I did a series of throw tests last evening on the cheapo plastic optical illusion reflector (mine is even unbranded chinese, I read above that there's quality differences even in these toys). I used a dedomed XP-E2 on 10mA current as light source and found that with the full size reflector there is not one focus, the middle part of the reflector does something completely different than the edge. Which is of course not good for throw. I wonder if this is inherent about a parabolic reflector used at these steep angles or that these optical illusion reflectors are particularly bad?
But this is the reflector I'm going to make a light out of, so I checked the hotspot brightness while varying which part of the reflector is used, from full size down to just 50mm diameter. I did this by laying the reflector upwards on the table and pointing the beam to the ceiling, and by placing pieces of cardboard with varying holes on top of the reflector I varied the reflector size. The led was positioned in the optimal focus point (that gave the highest spot brightness) using my XYZ-table. The highest luxreading in the hotspot was measured each time with a luxmeter.
Here are the luxvalues that I found, the numbers themselves have no exact meaning but they are a measure of spot brightness:
Full size (139mm)
946
90mm
898
80mm
881
70mm
865
60mm
858
50mm
663
As you can see, in this reflector it is quite pointless to go larger than 60mm (huge size increases with hardly any throw gain), the focal distance being 48mm this is about a 60 degrees cone of the led-beam. I was a bit surprised a this and I wonder if this is a general thing with recoil throwers, or that it is just a sign of using a bad reflector.
Fact is that 1) the (dedomed!) led has the highest brightness straight forward and clearly less close to 90 degrees to the side (it is in the led datasheets but I illustrated it a bit more visually below by positioning the used dedomed XP-E2 in the middle point of a piece of paper bended in a circle), and 2) that the imaging quality of the reflector is decreasing when going from the middle to the edge (I checked that by blocking defined parts of the reflector and watching the effect on the hotspot). In other words, it is a challenge to get enough light that is reflected by the edge of the reflector in the direction of the hotspot.
Extrapolating these results to the bigger 9-inch Mirascope reflector, a 90mm diameter section of that one should make the best size if a recoil thrower would be made with it.
I'm still overthinking what the influence has been of using the very small die dedomed XP-E2 (which was at hand) compared to leds with bigger dies.
Did you test out the mirasope before using it for the tests?
If the image of the mirasope (when used as intended) is highly distorted that could indicate problems with the reflector quality.
The mirasopes are supposed to be perfectly parabolic, so if your luxmeter was centered when you were doing your measurements the outside of the parabolic reflector would be reflecting light straight forward which would land outside the luxmeter. Basically the larger mirasope diameter should be making a larger spot, not a higher intensity spot, unless you measure with the reflector focused to a point at several meters distance.
Do you have any pics of the spot with the different sized holes we could take a look at?
The mirascope worked fine, my six year old son was amazed by it and demanded that I would not destroy it and make a flashlight out of the reflector :person_facepalming:
I have to think about the ray direction coming from the edge of the reflector compared to the centre. If what you suggest is indeed going on, the contribution of the edge of the reflector to the hotspot brightness would increase with imaging distance (which was just 2 meter in the test).
The recoil reflector should work in the same way, in principle, to a standard orientation reflector.
This thread has relevant discussion: Reflector width vs depth for throw?
The beam of a parabolic reflector can be thought of as an image of the LED, sort of the same as the beam from a aspheric lens light. Except the reflector has different focal lengths and so the beam is actually a superposition of the different sized and oriented images of the LED produced by the different parts of the reflector.
The light reflecting from the outer diameter of the reflector should produce the smallest beam since the focal length there (distance from LED to reflector) is the largest (see demonstration done by djozz in the above linked thread). The fact that the reflector in djozz’s present experiments is behaving differently for the inner and outer sections is indicative of the reflector just not being perfectly parabolic.
Well Djozz, that all sounds a little disappointing…
Let’s say i hope your mirror works better as a mirascope…
I expect mine in the mail tomorrow, i hope it may be better, but they all look rather identical, maybe there’s only one factory in China that makes them. (?)
If mine is bad too, it’s probably the end of this project for me, unless miraculously a source of proper affordable parabolic mirrors suddenly pops up…
In your last picture is see that your LED does shine about 135° wide, with the ‘best’ light within some 90°.
But it’s hard to really tell by the picture.
But i think it explains why a zoomy has a brighter hotspot (in focus) than a regular light.
If you do a ray trace of a parabolic reflector you will see that when focused to infinity (aka not converging or diverging) the LED is at the focal point, and all rays are collimated.
In this case, a larger reflector will make a larger spot (eg. a 6" diameter reflector will make a 6" spot, a 9" diameter reflector will make a 9" spot)
In either case the intensity will be the same, but more of the light is being collected in order to make the spot bigger with higher lumen output overall (same intensity, more area = higher lumens)
I would definitely suggest doing tests at large distances (eg 5m or 10m or around there), the converging beams should be a lot more accurate and form a smaller point. Pictures of the projected spot or beam would also be able to show if te reflector is collimating correctly.
One trick I love to use to make sure by optics are collimating is spraying the air with a water sprayer (fine mist) which allows me to see exactly which rays are going straight forward, which are converging, etc...
