LED there be Light, a D.I.Y. LED ringlight

Please have a look at
http://users.pandora.be/cisken/LED_ringlight/LED_ringlight.html

It took me hours to compose this report. So I really don't feel like
translating it into Dutch, German and French. You'll have to read it in
English, linguistic mistakes and all (I'm Flemish).

Instead, I'll go play with my new photo system. <wide grin>

Francis

P.S.
Flemish:
1. European language (similar to Dutch)
2. native to Flanders, part of Belgium.

 

Thank you.

Within a range of 4 to 10 cm I get a pretty unifom colour. Closer than 4 cm I
get a central dark spot and farther than 10 cm I get a central yellow spot. To
broaden the usefull range to 15 cm, I plan to build a ringlight with +/- 78
LEDs, i.e. an inner ring, a middle ring and an outer ring.

The camera's internal software compensates the white balance very nicely.
Now that I'm getting some experience with it, that is.

Because the ambient light affects the incident light, I have to set the white
balance for each and every series of shots with LED light.

 

Perhaps you could mount some sort of mild diffuser in front of the LEDs to
help spread the light more evenly?

 

Thanks for the link. That's exactly my cup o' tea.

 

I don't think so.

AFAIK Only the tag and slot idea is originally mine.
All other design specs I gathered on the net.

 

Hi Ciskay,

I looked at your web page for the digital camera ringlight. Looks like a
good job and is something I hope to do one day.

You plan to build another for higher brightness. I have a suggestion on
how to get more brightness from your current ringlight. First, it's not
clear how you regulate the current through the LED's. Are there series
resistors? What is the current in each LED and are they matched? Are you
just putting 4.5V across each LED and hoping for the best?

If you operate the led's in a pulsed mode you can get more output. Here's
one posibility: when the unit is turned on the LED's operate at a low,
constant current. This would serve as a modeling light and conserve
battery power. When the pushbutton is pressed the lights operate at high
current for 1/2 second (555 timer IC). This is long enough to manually
syncronize with the shutter. They should have no problem operating at 2X
their constant rating (I've done some work testing LED's for a gloss
meter, they are pretty robust). LED efficiency is generally higher at
currents moderately above the continuous rating, so you likely will get
more than one stop of light gain. I'll make a schematic if you like.

Tim

 

I meant matched in current draw. It would be good to check the current on
each LED. There might be considerable unit to unit variation. LEDs are
preferably driven with constant current since there is also voltage
variation with temperature. OTOH, leaving out the resistors is more
energy efficient. The gloss meter work I did was 15 years ago and with
green LEDs, so maybe things are better now.

That would rule out pulsing at higher currents, unless the pulses were
pretty short. The color change is probably due to chip heating. The light
from the chip shifts to a color that does not excite the white phosphor.

The only improvement I might suggest for the Mark 2 is a more efficient
DC-DC convertor instead of the linear regulator.

Tim

 

Gene said in a mail:

Super performant LEDs are available, but I don't know where to find them in the
US.

https://www.flugmodellbau.de
search for leds ultrahell
and
http://www.led-welt.de/
search for LED, weiß, 5mm, 10.000mcd

 

Next problem, a reasonably priced source of white leds!

http://lsdiodes.home.comcast.net/
Look what an internet search dragged up - $0.60 ($0.75 for higher
output)

I bet they aren't as cheap (say) in the UK - £-$ if you're lucky!
But with shipping at $2 anywhere, any quantity.

Is this too good to be true - are these guys for real?

 

I think it would be difficult to get the colors blended uniformly in a ring
light this way.

Tim

 

See for color rendition:
http://www.4and1.de/ringlicht.htm
and
http://www.4and1.de/weissabgleich.htm

Commercially available ringlights for white light all have white LEDs
only. I haven't seen any mix of RGB.
Amongst others there are:
http://www.volpiusa.com/machinevision/data_sheet/led_ringlight.html
http://www.volpiusa.ch
http://www.dentalfotografie.info/
http://www.dentaleyes.de/

I also received this mssg:

Francis Kennis, aka E Pericoloso Sporgersi

 

Here's the only sources I know Francis.
http://theledlight.com/led-specs.html and
http://www.whitelightLED.com/ cheap source.
Gene

 

... Or you could use a collection of red, blue and green

Somewhat OT:

I know it works that way (TV sets, CRT-monitors, TFT-screens and apparently LED
videowalls too), but I'm still confused.

