Fresnel Lens - Be Safer In UK & Overseas - Blind Spot Visibility Increased (1 Viewer)

[Two on Tour]

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[Nixdorfer]

Is the small one the 21x12cm? with sticky tape round the sides

 

[Two on Tour]

Is the small one the 21x12cm? with sticky tape round the sides

Yes.

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[autorouter]

That's what they were originally designed for AFAIK.

They are commonly used in Overhead Projectors, as used to project slides in a lecture or presentation. They are used to collect and even out the light from the projector lamp. Not the actual projection of the image on the screen, which requires a sharper higher quality lens.

 

[jezport]

Be careful with a lens, yes even a fresnel lens. Very easy to start a fire.

 

[autorouter]

Be careful with a lens, yes even a fresnel lens. Very easy to start a fire.

Good point. The heat-collecting power of a lens depends on the area. Double the width of a square lens, and the fire risk goes up by 4 times. Close the blind over it when leaving it in the sun.

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[Two on Tour]

Be careful with a lens, yes even a fresnel lens. Very easy to start a fire.

Yes I can see that a Fresnel lens can start a fire, but very unlikely with one fitted to a truck or motorhome, why, well the Fresnel lens used to start the fire in the video is designed with a circular pattern to take sunlight in straight onto the face of the Fresnel lens then bend the light to a focal point so concentrating the sunlight as does a magnifying glass.

I the case of the Fresnel lens designed for a lorry, it's job is to bend the light so the image is seen and magnified from an angle down from the height of the lorries side window, so the sunlight would have to come up in a reverse path to then concentrate it's light at it's focal length which is very unlikely. Also the focal length of the Fresnel lens designed for a lorry is way past the driver otherwise the driver could only view the image in one precise spot. As the focal length is effectively out past the drivers side window there is nothing for the concentrated light to act on.

I the case of a motorhome, the Fresnel lens is turned through 90 degrees rearwards and for the sunlight to act on the Fresnel lens it would be very low in the sky and being very low in the sky it will be very weak.

 

[Wombles]

To allay any fears about fire risk - here are the facts from the manufacturer Lens-Tech:
"I am pleased to confirm that our Fresnel vehicle safety lenses are wide-angle lenses with a specified negative Focal Length and an off-centre Focal Point so they cannot be used as a magnifying Fresnel solar lens.

For your information - In order to start a fire, a magnifying solar lens with a positive Focal Length and a central Focal Point would be required in order to concentrate the sun’s light rays into a focused ‘hot’ spot. In other words, a completely different specified ‘concentrating’ lens, as opposed to a wide-angle lens which is used to achieve vehicle safety views.

So, we can categorically state that our Vantage Point LCV Fresnel vehicle safety lenses cannot focus sunlight in order to generate any meaningful degree of concentrated light/heat into a ‘hot’ spot in order to cause a fire."

 

[autorouter]

To allay any fears about fire risk - here are the facts from the manufacturer Lens-Tech:
"I am pleased to confirm that our Fresnel vehicle safety lenses are wide-angle lenses with a specified negative Focal Length and an off-centre Focal Point so they cannot be used as a magnifying Fresnel solar lens.

I didn't know that vehicle safety fresnel lenses had a negative focal length. If so they are safe, and there is no fire risk. Makes sense now I think about it - just like convex rear view mirrors, that also have a negative focal length.

The only Fresnel lenses I've had dealings with are OHP condenser lenses which have a definite positive focal length, hence the confusion. With these, the image would be upside down, not much use at all.

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