You are so ignorant. This whole time you didn’t even know how a flashlight worked. The bottom picture with the more yellow tint is dedomed. Why is there a brighter hotspot and less spill? Because there is no dome to concentrate the light forward and create the spill. Instead, MORE light hits the reflector which forms the hotspot.
tldr; the light which hits the reflector forms the hotspot, anything that doesn’t forms the spill.
Now reflect on how stupid you sound. Learn your stuff before you try to argue. What a waste of my time.
The reason why dedoming “works” is IMO not trivial. At the distance that the cree lens is placed at, it doesn’t even focus all that much. It seems the primary purpose of the lens is to provide a medium with higher index of refraction to interface with the dense material (sapphire) above the semi material. My hypothesis is that at the off-axis angles that actually hit the reflector, the varying degree of magnification across the die due to the lens (you can see this by looking at an xml from the sides) tends to also make the focal point of the virtual image of the die less consistent to the reflector (ie whereas a point on a flat die will always appear at the same spot, the same point behind the lens tends to vary with the angle to the axis). I think a ray tracing simulation of some sort would be necessary to verify this since light is really a vector instead of just scalar value.
If all the dome did was focus the light so more of it went forward instead of to the sides then the end result of de-doming should be more throw with a reflector, but LESS throw with an aspheric lens. But this is not what happens. Instead, de-doming significantly increases throw with both reflectors AND aspheric lenses.
This is what I think is happening:
The dome acts as a magnifying glass that increases the effective area of the die, but reduces its effective surface brightness. Since the light isn’t coming from such a focused point source it is harder to focus and gives less throw in both aspherics and reflectors. When you de-dome the emitter, the light is coming from a more concentrated point source. Effective surface brightness increases and you get more throw regardless of what type of focusing system your light uses.
However, without the dome in place, more light is lost to internal refraction inside the LED crystal. This leads to an overall reduction in lumens.
@agenthex: What do you think would happen if you use dedomed XM-L with TIR optics?
I might be wrong, but didn’t see anyone has tryed out what happens with TIR.
Yup, magic. And no, I don’t believe frequency is conserved(this is not monochromatic light) so there are multiple frequencies involved which can be refracted/reflected differently. Refraction occurs whenever there is a change in medium density(dome to air). Reflection back to the emitter (and reabsorbtion/loss of output) is maximized at 90(flat cut of dome parallel to emissive surface). Dome shape MAY be a design feature to minimize refraction and maximize light transmition through the dome/air interface. Some of the higher frequency light may be reflected back and reabsorbed by the emitter. I have not yet been through this entire thread yet as this topic only recently became of interest to me so I applogize if this response is dated.
+1. This is precisely what has me interested in this thread. Could this be a way to use a cool white emitter with a tir optic to obtain a more focused neutral/warm beam.
Edit - response to Ledoman
People should realize that the reflector does not only reflects the light but it also curves the light. To converge the light into a single hotspot. That why the light from the reflector creates the throw like Slewflash said. The light from the die itself is still diffuse and that makes the spill.
TIR is designed using sophisticated ray tracing for a specific output profile, and I think much more sensitive to this given the way they work (ie more internal reflections). Without trying it all I could say is that it would probably be more different than originally intended compared to the simpler optics.
Your are correct in the bolded text above. The vast majority of the IR is at the interface between the emitter top layer and dome (and by dome I’m including the phosphorus gel), before the phophorus layer unless that gel itself has crazy optical density. The way the dome is shaped, IR at the outer surface should be much less (since the surface is perpendicular to the light. IOW, more is lost before being made into the spectrum.
Almost all the light lost to IR would be in the higher freq part of spectrum, ie blues.
In summary, the implication of this is quite interesting since it demonstrates that far more light is lost at the first interface w/o dome, because the dome decreases the loss at the first interface while increasing it at the second (given the second doesn’t exist without the dome). In fact, the disparity is so large that even though the outer face of the dome should be making the light warmer (by removing some of the higher spectrum), it’s dwarfed by the higher spectrum content lost without it.
I wonder if using tir optics has a greater prismatic effect than using reflectors. Is the off color corona more obvious? I’ve always liked optics for their ability to focus in a shorter depth but this may come at a price. One might have to compare a reflector and optic with very similar sized hotspots to tell. In other words, if de doming causes a frequency shift, does an optic make it worse?
It would be nice if anyone who already dedomed some emitters could give it a try with TIR optics. One doesn’t need to mount the optics. For the first impression It might be sufficient just to hold the optics in front led by hand. Of course mount it somewhere would be better.
Ok, do these pics help anything? One domed, one dedomed, no reflector just bare emitter. First is with dome. Blueish light coming out the top, then yellowish out the sides
Second is de-dome, yellowish everywhere.
You decide. I have only a Nikon S9100, no manual settings. Great for pocket zoom take on zoo/vacation trips, not so great for beamshots. If anyone can do a more standardized comparison you may solve some arguements.
Has anyone tried to “de-dome” a generic 5mm keychain light, hard plastic, LED dome?
With dome, it makes pretty clear spot, sand it down to a straight surface = pretty even spill
Not too expensive method to test out for yourselves
BUT:
Has anyone been thinking how a LED would throw, if there was some kind of “shape” inside the dome, which would force all light not ahead but to sides -> reflector?
I know, there are some recoil throwers. A bit like a recoil inside dome but to sides?
Whatever, I misspelled it. It’s basic photon-photon luminescence with re-emition at lower frequency (ie blue -> y/g/r).
The sky is blue due to Rayleigh scattering, not photoluminescence, which why the sunset is RED even though it doesn’t exactly travel through less of the atmosphere.
De-doming decreases the lumens while increasing throw. Confirmed to be the case with lights that use reflectors as well as reflectorless lights using aspheric lenses. I see no reason why a TIR optic would give a different result. A TIR is after all, just a kind of reflector. The only difference is the light shines through the inside of the optic and refracts off the back of the optic instead of reflecting off a silver-coated piece of aluminum.
The dome does 2 things: 1. It acts as magnifying glass, magnifying the effective size of the die. The die appears to have more surface area, but at the same time its surface brightness goes down. When the dome is removed, the die appears to be much smaller and the light coming from it appears to be much more concentrated. All flashlight focusing systems work best when the light is coming from a single point as opposed to a broad area. As such, removing the dome allows for better focusing with all kinds of reflectors to the side of the LED and with any lenses in front of the LED. Better focusing = tighter beam = more throw.
2. The material of the dome reduces internal refraction where light emitted from the LED is refracted back into the LED crystal. Less internal refraction = more light actually gets out of the crystal instead of being absorbed as heat = more lumens. This is the primary purpose of the dome: Significantly increase lumens by decreasing internal refraction.
The effect isn’t necessarily the same with reflectors and lenses, per hypothesis above (given one maintains a far more constant focal length than another), as the oddity seems to be with the effect of the dome itself. This may or may not apply more or less to TIRs. The role (or rather pointlessness) of “prefocusing” or whatever was confirmed by people who know what they were doing on cpf. There’s also a physical limitation to the ability of optics to focus.
A pre-focusing lens can also decrease angle of light coming from the emitter, thereby more light on primary/larger optic, resulting in larger hotspot of equivalent per unit brightness. The cree dome doesn’t seem to do this (or do it enough), though, which is probably why it cut from theoretical optimal throw.
The above might be an unavoidable side-effect of doing this optimally, while minimizing IR from the surface of the dome itself (ie normal surface to output).
