I’m a TIR lenses hoarder. I find it more satisfying than owning flashlights. Up to this date I have thousands of them. From expensive Gaggione to unknown dirt cheap lenses from Aliexpress. Sometimes when it comes to modify a light I had hard times indentifying those marked plastic bags. And in most cases, not all lenses stored in the same marked bags.
The hardest thing was to identify between close beam angles without put them into use. Big dies LED such as COB, XHP50/70, and quadtrix E21A, are all less sensitive to slight variation in optics geometries. And usually the minute differences can only be seenor perceived later after several days of uses. This is problematic especially in hard to replace (or reach) light engines.
Normally, you can see the difference just by eyeballing each lenses under magnifying glass. But some have very slight difference which only easy to spot used with small die LED. Below is the example of two almost identical plastic TIR lenses with OEM rated FWHM (with 1mm2 LED) of 40° and 60°:
Check if you can spot which is which. The only obvious thing is the right one was used and looks dirtier
From the side, you can see very small difference between the center lens focal point by their curvatures. It’s easier to check in picture than in real life. In reality, it’s VERY HARD to see.
So I found the easiest way is to place them on top of diffused even lighted surface. In my case I use my smartphone “front flashlight” screen. There are many free applications to convert your screen into a homogeneous soft light panel.
Still no real differences……
But when you flipped them upside down, it’s now so much easier! The left one is the 40°. The 40° center lens has shorter focal lenght needed to collimate the beam more aggressively than the 60°. Thus, making the small surface micro lenses looks bigger.
[Clemence]