BLF LT1 – The Video. Staring PWM, Frequency, and Tint.

Other Types of Lights LED Lanterns
1

The LT1 has many tricks up its sleeve. I thought it might be interesting to take a look at what’s going on as far as the PWM and Duty Cycle during the various Tint and Brightness combinations.

It is really difficult to describe, even with still shots from the oscilloscope.
So I shot some short videos of what going on while all the magic is happening.

Unfortunately my old iPhone can’t be fixed as far as color or exposure. So the Auto settings kind of hide what the light is doing.

0.00: Middle Tint Setting, Ramping the Brightness.
The PWM Duty Cycle reaches 54% Duty Cycle.
On Low, the Frequency changes to 3.9kHz, with a very short Duty Cycle.

0.46: All cool LEDs only, Ramping the Brightness.
Notice the light at full brightness has no PWM. It’s running a DC Offset.
As the brightness decreases, the Duty Cycle lowers. The Frequency stays at 15.9kHz.
At the lowest brightness, the PWM again drops to 3.9kHz.

1:40: A Middle Level Brightness setting, Ramping the Tint.
Very unique PWM waveforms are created by the Duty Cycles of the 2 types of LEDs
Interacting with each other.

2:15: Ramping both the Tint and Brightness.
I changed the time frame on the scope to zoom in on the waveforms.
Starting at Warm, Low Brightness.
Ramping to Full Brightness, Cool Tint.
Notice that at Full Brightness, the light is not running pure DC Offset.
There is still some Duty Cycle being seen from the LEDs.
I miss-spoke in the video. The full cool bright is a DC offset, but there is some artifact left from the way the Duty Cycles of the LEDs are interacting.

There are some really interesting waveforms being generated by the Duty Cycle of the warm and cool LEDs stacking on top of each other.

Taking a look at the ends of the tint ramp - The single square wave Duty Cycle, is (I assume) an indicator as to why some have reported that the light has the longest runtime when at those tint settings.

All the Best,
Jeff

2

outstanding post
I learned a lot
thank you

is the PWM invisible to the eye

is it invisible to the iPhone too?

3

AFAIK from all the posts I’ve read about the LT1, there is no visible PWM under most conditions.

4

Thank you for the kind words.
PWM and camera shutters are funny bedfellows.
On some lights, I can see it on the iPhone, but not really see it in the light itself.

One the LT1, I didn’t notice any PWM on the videos.
By mine eyeball, at the faster 15.9kHz PWM, all I see is light.

At the lowest brightness setting. The PWM is at 3.9kHz, and Duty Cycle is very short at 1.3%.
This is just in the realm where (I think) I can just detect something by eyeball.
But only in a full dark room, and moving something very fast through the beam.

But perhaps I am fooling myself. A double blind experiment is hard to do alone.
It certainly is not bothersome – IF - I can actually detect it at all.
There are some more O-Scope shots in this POST:

So, I really think my answer is, I can’t see it with my eyeballs.
All the Best,
Jeff

5

Also meant to say,
That when the light ramps up for full bright,
The DC offset could also be thought of as a PWM running a 100% Duty Cycle.

It would be interesting to see the block of code that is dictating the behavior of the Frequency and Duty Cycle down near the lowest brightness. And near the top.

If I was quick enough, I though I saw a short blip at around 7kHz just up form the lowest brightness level.
Before it jumped back up to 15.9kHz.
All the Best,
Jeff

6

people differ in their awareness of PWM

50% dont notice, 25% notice but dont care, and 25% choose to avoid pwm that produces dots when waving the light…

HDS uses PWM at 454 hz

when I asked Malkoff what PWM speed his lights use, he said
“The PWM frequency is 310 Hertz. Most people can’t see it.”

some flashoholics claim 10kHz is their detection threshhold for PWM:

7

345Hz? Depending on the Duty Cycle, I can sure see that easy enough. If the Duty Cycle was too low, I would not like to have to use it for long periods of time. Below 25% and I wouldn’t like to use it at all.

The Nitecore TUBE on low is 503Hz, but with a short 1.63% Duty Cycle.
I see that even with some indoor light as a background. I don’t find that terribly obnoxious for short periods of time. But I sure can see it all the time. I would hate to use that as a sole source of illumination.

Once the brightness ramps up a bit, and the PWM jumps up to 3kHz, I can still detected it, but it doesn’t really bother me. But again, I don’t use the TUBE for long periods of time in total darkness.

I’ve got a cheap Rayovac headlamp that has a 235Hz PWM with a little over 50% Duty Cycle.
I can always tell it’s there, but it’s not really bothersome as long as there is some ambient light in addition to the headlamp. I could use it it total dark, but I’d rather have something nicer if that was an option.
On low, it’s running at 188Hz, with about a 20% Duty Cycle. That’s intolerable! Under any conditions.

With TK finding anything below 10K bothersome, I must assume that the only way to get the LT1 dim enough was to use the 3.9K PWM.

The SP36 in moonlight is about 4.8kHz with a short Duty Cycle.
It’s one of those that I really can’t detect it. Sometimes I think I can, but it’s kind of “am I really seeing it?” It certainly is no bother to use in full dark. But once again I’m not using it for detailed work or for long periods of time. Again I wonder if using it for long periods would invoke eye fatigue or headaches?

And I think the environment and use plays a big part. Mostly my full dark use is roaming around the house at night. Headlamps and flashlights are usually used to light up dark areas inside where I’m working where the indoor lights just don’t give me enough photons to get things done.
The addition of background illumination greatly reduces the negative effects of crappy PWM in my case.

I think a real test of tolerance would be working by the light alone without any background illumination. For an extended time in different scenarios.
Reading at night.
Up close working on detailed stuff. Where you’re moving both objects and your eyes around.
Wandering around the house where there are plenty of surfaces to reflect and reinforce the light.
Outdoors walking in pitch black.
Searching outdoors where the light is constantly in motion.
Watching some kind of outdoor activity where lots of movement is going on.

I also wonder if the PWMs that are just on the edge of (or just beyond) what I can detect, leads to eye fatigue over time.
All the Best,
Jeff

8

excellent points, thank you for the details

I agree PWM is most eggregious when used as sole ambient source for long periods

Ive known of someone that lived in a cabin, and used a Zebra for ambient, as much as 4 hours at a time… They reported migraines.

Ive adopted the 3kHz level as a possible threshold, beyond which biological effects may be negligible

grateful for your comments about duty cycle, It helps me understand a criteria Im new to

really like your live action scope… your video Rocks!
thanks for taking the time
I had not seen the LT1, Im impressed

9

No problem.
I love rambling on about lights, and you can tell by my POSTs”
I ramble - Therefore, I am.

Just for Fun. Here’s a video I did a while back that shows the PWM of a headlamp of mine and the TUBE.
You can see the Duty Cycle ramp up and down. And watch the Freq. on the Display. The lowest window shows the Frequency spectrum. All the peaks above the actual PWM Freq. are caused by harmonics due to the shape of the square waves.
And you can hear the PWM sing in the audio frequencies.

TURN THE VOLUME DOWN first…

All the Best,
Jeff

10

fortunately I dont own any lights I can hear… that would harsh my high