Hello
[...]
Anyway, I would like to ask your opinion on something.
Back in the days of CRT's the refresh rate of the monitors was quite important for me because
flickering is affecting me and makes me dizzy. I don't like flickering... 
All the CRT monitors
I bought had more than 100Hz refresh rate which made it more comfortable than the 60, 75Hz etc.
At CRTīs is the refresh-rate equal to the light-emitting-rate. With CCFL/LCDīs it is a little different.
Now we have the new LCDs of course and nobody mentions monitor frequency any more.
Nope. You can buy LED-TVīs with 100, 200... 400 Hz refresh-rate of the display without any problem.
But there is a big difference to the CRTīs: The light-emitting-factor ist the PWM-Frequency of the Backlight,
especially the Duty-Cycle, not the display-refresh! (Of course: Except, thatīs a higher refresh-rate need a higher PWM-rate.)
Indeed, I can work more relaxed on LCD's but still there was something "annoying" about them.
I swear that I could detect very high frequencies that made me feel uncomfortable after some time
working on them, despite the fact that everybody said that there is no flickering.
They are wrong. Itīs a well known problem with some LED-Backlights.
So, I decided to check it. I bought a multimeter, and together with a solar panel I had from a
broken car battery charger I made an optical frequency meter.
Cool idea!!!
To my surprise, when I measured the monitor I am typing right now, I got 157 Hz !
I immediately liked that meter I just made! :-) I started measuring everything. My SONY laptop
that was very pleasing to my eyes had a 210Hz refresh rate despite being 10 years old!
Everybody that worked on that laptop said that the image was like printed paper.
Some led lighting I've installed and did not like, had an 100Hz flickering, which is exactly double
the mains frequency and its probable a result of bad rectification. I also measured successfully
a UV light source.
157 Hz are for sure much to low! It seemīs, the laptop had in his origin design a CCFL-Backlight.
Why? Letīs look, how the displayīs work...
First: The LCD, which is showing the picture, is with CCFL and LED Backlight the same.
As long there is no changing in the picture to show (reading a text... ), as long the display looks steady.
(Because it switched at every refresh the same pixels on and hold them until the next refresh. There is
no flickering at all.)
If the picture is changing (scrolling the text... ), the display is actualizing at the next refresh.
Standard at laptops:
The display refreshes 60 times per second ( =60 Hz), regardless there is anything to actualize or not.
Thatīs the refresh-rate.
Now we add a backlight. First the CCFL.
The Brightness of the display is controlled with PWM. The frequency of the PWM-Signal is always the same.
Important is the Duty-Cycle. At full Brightness you have a duty-cycle-ratio of 100:0 (say 100%) on-time.
The Backlight is lighting up all the time.
At half brightness you have a ratio of 50:50 (say 50%). At half time the backlight is full lighting,
in the other half of the time it is darker (not black!), because a CCFL has some after-glowing when it is switched off.
The Contrast between on and off isnīt very hard. That avoids a big part of flickering.
Now we change to a LED-Backlight.
The Brightness is also controlled with a PWM. It works the same way. But there are two differences:
A LED-Backlight is brighter as a CCFL. To produce the same result in absolute brightness as the
CCFL with 50% duty-cycle a LED needs (for example) only 33%.
Much more important: If the Backlight switched off, the LED`s are immediatly black! There is no after-glow.
Thatīs making a very hard contrast.
In the first case (CCFL) you have a backlight, which is half the time full bright and half the time dimmed.
In the second case (LED) you have a backlight, which is at 1/3 of the time at full brightness
and at 2/3 of the time absolute black.
The On/Off-Cycle of the backlight is building the Backlight-Rate.
A CCFL-Backlight work fine with (at least) 150 Hz.
To avoid flickering with a LED-Backlight is a rate of (at least) 250 Hz needed.
Unfortunatly are the manufacturers not giving informations about the backlight-rate.
It seems, your little device should be very useful when you buy your next laptop...
Today it seems to be the only way to make the display a little less flickering, to increase the brightness-setting
(except it is at the max). Perhaps it helps a little...
You can, just for fun, fiddle with this setting and look what the 61E is showing at the value of the duty-cycle (Function "%").
1) Does anybody know if they are selling any measuring devices like the one I've just made?
I could not find one, and I think its very useful and easy to make. Actually I've already ordered
the tiny UNI-T UT120T multimeter and I am searching for a small solar panel in order to combine
them in a single portable measuring device. I can even replace the multimeter battery with a
high capacity rechargeable one that will be charged from the solar panel.
Iīve never seen. Probably there is no big market and because it is easy to build yourself.
(I have a deja-vų - a DMM, powered from rechargable cells which feeded from solar-panels...
possible that Iīve seen that anywhere before?)
2) Does anybody know a way to find out the frequency range that a solar panel can measure?
Specifically the upper limit. Do they have an upper limit?
It belongs to the frequency do you want to measure.
Probably are solar-panels not the first choice. Because they are optimized for producing current,
not to follow quick changes in lighting.
Perhaps it is interesting to measure their rise-time. But I canīt do it myself. Iīve got no solar-panel at hand.
3) The UT61E has a (hz) mark on Volts & Amps measurement. It also has a dedicated (hz) switch
position! The frequency measurement works only on the (hz) switch position.
Do you know when to use each one?
My guess is that the (hz) on the Volts & Amps works only when measuring an AC source and
the (hz) dial position works only when measuring DC ripple voltages. Am I correct?
Sorry, no. With the function at the dedicated rotary-switch position is the DMM more sensible at 3,3 to 5 Volts (common digital-signal-level), is responding up to 220 MHz and has an upper limit of 30 V. When you using the second function Knob at measuring Amps or Volts, then the sensivity is controlled through their range and it is responding only up to 1 kHz.
For more details: Please RTFM.
http://www.uni-trend.com/manual2/UT61English.pdf4) Is there any better way to do the same thing? Maybe the BPW34 photo-diode instead of the
solar panel?
Perhaps it is a more effective way to build it with a photo-transistor.
You need only the photo-transistor, a voltage-source (DC), a resistor, some wire and something
(e.g. a plastic-tube) as a housing to avoid anbient stray-light.
That gives you a quite nearly digital signal and you can measure up to MHz.
Sorry for my the lengthy first post but I wanted to share something that I thought was very
interesting.
Sorry for my long answer, but I donīt know, how much you know, neither the level of the other readers.
Iīm still hoping, my english is good enough...
Thank you.
No problem. Youīre welcome.
Hartmut
