Jennie took part in a unique experiment with a special ultraviolet (UV) camera

We all know we need to slap on the suncream to protect our skin against signs of ageing, such as sunspots and premature wrinkles, and skin cancer.

But how protected are you really? Not as well as you might think, as I discovered after doing this unique experiment with a special ultraviolet (UV) camera.

For as the pictures on this page reveal, when it comes to shielding our skin, it's not simply about using the right level of sun protection factor (SPF). It's about how you apply it and the type of product - cream, oil, spray - you use, as there are different techniques for each.

The images were produced with the help of Cancer Research UK, using a camera that shows how much UV light is reaching the skin. The darker the skin in the picture, the less UV light is being absorbed by it, which means the skin is better protected.

The darker the image, the better. The pictures where the whole face seems lighter show that, despite some overall protection, there are still some UV rays reaching the skin. The bright white patches you see are areas of skin where a lot of UV light is getting through, which is bad news.

One of the first mistakes people make with their sunscreen is using too little.

As a rough guide, the British Association of Dermatologists recommends at least six full teaspoons of suncream for your whole body: one teaspoon for each leg, one for your front, one for your back and half a teaspoon each for each arm, face and neck. That's around a fifth of a standard 200ml bottle of suncream per application.

There are two types of ultra-violet light to worry about: UVB, which is what causes burning, while UVA is the ageing type. Both are linked to skin cancer.

'The SPF on a suncream bottle relates to how it protects you from UVB and sunburn,' explains Sarah Williams, health information manager at Cancer Research UK. 'UVA penetrates deeper into the skin, so you won't see its effects as a pink burn, but it can still cause damage.'

UVA protection is usually listed on products as an out-of-five star rating.

'Most modern suncreams contain elements of both, and this works incredibly well,' says Dr Justine Hextall, consultant dermatologist at Western Sussex Hospitals NHS Trust.

But even the best technology for protecting skin is no good if you don't use it properly. A common mistake is to use a high SPF but not reapply it frequently. 'It's probably better to use plenty of a factor 15 and reapply it regularly than to use a factor 50 just once,' adds Sarah Williams.

Even then, it's how and where you apply it that also really matters, as these pictures reveal. In this experiment we wanted to see how different formulations of suncream - from everyday moisturisers that many women rely on to once-a-day suncreams - protected the skin, and whether the way they're applied makes a difference to that.

Our results may make you a little less slapdash about applying suncream . . .

1 - Everyday moisturiser with SPF 15

No7 Protect & Perfect Intense Day Cream SPF 15, five-star UVA, £24, from Boots.

No7 Protect & Perfect Intense Day Cream SPF 15, five-star UVA, £24, from Boots

Like many people, I like to think I'm quite careful in the sun.

At the age of 28 and pale-skinned, I don't have any noticeable patches of sun damage, although under the UV camera there is a smattering of freckles across my nose that aren't normally visible. This is a sign of sun damage in the deeper layers of my skin.

My skin isn't too bad, apparently - a sunbed user would show much more visible damage under the UV camera.

Like many women, I use a daily moisturiser with an SPF of 15 in it. But this hasn't been enough to protect me.

In theory, an SPF 15 in a daily moisturiser should be just as good as an SPF 15 sunscreen. In practice, however, you are likely to put on less moisturiser than you would a sunscreen, says Dr Hextall.

The hairline is a common area to miss with suncream

'A product's SPF is worked out in laboratory conditions,' she explains. 'They test 2mg per cm sq of skin, which is likely to require quite a thick layer of cream and be cosmetically unacceptable. Therefore, most people apply a quarter of this - about 0.5mg per cm sq of skin - and with daily moisturiser, most would apply even less.

'On an overcast day, if you were only going to be outside briefly - perhaps while walking from your car to your office - an SPF 15 moisturiser would be fine. Though, personally, I wear a separate factor 50 sunscreen on my face all year round to protect against sunlight's ageing effect.

'And if you're sat by an office window all day, UVA rays can still penetrate glass.'

What this picture shows is how easy it is to miss bits. The white gleam on my cheeks suggests no protection there, and I've missed the inner corners of both eyes.

