The Problem
This is the followup to Post #1661. The problem is that I frequently have to crane my neck to see traffic lights, in my wife’s Prius Prime, owing to the steeply sloped windshield.
The inability to see stop lights is hardly a new problem in the American auto industry. In that prior post, I reviewed the century-long history of inventions that would let you see above the top edge of a car windshield.
I noted that in the modern era, you could solve this problem with a $30 dashcam. But, really, where’s the joy in that?
Instead, I turned my back on that obvious solution and decided to come up with an optical device to let me see above the top edge of the windshield.
The design criteria for this stoplight-viewing device are:
- Not hand-held.
- Not permanently in the field of view.
- Not permanently mounted.
- Adjustable.
A new solution to an old problem.
My solution is a negative Fresnel lens, mounted to the sun visor so that you can flip it down when you need it, and flip it up out of the way when you don’t.
In this case, a “negative Fresnel lens” is a flat plastic lens sold as an aid to seeing around blind spots on vehicles. (Negative refers to negative focal length, meaning this isn’t a magnifying glass, it’s a “shrinking” glass.) Typically, these are used by large vehicles as an aid to backing up. The lens allows the driver to see objects that can’t be seen directly through the back window of the vehicle.
Below, note that the top of the cloud is obscured by the roof of the vehicle. Yet, you can see the top of the cloud in the shrunken image in the Fresnel lens. This is precisely what I want to happen, for stop lights obscured by the roof of my car. I want to use a negative Fresnel lens to pull them into view.
Source: The lens I bought for this project, for about $10, on Amazon.
Some variation of this technology is used on the LightInSight. This is an aid to viewing stoplights consisting of a long, narrow Fresnel lens designed to be stuck to the of the inside of the windshield. The product illustration below is completely unclear, but the LightInSight does exactly what the lens shown above does: It pulls images from above the top edge of the windshield down into the driver’s view.
Source: Amazon.
From my standpoint, the LightInSight has a couple of drawbacks. First, it’s permanently in the field of view. I don’t want that. I want it out of the way when I don’t need it. Second, Fresnel lenses fail when viewed at sufficiently shallow angles. The higher the power of the lens, the sooner that happens. I feared that the LightInSight, however well-designed, was not going to be usable on the extremely sloped Prius windshield. Or, if it did, it would have to be a relatively low-power lens, and provide only a modest boost to visibility above the roof of the car.
Instead, I wanted a relatively high-powered negative Fresnel lens, mounted perpendicular to my line of sight. But mounted so that I could put it away when it wasn’t needed.
Finally, I rejected the use of a cheap positive (magnifying) Fresnel lens. That would have made fabrication a lot easier and cheaper, but it would have produced an image that was upside-down and side-to-side reversed. To me, typically facing a string of lights at a multi-lane intersection, that just seemed like a recipe for an eventual disaster.
The rest is just tinkering.
Results
Other than the Fresnel lens, I tossed this together from scraps lying around the garage. Size, shape, and method of attachment were therefore more-or-less determined at random.
Here are the materials. The flexible Fresnel lens needs some sort of clear, hard plastic sheet to be affixed to. I decided to tape the lens to the plastic sheet with clear packing tape. And I decided to have this rest above the sun visor, held on with a couple of pieces of elastic, run through holes drilled in the hard plastic.
The only thing that is even remotely tricky is that the Fresnel lens is not uniform. By design, the bottom and side edges do a much better job of pulling images into the field of view, compared to the top edge. And after you cut it, you want to be looking through that external edge to find your stop light, not through the (much weaker) center of the lens. The upshot is that you want to cut your piece out of the bottom of the Fresnel lens, and you want to mount that so that the edge of the original lens ends up where the holes are drilled in the plastic.
Below I show the first test. It sits above the sun visor, held in place with two piece of elastic. To deploy it, pull it forward and let it hang off the front of the sun visor. When you are done, slide it back into position above the sun visor.
In the three pictures below, I’ve circled the one-way arrow to keep you oriented.
The first picture is the intersection, as seen when sitting up straight in the driver’s seat. The light is obscured by the roof.
Second picture show the traffic light from the “slouch and crane” position. Normally, I’d slouch in the seat and crane my neck to watch the light.
But with the Fresnel lens, I can see the light without slouching. This may not look like much in the photo, but it was perfectly adequate for monitoring the light to see when it turned green. No slouching required.
This will win no beauty awards, but it works, and it’s unobtrusive. When not in use, all you can see of it is the thin pieces of elastic circling the sun visor.
This could definitely use some tweaking if there were any need for an improved version. First, it’s far larger than it needs to be. Second, I’d probably glue the lens down, rather than tape it. Third, I’d probably cut a section from the less powerful portion of the lens (the top), as the lens is far more powerful than it needs to be to provide a clear image of the light.
By far the biggest drawback — totally unanticipated — is that you have to focus your eyes on the Fresnel lens, not on the road. Beyond being an annoyance, that means you aren’t focusing on the roadway in front of and around you. When the light turns green, you then have just a split second to refocus on the roadway and check conditions. This strikes me as a significant safety drawback to this device. Enough that maybe I want to rethink the whole thing.
But the bottom line is that this does what it’s supposed to do. It provides a usable image of a stoplight that would otherwise be obscured by the roof of the car. Thus, I carry forward the century-old tradition of ad-hoc “signal viewing devices” that let you avoid craning your neck to see traffic lights.
