet, this does not seem to have entirely pleased Riley - she took several unsuccessful legal actions against the appropriation of her designs.

To complicate matters further, if you go to see the current Green on Red exhibition of her new paintings and gouaches, the two large oil paintings included in the show have not actually been painted by Riley.

In fact, what Riley does is create the design - precise measurements and sketches - for her paintings, which are then painted by assistants. She has been doing this since the early '60s because, for her, the actual act of painting is the least of her concerns.

"The deeper, truer personality of the artist only emerges in the making of decisions - in refusing and accepting, changing and revising, " she says.

Her interest lies in the design - the balances and contrasts of shape, colour and form - and how this interacts with the viewer's eye.

Drawing attention to the act of looking is central to the experience of her paintings.

Riley, who was born in 1931, studied at Goldsmiths College and then the Royal College of Art in the 1950s. However, she left the latter early to care for her ill father and, soon after, she had a nervous breakdown. Following her recovery, she was, by the end of the '50s, beginning to develop her own artistic style, influenced by the colour theories of pointillist artists such as Seurat. It has been suggested that the development of her rigorous, abstract approach at this time emerged after a failed love affair. "My personal life is not at all exciting now. It was the reverse at one time and I found it was bad for my work, " she said recently.

In 1962 Riley had her first solo show in London and then, in 1965, she burst onto the international art scene when she participated in a group show at Moma in New York, The Responsive Eye. Her dizzying paintings featuring geometric waves, circles, stripes and zigzags brought Op Art to the forefront of the avant-garde. Indeed, the overwhelming effects of these works have often induced headaches, dizziness and nausea in viewers. As Riley once explained: "They must stealthily engage and disarm you."

But so enduring are the images of Riley's work from the '60s that it is easy to forget she has continued to work right up to the present day.

She gradually began to introduce colour into her paintings, particularly after a trip to Egypt in the '80s, and the jolting, vertigo-inducing illusionism of her earlier work has given way to a more subtle exploration of the effects of shape, colour and pattern on perception.

Her latest work is also more evidently an abstract evocation of nature. Riley once said: "If I am outside in nature, I do not look for something or at things. I try to absorb sensations without censoring them, without identifying them."

In several works at the Green on Red, pastel shades of green, peach and cream pervade, while verticals interact with curving diagonals to create the impression of a forest, or a tree gently swaying in a breeze. The large 'Blue Painting (Painting with Verticals Cadence 5)', meanwhile, with its undulating, wave-like blues, reds and purples, could almost be a stretch of lapping, reflective water.

The optical effects are still present in the interaction of colours and shapes, but the dizzying bravura has given way to a calming, meditative softness. Three of the smaller gouaches in particular - 'Revisions of May', 'Bassacs' and 'xxx' - are all the same pattern but subtle colour alterations create dynamic, polyphonic changes of effect.

These uplifting new works, created by a woman in her eighth decade, are essentially celebrations of life itself. As Riley reflected on the creative urge: "An artist feels 'a need to do something' about the very fact of being alive, rather like a bird feels the need to sing."

'Bridget Riley: New Paintings New Gouaches' at the Green on Red until the end of this month