Skin cancers often occur around the eyes. Other high-risk areas include the bowl of the ear and the temples. My hairline also isn't protected. 'The hairline is a common area to miss with suncream,' says Dr Anjali Mahto, consultant dermatologist and spokesperson for the British Skin Foundation. 'Skin cancers can develop on the scalp. Men with thinning hair may should ideally wear a hat.'

So make sure suncream is applied right up to the hairline and in your parting.

Doubling up by using a foundation containing SPF over your SPF moisturiser can ensure better coverage across your whole face, suggests Dr Hextall. 'You may be more likely to put foundation in areas where you put less moisturiser - around the nose, for instance, or under the eyes.'

2 - Standard suncream

Nivea Moisturising Sun Lotion SPF 15, four-star UVA, £11.99, from chemists and supermarkets.

Nivea Moisturising Sun Lotion SPF 15, four-star UVA, £11.99, from chemists and supermarkets

I applied about half a teaspoon's worth of this sun lotion - what the British Skin Foundation and the British Association of Dematologists recommend you should use on your face - using a mirror to ensure a more even coverage.

Yet the picture clearly shows I've missed a patch around my nostrils, and there are streaky areas on my cheeks, suggesting these aren't shielded from UV light.

'Around the nostrils is a common area for skin cancers - especially the crease where the nose meets the side of the face,' says Dr Bav Shergill, consultant dermatologist and spokesman for the British Association of Dermatologists.

'Think of the skill involved in applying an even layer of make-up. With foundation, you take care not to miss bits as you will look odd. It should be the same with suncream, but, unlike foundation, there's no way of checking you've got an even application.'

You must therefore reapply suncream regularly, adds Dr Hextall: 'If you miss a spot the first time round, you'll probably get it with the second coat.'

3 - Once-a-day sun lotion

Riemann P20 Once A Day Sun Protection, SPF 20, three-star UVA, £13.99, from Boots.

Riemann P20 Once A Day Sun Protection, SPF 20, three-star UVA, £13.99, from Boots

This promises protection lasting ten hours, even after swimming unless you've towelled yourself down 'vigorously'. The clear liquid is fairly thin and runny.

It was very easy to apply and dried quickly. However, it says not to use the lotion around the eyes (some chemical sun blocks can irritate eyes), so I duly avoided this area.

'Once-a-day formulations contain the same types of UV protection as other lotions,' says Clare O'Connor, UV technical expert at Boots. 'However, they also contain molecules that grip to the skin better.'

This may explain why - eye area excepted - the UV coverage offered by P20 seems more uniform than some of the other products I tested.

'Even if a product says you don't need to reapply, I always would,' says Dr Hextall. 'You're still going to sweat or towel-off after swimming, and some of it will come off.'

There's an unprotected patch on my lip. Skin cancer can form here... and may be more likely to spread

Not being able to use this around the eyes is far from ideal. Last month, researchers from The Wellcome Trust Sanger Institute who analysed apparently healthy skin samples from people's eyelids found that 25 per cent of the skin cells had mutations to their DNA that are linked to cancer.

Eyelid cancers account for about 5 to 10 per cent of all skin cancers, and most frequently occur on the lower eyelid, says Dr Mahto.

All the experts recommend wraparound sunglasses with a UV filter as the best way to protect the eye area - especially if you find it hard to put thick sunscreen on this delicate area. You should also reapply suncream around the eyes if you've been wearing glasses, as they can rub away cream where the frames touch your face.

If you find suncream runs, irritating your eyes, Dr Hextall suggests using a physical UV blocker such as pure zinc oxide (a thick, white paste of the kind surfers use) as this is less irritating than chemical UV‑blockers. 'It's what I recommend for sensitive skin, too,' she says.

The picture shows there's also an unprotected patch on my lip, possibly because I rubbed the cream off. People often forget to protect their lips, but non-melanoma skin cancer (the most common type of skin cancer, which develops in the top layers of the skin), can form here, says Dr Mahto. They are most common in fair-skinned men over 50.

What's more, skin cancers on the lips may be more likely to spread, says Dr Hextall, who recommends using a lip balm with SPF and UVA protection.