Very convenient way of checking the collimation that doesn't require a foggy night :P
with short distances (table top to ceiling) it will work that way though.[edit] Well,it depends of course how you focus it (paralel or spot)
Some beamshots an wall- / ceiling-shots would have helped maybe.
1) when I do ray traces I can offset the emission angle to see how the light coming from the edge of the LED (off axis) behaves.
I will post screenshots later today. The result is larger reflector = less beam divergence.
2) as long as the parabola equation is the same, and the parabola is focused to infinity (aka collimated) then a larger area increases the total light collected and increase the spot size, not increase the intensity. it only increases the intensity if you focus it to a close point rather than to infinity. Again, I will post screenshots later to illustrate what should happen.
1) that makes sense.
2) When reflector lights are focused properly to a distance far away (which is approximately infinity), the beam intensity is proportional to the area of the reflector. If your calculation says otherwise, it is mistaken or not relevant to how our flashlights are set up.
Also, what is the ray tracing software you are using; I would like to play with it.
I'm really confused why you think this, can you explain plz?
By beam intensity do you mean the total intensity of the whole beam in general?
Increasing the reflector size will make the total % of LED light reflected greater, but also the area (beam cross-section) so the total intensity remains about the same when measured with a luxmeter, because a luxmeter only measures a single point.
Just imagine taking a reflector and cutting some inches off the circumference, then the inner circle still reflects the same amount of light it did before, but the projected spot is smaller due to the outer circle being cut off.
Instead of O you end up with o and both have the same intensity, the left one has more lumens.
When focused to a point, THEN the left one will should higher intensity.
I can't wait to get home and make some pics because all this stuff is confusing and easy to misunderstand :P
The thread I linked in post #127 should explain. To summarize, a way to think about and predict the throw (cd) of a flashlight is through the equation, I=LA, where I is the luminous intensity (cd), L is the luminance (cd/mm^2), and A is the effective area. Luminance is like the surface brightness of a surface, and when there is more surface area, the resulting luminous intensity (cd) is greater, according to the equation I=LA. When an LED is properly focused in reflector, the entire surface reflects the LED so it is like the entire surface of the reflector has the same luminance as the LED. So then the throw is proportional to the reflector area.
The inner section of a reflector makes a large beam. The outer section of a reflector makes a smaller beam. When you add them together the intensity at the beam center is the sum from both sections of the reflector. See demonstration of this in the thread I linked in post #124.
I think there is some confusion regarding exactly how and when a reflector is focused. When you say the outer part of a reflector makes a larger beam, that might be true very close to the reflector, but far away from the reflector, where the image is being projected (and this is the part of the beam that is relevant for us), the beam from the outer part is smaller.
I’m quite familiar with common flashlight reflectors and aspheric lenses, and this recoil imaging is new to me.
With a common reflector, the outer part does the narrow hotspot imaging, the deeper part in the middle makes the blurry corona.
This recoil reflector, this cheap one that is, seems the other way around: the middle part produces a sharp image of the die, the outer part produces a blurry corona that at another plane focusses more tightly into something undefined, but not as tight as the sharp die image that the inner part makes. Quite a mess really, that cleans up a lot if you leave out the outer section of the reflector.
I think that you are assuming you change the reflector or LED position according to the reflector size (eg large reflector will have the LED at a farther focal distance than a small reflector) Like what happens on small vs large head flashlights.
What is happening when you block out part of a reflector is not this, because the LED remains at the same focal distance, and the curvature of the reflector doesn't change. All that happens is you remove area from the edges of the reflector.
This picture
Shows exactly what should not happen, this is a bad reflector and it is trying to focus everything to the middle point.
It could also be the bulb is not at the focal point. Either way, this is not what you want with a parabolic reflector.
I had a spotlight like this before, at night you could see the beam converging to a spot and then diverging, not collimated at all.
With a collimated parabolic reflector, the thing you see coming from the rightmost image should look exactly like that when projected on a wall.
Djozz, maybe I’m wrong, but it seems like the reflector should behave similarly in both orientations (standard and recoil). I think your observations just indicate the reflector is not good.
Have you ever seen a flashlight behave like this? Do you mean at infinity it should look like a ring or just very close to the reflector? Unless you have seen something I haven’t, I think you are just overestimating the extent to which an LED flashlight beam can be “collimated”.
Not handheld flashlights. Sky beams and other projectors do.
That's why the metal bars that are in front of the lamp cause a visible dark line to appear in the beam.
The glass reflectors are near perfectly parabolic and focus the light straight forward, not converging or diverging.
Yup it's yours :) you've done a lot of good tests.
The first two parts of that image clearly show how the center/middle of the reflector is too close to the bulb/emitter to give a good beam, and how the outside of the reflector is at a good distance but is not a consistent parabola.
Glass or electroformed reflectors are much more consistent and properly collimate the light :)
Sigh… (again)
It seems we’re back to square one:
Where to get a proper reflector.
I don’t expect my mirascope to be better than Djozz’s.
I’ll pick it up tomorrow so then i will know for certain…
