Since I'm aware of Yuji leds I wanted to see for myself what that ultra-high CRI was looking like in reality. Especially their VTC series which is 98CRI and based on purple leds instead of blue (to get rid of the cyan dip in the output spectrum) fascinated me. But two reasons kept me from buying them: 1)the high shipping costs plus that they are sold per hundred pieces, and 2)they do not make high power leds so the output in a flashlight will always be underwhelming.
Now recently they started selling a new type of their VTC-series leds: the D50 (beta-)version, which has the same 98CRI but with the phosfors tweaked a bit to make them (partly) compliant to ISO3664:2009, which defines the D50 standard illuminant published by the CIE. A good enough reason or not, this was my tipping point for buying a batch of these.
So what is a standard illuminant? (I'm not more than slightly ahead of you, but I suspect that maukka is more educated than me in this). Wikipedia: A standard illuminant is a theoretical source of visible light with a profile (its spectral power distribution) which is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting.
And the D50 standard illuminant represents "horizon"daylight of 5000K (as opposed to i.e. "noon"daylight of 6500K). So scientists took numerous samples of horizon daylight and averaged the spectra into the D50 standard. This led is a close D50 illuminant but apparently not close enough to fully comply to the norm. Here's a graph comparing the spectrum of this led to the D50 spectrum (taken from the Yuji website, more data there):
The tested led is a 5730-size midpower led specified for 150mA and about 38 lumen. Not only the output is flashlight-unfriendly but also the led-shape, and tint-variation in the die will guarantee a less than ideal beam profile when used in a reflector light (although not worse than the XP-G3, I checked but made no picture):
A plus is that this led-size can be succesfully used on a standard XP-board. it works fine but the source of the light will be off-center a bit:
(the image on the right shows the led in a small zoomie that will be discussed below).
So how's the output? I did a test! As a habit I used a DTP-board for the test, but the thermal resitance of this low-power led is so high (23 degC/W) that the type of ledboard will unlikely be the bottleneck for thermal performance, any non-DTP board will probably give the same results.
The output test was done like all my more recent emitter tests. I described it in detail in my XP-L test. , with two minor differences that should not matter significantly for the results: I used my Integrating sphere no. II instead of no. I, and for the current I used a clamp meter, which appears to measure 0.1A lower than the power supply current-reading that I used before.
In summary: 1) just one led was tested, reflowed on a DTP copper board (KD-light 16mm) 2) I used my large version II integrating sphere with high quality luxmeter, 3) the output numbers and voltages were measured with the led close to 'steady state' for each current, so warmed up and settled, you should be able to get these numbers in a well heatsinked flashlight. Mind that these are output numbers of the bare led, in a flashlight there will be losses from light obstructions, lens and optic, 4) output is in 'djozz-lumen' defined as 1/550 of the output of my Sunwayman D40A on high setting, which I hope is close to the real lumen, but at least is consistent over all my emitter tests done in integrating spheres.
Here's the graph:
What can be seen?
*It is nice that the led can handle quite a bit more current than the specified 150mA, it runs it up to 300mA without problems, with 65% more output. The led was still alive at 600 mA.
*40 lumen at 150mA is also over specs, although my 'djozz-lumen' is still not officially calibrated to a standard light source.
*the voltage curve is quite steep, the led can even run succesfully direct drive on a li-ion battery although I would call that overdriven.
*as a light source in a flashlight, with losses by optics and light obstructions and such, it can provide a bit over 50 lumen maximum.
*300mA drive current is AAA-battery with boost driver territory :-)
So now what to do? I reckoned that i would make a little D50 flashlight with one of these leds, and the nature of this led made me decide to use a zoomie, to be able to evenly illuminate something in flood setting. Spot setting of course spoils any standard illumination with the ugly die projection. I used this über-cheap AAA-zoomie:
It has just 1 mode and runs on a standard AAA alkaline battery or AAA NiMh battery. I have not tried a 10440 li-ion in it yet, might kill it! I reflowed the Yuji-led on the tiny stock ledboard, re-shaped the plastic disc that presses the ledboard on its shelf to fit the odd-size led and extract a bit more light.
And it works fine, in flood a nice even illuminated circle of about 30 lumen, and the ugly die projection in spot. And the ultra-high CRI light that comes out is almost boring, so natural is the appearance :party:
In all other aspects the performance is poor: for those 30 lumen, the Eneloop delivers 1.2A, which is an ultra-low system performance of 21 lumen/W (my most efficient lights are close to 120, with very low CRI of course). This is caused by the inefficient led, the fact that it is a zoomie, and the cheap boost driver eats a lot of power too.
I made a beamshot that compares the light to a flashlight with 83 CRI 5000K Nichia 219C leds and with my eyes I can see the difference well enough, but photo's (and here my phone camera is also to blame since UV-over-exposure damaged the center part of the imaging chip, leading to a slight colour deviation) are a very poor representation of what you see with your own eyes. Left 83 CRI Nichia, right Yuji D50.
Conclusion
So that was fun, to see and test one of these VTC-series leds. Hardly useful for flashlights but as you see you can make a weak light out of them (perhaps reflow 4 of them on a MT-G2 board will make it a bit more powerful). But I like the little flashlight that I made, the light it gives is really great and I will treasure it as something special. Btw, I bet this led fits that small and flexible IKEA USB-light too, or you can do some household ledbulb surgery and replace the leds in there.
I hope you liked reading this test :-)
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Edit August 2017: in post #27, clemence describes a nice build with these leds, including tint pictures, and in post #29, maukka does his great tint and CRI analysis :-)
