Automotive IR emitter

mech

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(Z12 at max IR, versus IR LED pod with 15° optics)

I figured this would be relevant to the LPR forum. I picked up a Black Oak IR pod for a LPR I'm making for a buddy, and asked them to set it up with all 15° optics for max throw. Currently it's bolted to the chin of a big ol' housing as shown below. In the video above, I flipped my Z12 to B/W mode, maximum IR, and then aimed the apparatus below into the scene for comparison. The far end of the lot is about 85 feet from the camera.

Still shot:

10-05-2019@20_28_13.jpg

10-05-2019@20_36_00.jpg

For $85ish, this could be a contender if you need a very long-range IR to complete your plans for world domination (and are able to get 12-volt power to it). I think they could also set one up with all flood optics if that's what you wanted.

upload_2019-10-5_20-18-48.png

If you're wondering about the corona effect in the video, that is from the camera's own IR reflecting off the side of the building it's mounted to. If I normally ran it in IR mode, I'd do something about that, but it usually runs color as seen at the start and end of the video.
 
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mech

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It's currently set like this (general parking-lot duty):

upload_2019-10-5_20-54-46.png

I can redo this with a fast shutter speed that would be typical of a LPR if you're interested. Let's do 1/2000th, and I'll zoom BI in on that Honda in the corner.

I zoomed in a bit (I can't easily re-aim the camera to scope in on the Honda like a LPR scenario), let the camera focus at a slow shutter speed, then switched to 1/2000th with the settings shown below. The camera's one floor up from me, so I can't get the IR close to the camera itself right now, or else you'd see a huge boost in reflection off the license plate. Still, it manages to show some of the fence and tree over there.

upload_2019-10-5_21-6-27.png
 
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EssDeeEll

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I'm not sure why but for me at night no amount of extra IR seems to make any difference when my shutter is 1/500 0-33 gain and some HLC.

Maybe I'm doing something wrong but that was my observation in my setup.

EDIT: Must add that I am capturing diagonally, I've yet to ask my neighbour opposite if I can place an IR floodlight on her upstairs window pointing to my capture zone from opposite like a triangle, bouncing off the plate into the lens.
 

mech

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Are the license plates retro-reflective? If not, I imagine very intense illumination will be necessary.
 

EssDeeEll

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Are the license plates retro-reflective? If not, I imagine very intense illumination will be necessary.
I'm not sure. The retro reflective surfaces I've seen tend to have some structure visually, indentations and undulations and such.

Our plates are more like the stuff they use to make reflective painted road markings. Like a paste with reflective sand that glows more than it reflects.
 

mech

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If the plates do have a retro-reflective nature, then they'll selectively reflect more light back toward the source, so having the camera look directly down the IR beam would get the strongest reflection. This is why the Honda's plate in my video doesn't get brighter when the IR cannon hits it... the camera was nowhere close to the IR. So if you do have an extra IR emitter, try putting it right next to the camera, the closer the better. Depending on distance and the power and spread of the emitter, it may not help a lot if it's a wide-angle emitter, which is where the interest factor of this 15-degree high-output emitter comes in... lots of power, packed into a very tight beam.
 

EssDeeEll

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It's a 150 LED floodlight thing - more of a 6ft bright spot with a 30ft aura.

It's either too much or too little in most of my other applications.
 

J Sigmo

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All of the license plates I've seen use small glass beads embedded into the paint which act as tiny retroreflectors.

So you DO want to have the camera looking as close to straight down the IR beam as possible.

But the downside of having a light source near a camera is that blowing dust, bugs, etc., near the camera and emitter end up brightly illuminated and close to the lens, which can be trouble.

That means that you may wish to mount the IR blaster slightly away from the camera even though that's not optimal from a retroreflective perspective.

At longer distances, though, the error in angle can be slight despite having the blaster and camera separated enough to help with the bugs, dust, snow, and spiderwebs.
 

EssDeeEll

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All of the license plates I've seen use small glass beads embedded into the paint which act as tiny retroreflectors.

So you DO want to have the camera looking as close to straight down the IR beam as possible.

But the downside of having a light source near a camera is that blowing dust, bugs, etc., near the camera and emitter end up brightly illuminated and close to the lens, which can be trouble.

That means that you may wish to mount the IR blaster slightly away from the camera even though that's not optimal from a retroreflective perspective.

At longer distances, though, the error in angle can be slight despite having the blaster and camera separated enough to help with the bugs, dust, snow, and spiderwebs.
Right now I have the floodlight on the dashboard of my van pointing directly at the plate, but my camera is at an angle. No difference in plate brightness.

I think you're right about the fact it has to be right down the lens - there's not much more perfect than the IR LEDs built into the camera itself. At least that's the case with this cam I have.

I'm looking in to the actual material - 3M obviously dominate here and have a product called "Scotchlite" which is the stuff you get on hi visibility work vests and cycle jackets. It has a distinctive blurry look depending on the angle or the viewer and the light source - a true retro-reflective material. I can say with 99% certainty that this is not the material used in the plates but it may be in the series somewhere as "engineer grade white/yellow" look very close, perhaps even the 610 series.


