LED lights and car number plates

[blut wump]

I was just out walking the dogs since my beloved spouse is away for a few weeks and noticed an odd effect while I was out.

Since walking multiple dogs causes me to run out of free hands very quickly, I was using one of those LED head-lights. As I returned to the house, the light played briefly over the cars and I noticed that the number plates shone back at me very brightly. I could barely make out the cars in the dim light but the number plates plates were more than merely readable and were shining brightly back at me.

I went into the house to fetch a normal torch (flashlight) and under its light the number plates were no more illuminated than the rest of the cars. Flicking on the LED head-lamp gave me bright number plates shining in the darkness.

I assume that something related to polarized light is involved here but (a) I wasn't aware that LEDs gave out polarized light, not that I'd thought about it before, and (b) I have no idea why polarized light would have this effect.

Can anyone here explain this? I suggest you go out and try it. I used to have a few other LED light-sources but I gave them to my nephews else I'd go back out to gather more info. Actually, there's one on my phone...

In the spirit of things, I'll donate 10,000K (before taxes) via the store to the best answer. I'd make this a K-thread but as a lowly non-Plat I can't do that.

Thanks in advance.

 

[blut wump]

The LED on my phone has the same effect as does a red LED I have. At a distance of over 70 yards, I couldn't even make out that there were cars there but the number plates still shone back brightly.

I'm wondering whether the material of the number plate is being excited in some fashion by LED light. If so, the material is maybe as nerdy as I am.

 

[MTS]

The number plate is made with a special material that causes light coming straight at it to reflect straight back. With a regular flash light the light is a little more dispersed then the led (where they also put special side coatings to make the side light travel straighter). With the led light traveling straighter then the normal flashlight the number plate material will reflect more straight light back at the user. Lazers are the ultimate straight beam of light, so if you take a lazer and shine it at a number plate or stop sign it should reflect very very brightly.

 

[blut wump]

I'm not convinced that this is all there is to it. With the (fairly bright) torch shining directly at the car I don't get the bright, shiny look at all from the plate.

At a distance of over 50 yards with a LED light not shining directly at the cars and the light so dim that I can't even see the cars, the number-plate is brightly illuminated. It seems to spring to light as soon as any LED light hits it.

Collimation of the light might be a factor but...

 

[blut wump]

On a maybe related note, do you know that if you shine a beam of light into the interior of a cube then the returning beam is parallel to the incoming beam?

Similarly, if you throw a bouncy-ball between two parallel surfaces, say under a table or, even, between a wall and a door open parallel to it, the ball comes straight back after bouncing between the surfaces.

 

[MTS]

I'm not convinced that this is all there is to it. With the (fairly bright) torch shining directly at the car I don't get the bright, shiny look at all from the plate.

At a distance of over 50 yards with a LED light not shining directly at the cars and the light so dim that I can't even see the cars, the number-plate is brightly illuminated. It seems to spring to light as soon as any LED light hits it.

Collimation of the light might be a factor but...

that was my guess...I'm not sure if it would be all LED lights...like the phone, but im actually going to try it out sometime

 

[mikefear]

http://www.nightfirepatches.com/reflective_patches_decals_how_it_works.html

http://72.14.209.104/search?q=cache...+reflective+material&hl=en&gl=us&ct=clnk&cd=6

Durable, reflective nighttime visibility. Using highly durable, industry-standard Scotchlite or equivalent sheeting manufactured specifically for reflectorized license plates, we can turn your standard EMS, police, or firefighter license plates into ones that light up at night. Embedded lens elements enclosed in the transparent resin provide reflective visibility from both head-on and off-center angles. Over the years, this material has become an industry standard for reflective license plates, providing higher reflectivity and better nighttime visibility than the alternative beads-on-paint method.

 

[silver_shadow]

that should result in decent reflectivity even in dim light then right?

 

[mikefear]

yeah.. hit a license plate with a laser-pointer and watch what happens.. it's the same material used on stop signs and most other street signs

 

[blut wump]

Interesting but why don't I get the effect with a normal torch?

 

[mikefear]

maybe the light is just too diffused... dunno

 

[gjohnson5]

God, we need some new people to post on this board cause I'm K hitting the same people...

 

[blut wump]

http://www.nightfirepatches.com/reflective_patches_decals_how_it_works.html

http://72.14.209.104/search?q=cache...+reflective+material&hl=en&gl=us&ct=clnk&cd=6

Durable, reflective nighttime visibility. Using highly durable, industry-standard Scotchlite or equivalent sheeting manufactured specifically for reflectorized license plates, we can turn your standard EMS, police, or firefighter license plates into ones that light up at night. Embedded lens elements enclosed in the transparent resin provide reflective visibility from both head-on and off-center angles. Over the years, this material has become an industry standard for reflective license plates, providing higher reflectivity and better nighttime visibility than the alternative beads-on-paint method.

Thanks. This one's a winner.

As to why I wasn't getting the effect with the normal torch: I was standing too close for the brightness of the torch. The bright torch was illuminating the whole scene so well that, effectively, everything was bright, just the same as when looking at number plates during the daytime they don't stand out as super-bright.

I wandered off down the field for a goodly distance and then pointed the torch in the direction of the cars. The number-plates shone out in the darkness.

 

[MTS]

Thanks. This one's a winner.

As to why I wasn't getting the effect with the normal torch: I was standing too close for the brightness of the torch. The bright torch was illuminating the whole scene so well that, effectively, everything was bright, just the same as when looking at number plates during the daytime they don't stand out as super-bright.

I wandered off down the field for a goodly distance and then pointed the torch in the direction of the cars. The number-plates shone out in the darkness.

 

[blut wump]

I thought at least someone would go out to reproduce my results.

 

[MTS]

I thought at least someone would go out to reproduce my results.

will do...ive just been really busy the last few days and i keep forgeting..i'll try a few different led lights and let you know

 

[MTS]

yeah i see what you mean...i noticed with different color lighting on different colored plates and signs, shined at different intensities also. I tried blue, red, white, yellow, leds and the traditional blub flash light.

at least that is what my brain perceived