I was discussing with a work mate recently about lighting for the theatre that he works in. He was saying the air conditioning is directed at the audience so there staff working backstage and in particular up on the lighting swelter.
Of course, I suggested switching over the lights from Incan to led. He said this has been suggested before and declined because the colour rendition or tint (I can’t remember his specific words) changes as the light ramps down to off.
I asked if good pwm would help but kinda didn’t answer going on about something else. I’m not sure if he knew what he was talking about.
So I have a two part question. Does led change tint at different power levels and if it does is pwm or constant current better at reducing the variation?
The tint changes with changes in both forward current and temperature - see for example https://www.nichia.co.jp/specification/products/led/NVSL219B-V1-R90-E.pdf, page 14. Using PWM with different duty cycles will keep the same forward current, but the temperature will change due to change in energy consumption.
Thanks MiG0. So the way I read that is the nichia tint will jump a maximum of 1 point on the Y axis on a 70C temp difference and half a point on both axis from current change. As far as I’m concerned that would not be noticeable. I don’t really know much about stage lighting but I would assume the same applies, it wouldn’t really be noticeable….?
It looks to me that for stage illumination you will need quite substantial number of LEDs. You can change the amount of light by changing the number of LEDs activated. In this case with proper thermal design you should not encounter any perceivable tint shift.
Yes, tint does shift slightly, but it isn’t noticeable to my eye. I’ve taken photos of my lights on “moonlight” mode, as well as maximum brightness, and (after compensating for exposure of course), the images look exactly the same. I can’t tell there’s any difference in tint. There probably is some, but it’s not apparent.
However, LED lighting may not be the best for stage lighting. It just doesn’t have the “warm” tones that good incandescent lighting has, and it might be more difficult to get it as focused (in the case of spot lighting), or as bright.
Whatever the case, stay away from PWM, unless it’s really fast. That can be very annoying to some people.
I think the issue must be that the (black body) color of an incan. changes as it shuts off, and with dimmers. That is a much bigger effect, unless the LED gets so hot the phosphor stops working and it turns blue. Fading to softer color as they are dimmed or turned off is a nice feature of incandescents that they will probably get around to imitating with LEDs some time.
Absolutely agreed. I am not really sensitive to PWM though many people are. I see it clearly in my EDC light but it’s no problem. Some Cadillac automobile taillights annoy the heck out of me- I can’t look at then for more than a few seconds before my vision starts getting jumpy and even in my peripheral vision they have a similar but lesser effect. Other lights I have with fast PWM, I can’t even see it and I can use them for as long as I want.
With a really large number of LED’s, ramping by adding or subtracting a small percentage at a time would probably work fine but a 5% (one in twenty) jump might be too much. And that many LED’s will have a considerable initial cost. Modifying the PWM for ramping purposes works pretty good when the lowest mode is still fast enough to not be ordinarily detectable.