'Anything you use in this area will rub off more quickly as we talk, eat, drink and lick our lips, so it may be easier to reapply a protective lipbalm. Some lipsticks also now contain an SPF.'

4 - Invisible spray

Garnier Ambre Solaire Clear Protect transparent spray, SPF 15, three-star UVA, £7.50, from chemists and supermarkets.

Garnier Ambre Solaire Clear Protect transparent spray, SPF 15, three-star UVA, £7.50

'With clear sprays, it's harder to gauge exactly where you've put it - and whether you've used enough,' says Dr Shergill. And this is exactly what the picture of me shows.

The attraction of a clear spray is you don't get the white streaks that you can get with a cream - especially when there's no mirror to check your face.

The first time round I sprayed one squirt directly onto my face and rubbed it in quickly. Worryingly, it made barely any difference under the UV camera. This picture shows how my face looked after three sprays: the coverage is still speckly and uneven.

Sprays may not be the best thing for the face, says Dr Hextall. 'Our natural response is to screw our face up in response to anything sprayed on it, which may mean you miss bits.'

However, both she and Dr Shergill say a spray is a good practical option for small children, but use a coloured spray so it's easier to see where you've applied it.

5 - High-factor suncream

La Roche-Posay, Comfort Anthelios XL, SPF 50+ with UVA, £15.50, from chemists nationwide.

La Roche-Posay, Comfort Anthelios XL, SPF 50+ with UVA, £15.50, from chemists nationwide

This was the highest factor suncream I tested, however this shouldn't make too much difference to the image produced by the UV camera - it simply means I could, in theory, spend longer out in the sun before my skin was damaged compared with a product with a lower SPF. The reason this image is the darkest is that very little UV light is reaching my skin. Not only was I meticulous in applying it, but the cream went on smoothly and evenly.

People often over-estimate the difference a high-factor suncream makes, explains Sarah Williams.

An SPF of 30 does not give twice the protection of an SPF 15. In fact, an SPF 30 will block out about 96 per cent of UVB light whereas SPF 15 will block out 93 per cent.

Despite feeling thicker than the Nivea lotion or the No 7 moisturiser, this absorbed easily, but the picture proves I still managed to miss the inner corner of my eyes.

To make sure you're properly covered, Dr Shergill recommends starting with the sensitive, easily missed areas of your face - 'the corners of the eyes, the crooks around the nose, the upper lip and sides of the lip', before doing the rest of the face.

Other common areas to miss are the feet, underarms and the backs of the knees.

6 - Wet skin spray

Piz Buin Wet Skin Transparent Sun Spray, SPF 15, three-star UVA, £18, Boots.

Piz Buin Wet Skin Transparent Sun Spray, SPF 15, three-star UVA, £18, Boots

This sunscreen can be applied straight after swimming onto wet skin. It sounds like a great idea for beach holidays, but how well will it cover you?

I tried it on a damp face, and although the water made it very easy to spread, it was harder to tell where I'd put the oil. And it showed with the UV camera, with coverage a little patchy, including on the tip of my nose, and I missed a large area around my upper lip.

'You need to pay attention to the upper lip, particularly the sides, as it can be quite sensitive to the sun - whereas the central bit of the upper lip is more shaded by your nose,' says Dr Shergill.

'Around the mouth is also prone to signs of ageing - it's more likely to wrinkle because we use the muscles here a lot,' says Dr Hextall. 'You don't want to add to this with lots of sun damage.'

'And the tip of your nose is more likely to burn because it's not shaded by anything on your face,' she says. 'A wide-brimmed hat can help, and sit facing away from the sun if you're outside. If I know I'll be in the sun, I use a stripe of zinc oxide on my nose on top of my other sunscreen,' she adds.

An oily preparation of sunscreen may not be best if you prefer active holidays either. 'A very oily product is more likely to block the hair follicles,' says Dr Hextall. 'This could lead to folliculitis or sweat rash, as sweat can't escape, leading to irritation.'

Clare O'Connor adds that you should never simply spray a product on and think you'll be covered. 'Always rub it in.'