Because the LCD screen will only allow light to pass through perpendicular to its plane the light has to be collimated to allow this. AFTER the light has left the LCD it is then focused on the projecting lens. You can argue about distortion all you want and are 100% right. But they’ve been making cheap projectors like this now for at least a decade. They’ve been using Fresnels in overhead projectors for decades.
As to Fresnel throwers… You can see in my pics I project an almost perfectly square die image - 200 yards into a brick wall. It’s rectangular now because I put a 2mm^2 in. I can’t capture it with my cell phone but at 40ft focused i could read the inscription on the MCPCB at 40ft. Possibly because the RLT moved on me and is illuminating the whole MCPCB.
So sure, I don’t expect a hi-def video projection. But you can search the thousands of YouTube videos of both DIY and cheap manufactued video projectors and see the image quality, while not great, is far better than you describe.
You weren’t talking about the LED when you said this, you were talking about a short throw projector:
“I think if you were building an ultra-short-throw video projector on the cheap, these could work great!”
A short throw projector needs to have an optical system that outputs light at a large exit angle, not collimate light from a light source with large incidence angles.
Also I see absolutely no retail digital projectors that use fresnel lenses on google, other than people that made a diy projector with an overhead projector and laptop screen.
And the only reason the image is even usable is because the light is going through a massive LCD that is 15” in size or something, not a regular projector LED which is usually less than one or two inches.
They must block such search results in Vancouver, otherwise you may attempt to rule the world…
Here’s some of dozens of YouTube videos:
And Instructables:
These guys tear down these Fresnel projectors:
Looks like Fresnels on both sides to me. My thought was to put the short focal range Fresnel on the front past the LCD.
I emphasize the phrase “video projector on the cheap.”
I’ve been around bowling for about 25 years. Back in the day they did not have computerized automatic scoring like they do now - automated scoring came about just before I started. What they did what give you a transparency with the score boxes and a yellow wax pencil. You set the transparency on the “Tele Scorer” which is basically an overhead Fresnel projector with a Fresnel underneath the table and overhead in a box with a mirror inside to face the projected image forward onto a screen.
OK, here’s a Fresnel lens designed to collimate 14 LEDs:
What performance would I expect from it?
cheap plastic fresnel = bad throw
lots of beam artefacts from LED cross-talk i.e. shining at the neighbouring lenses
OK, I don’t think this particular lens is good.
But I find the general concept of multi-LED aspheric VERY interesting.
But first one would need to solve the crosstalk issue.
It’s easiest to do with baffles. With a proper baffle system between the LEDs there would be no crosstalk whatsoever.
But what if the baffles were actually collars, carefully machined to have the right aperture shape?
One could build a multi-LED thrower (actually I wouldn’t prefer 14 except for huge lights. 3 would be more like it) with
high output
very high throw (collars!)
high efficiency
quite short focal length
Unlike multi-LED TIRs or reflectors (I’ll call them T/R) - these have no dead zones i.e. ones that don’t produce throw. However:
with fresnels reflection off ridges costs throw. And precision tends to be lower
with convex lenses - I strongly suspect the valleys between individual lenslets wouldn’t be very precisely made.
So the loss structure is different from T/R. It scales differently, f.e. 3-up is quite bad with T/R and should be good here. OTOH 7-up is quite good with T/R.
EDIT:
Again, I post here and within minutes stumble upon just the same kind of stuff. This time I got email with this:
You can buy adhesive backed vinyl with Fresnel patterns and metalized coatings behind; they use it for decorative stuff as well as cut for letterings and stuff with a prismatic effect. Which got me thinking… probably not practical, but one could do the opposite - take a Fresnel with a parabolic profile, put a reflective coating, and have a parabolic reflector that is essentially flat…
Fenix E16 is known for its unique flat optics. This kind of optics is called RXI and quite popular in literature but nearly completely absent from the market. It seems to stifled by patents.
I found 2 makers that manufacture such optics. Neither looks like the Fenix one so there are surely more.
Oh, I forgot that lens. It seems to indeed use the same working principle. Except that Carclo has really low transmission, I suppose it spills light everywhere. That’s not the case with metallized reflector lenses.
The Fenix looks more like a hybrid reflector than a catadioptric. The hybrid reflector has a TIR that directs the light from the LED into the reflector. Polymer Optics makea then with aluminum reflector and plastic TIR. Auer in Germany makes them out of glass.
It doesn’t look so to me.
First, it seems to have a small metallized ring at the front that I can’t explain otherwise than “it directs light towards the main reflector”.
Second - it’s just too flat and angles don’t seem to add up. I think it would generate extremely wide beam with light hitting the side reflector directly. And we know that it actually collimates fine. Not neearly as well as a TIR in Olight S1 Mini but the output is nevertheless quite throwy.
The catadioptric is literally the same thing, a hybrid tir+reflector with a metallic coating on the rear side and the center which reflects the light coming at incidence angles too large for total internal reflection. That’s what allows the optic to be so flat.
Go look up the carclo 10158 on google and you will see it works in exactly the same way.
I own 4 of them.