Hello everyone:
I have a UF-1504 host on the way from mtnelectronics, so I’ve been thinking about how to optimize the flashlight. My goal with this flashlight is of course to have a lot of throw (cd), but I’m concerned about the low light extraction efficiency due to the light not collected by the lens when it’s in focused “spot mode”.
So I am planning on using a small collector lens very close to the LED to collect a good portion of the light that would just be lost. My understanding is that while this can significantly increase the light collection efficiency, it will not result in significant changes to the beam cd. In ray optics language, this is because when the collector lens is placed very close to the LED (within the focal length), this produces a virtual image that is larger than the actual LED. The large lens then sees this virtual image and focuses it. The result is the beam with the collecting lens in place is larger, but close to the same cd. This works for me because it makes the beam wider and more usable. There is some talk about collector (pre collimator) lens in this thread: Uniquefire UF-1405 - A worthy zoomy?
The reason I made this thread was to more closely analyze the light collection efficiency. Aspheric lenses used in flashlights usually have a focal length approximately equal to the lens diameter. This means the cone of light collected from the LED has a 26.6 degree half angle. Now how can we estimate what portion of the total light from the LED lies in this cone? We can do this using the “typical spatial distribution” plot in the XP-L data sheet. This shows the luminous intensity (cd) as a function of angle. It is greatest at 0 degrees (right above the LED), and goes to zero at 90 degrees (down to the side, parallel with the LED die surface). From this plot, one can see that there is a lot of light at half angles greater than 26.6 degrees, so the lens is losing a lot of light, but this plot doesn’t tell the whole story. To get the total light as a function of angle, one must integrate the light cone (integrate around the axis perpendicular to the LED surface). Doing this effectively weights the higher angles more because there is more light in the cone at higher angles. This adds a factor sin(theta), and this factor multiplied times the spatial distribution plot gives the total light as a function of angle.
If we approximate the luminous intensity in the spatial distribution plot as linearly decreasing with angle, the graph in this link:
shows the total light as a function of half angle (in radians, so 1.57rad is 90 degrees). The area under the curve is the total light in different sized cones. The cone from 0 degrees to 26.6 degrees (0.464rad) has roughly only 25% of the total light from the LED. So a typical aspheric lens setup loses 75% of the light. Now if we use a collector lens close to the LED that collects a much larger cone, say with half angle 75 degrees (1.31rad), 94% of the light is collected.
Conversely, one can calculate the light collected by a typical reflector, which might collect all the light with half angle greater than 30 degrees (0.52rad). The light collected by this reflector would be 72, so 72 of the light would be directed to the hot spot, while the rest would come out as spill.
I just realized that the extra sin(theta) factor should be included which is why I made this thread. I think the analysis is correct. What do you guys think? Like I said, there is some talk of people successfully using collection lenses in the thread I linked above, but I could not find anyone estimating how much more light was collected.
I’m planning on doing some measurements before and after I modify my UF 1504 with a collector lens, so we’ll see how much more light it collects.