3M-Scotchlite-CC.jpg

It's listed as retro-reflective but I think there's a difference in that one is prismatic and the ones our plates use, aren't. Perhaps something like the following (figure 3)?

download2.jpg

I think ultimately what we are dealing with is something like this according to the information I found:

MATERIAL-MATTER-WHAT-IS-3M-REFLECTIVE-SCOTCHLITE-ZOOM-1.jpg

xms-reflective-technology.jpg

Note the beads sit half way into the face film in the top image? This would kind of explain why they can still be lit from an angle, some 45-50 degrees... and also why perhaps for this type of coating that no amount of IR you throw at them from whatever angle other from the lense direction itself, it won't illuminate anymore.

I see some sense in this as we are huge on ANPR in the UK. They are literally everywhere, on every major road, intersection, traffic lights etc - also on average speed check zones.
 

EssDeeEll

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Here's a police car with reflective livery - this was 1/1000 though. Think I'm better off at 1/500

dfd.jpg

Just going to have to accept there are some plates I simply will not catch unless I relocate the cam more in line with the direction of cars. OR, get another cam facing the other way since I have literally got a 10/10 on front plates over 24 hours. With two directions I increase the chances of getting a good shot. our law dictates that you must have front and back plates which is one less drama to deal with.
 

J Sigmo

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I wonder why the slight difference between yellow/white?
Perhaps the yellow pigment is on a layer above the actual reflectors, so it may absorb some light both coming and going, so to speak, making it a bit less reflective.

The interesting thing is the way they refer to their figures for the retroreflectivity.
Table A refers to the retroreflection as:

Minimum Coefficient of Retroreflection Candlepower per foot candle per Square Foot Candelas per Lux per Square Meter (0.2° Observation Angle).

With this footnote:

1Observation (Divergence) Angle — The angle between the illumination axis and the observation axis.

To me, the implication is that they expect you to be illuminating the retroreflector at an angle not more than 0.2° away from the observation angle. Or at least that's the angle they're using when testing to give the ratings. It would be interesting to see a graph of retroreflectivity versus "observation angle".

They do give some data points for the viewing angle with respect to the surface, but they don't say anything about how rapidly the reflectivity drops off as you move the light source away from the observation axis. They test with the angle between the two at 0.2°, though, which is pretty much straight on.

Without any test data for that, I have to think that viewing more or less straight down the illumination line will always be the best for this product.

It's probably somewhat forgiving, though. After all, a car's headlights are never too near to a driver's eyes. But what we do probably see is that when you're far away from a license plate, you get pretty good reflection because the angle between your eyes and your headlights is shallow, but as you approach the car, the angle becomes larger but at the same time, you're getting closer, so the brightness of the illumination gets better, too. Perhaps offsetting the reduced reflectivity in a rather pleasing way.

And the same may go for an LPR setup. You might actually do a bit better to place the illuminator slightly away from the camera so that when a car is distant, you have a smaller Observation Angle and better reflection, and then, as the car gets closer, the observation angle gets worse, but the intensity of your illumination is better because the car IS closer to the light source. And THAT might make the exposure to the camera remain somewhat constant, making exposure settings more forgiving. Sort of an unintended benefit!

But you'd probably want to optimize this for a particular type of retroreflector. Some may be very forgiving of Observation Angle, while others may be more "strict".
 

EssDeeEll

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There's definitely something up with the rear plates over here. Not all, but some. I'd say 2 in 20. They simply are not reflective to the standard I believe the law requires.

Saw one today while walking. The yellow pigment seemed very paint like and flat as opposed to a subtle sparkling sheen. I checked several cars including my own vans which are fine but there are the odd ones that I don't think are regulation.

I can only assume they are aftermarket/third party/rubbish imports from God knows where. Our MOT test is about to get a lot stricter next year with ANPR cameras at the test stations and more advanced equipment overall. I hope they include something to test the plates other than visually.

I have some glasses with little spot lights on the side - might dig them out and see what the plates look like with the angle of the light reall shallow.
 

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Here, people buy aftermarket covers to put over their license plates to defeat their normal reflectivity. This is mainly done to prevent police laser speed measurement from working, and also to prevent automated LPR systems from reading the plates.

These covers are illegal. But I still see them in use. It's like dark window tinting. Darker than some standard is illegal, but that doesn't mean people don't do it.

I don't think the cops put a lot of effort into testing and enforcement of these violations.


 

EssDeeEll

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Here, people buy aftermarket covers to put over their license plates to defeat their normal reflectivity. This is mainly done to prevent police laser speed measurement from working, and also to prevent automated LPR systems from reading the plates.

These covers are illegal. But I still see them in use. It's like dark window tinting. Darker than some standard is illegal, but that doesn't mean people don't do it.

I don't think the cops put a lot of effort into testing and enforcement of these violations.


It's a shame things like these are available so freely. The only deterrent is a high fine - $5000

I'm going to look in to how they work. It must be some sort of refraction.
 

EssDeeEll

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Already I've found something that makes me a bit happier....

"PhotoShieldis a clear plastic license plate cover that incorporates a thin diffusion lens designed to make your license plate unreadable and distorted to traffic cameras when viewed at an angle "
 
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