I distinctly remember a (too long ago)low-level physics class in school where
the teacher told about Newton, rainbows, prisms and the light spectrum.

To demonstrate he had this disk divided in 24 or so (a multiple of 3 anyway)
segments, alternately colored Red, Yellow (NOT green) and Blue.
When he set the disk to spinning fast enough, it appeared uniformly white.

Why is it R, G an B light instead of RYB to produce white light?
And why does CMY together produce black?

Please enlighten me.

 

http://www.4and1.de/weissabgleich.htm
It looks like the white balance can handle the problem of white LEDs
emitting not very much red light.

Thanks, Michael

-Michael Schnell, Krefeld, Germany,
mailto:

 

Re RGB:

It's not a question of physics but of how the Human eye works (it's three
colored, while many other mammals' (e.g. horses') eyes are two-colored).

The film material and the digital cameras (and any other video stuff) is
designed to be a fit for the human eye.

That is why we have the RBG color space (and all the other used for
different purposes).

-Michael

-Michael Schnell, Krefeld, Germany,
mailto:

 

The human eye uses color sensors that are (vaguely) equivalent to a set of a
red, a green and a yellow monochromatic sensor. The digital camera uses
color sensors that are (much more truly) equivalent to a set of a red, a
green and a yellow monochromatic sensor.

So if mixing colors by adding light you can produce any color from
monochromatic red, green and blue light. But a mix from green and red
produces the same sensation as monochromatic yellow light (RGB color space).
"Technically" white light contains any color of the rainbow.
"Physiologically" only three colors are enough to create light equivalent to
real white.

The the way the sensors in the eye are built you can also use monochromatic
green and monochromatic yellow light to produce the sensation of white,
though no red light is in the mix. Thus a red object (absorbing any light
but red) will look black in this "white" light. So white LEDs look white
though they only emit very little red.

The RYB and CMY colorspace is for subtracting color. You can produce any
color sensation by subtracting (filtering by a transmissive glass or a
reflecting surface) some of the R, Y, and B parts from white light. So a
green surface absorbs (much of ) red any yellow light parts.

-Michael

-Michael Schnell, Krefeld, Germany,
mailto:

 

So that's why Stevie Wonder sings:
"You are the Sunshine of my Life".

Thanks Michael,
it was already dawning on me, but now I'm fully illuminated.

 

It seems they're planning a kind of contest.
Here's my entry:
<quote>
Have a look at
http://users.pandora.be/cisken/LED_ringlight/LED_ringlight.html
To non-dentists this is patently insane.
Just kidding of course. But this one really is ridiculous:
http://users.pandora.be/cisken/LED_ringlight/congres_50_61.jpg
(Shot taken at the reception of a dental congress, celebrating the 50th
anniversary of the local dental association, of which I'm the secretary)
The red thing below my chin is a bicycle's rear light (2 LEDs, battery-
powered), set to strobe.
The text says: "THERE're full ones ..."

Even if you were to nominate me champion of insanity (I hope not), then just
let me know. Please don't send me 1.000 blue LEDs. Belgian customs officials
would probably get blue in the face and charge me taxes on the estimated value
(and belgian customs officers are notoriously thorough in value estimates).
Have a drink and toast my future mental health.

Cheers,
(unquote)

 

I have received an order of 25 of the 8000mcd white diodes. They are
in a clear case so the main beam is pretty narrow. Color uniformity
looks good. A rub on some 600 grit emory cloth would frost them
nicely. At 20mA (3.2V) they are damn bright. Anything over 3mA and you
can't look at them directly. I ran one up to 80mA (4.5V) for a few
seconds and there was no obvious color shift.

Tim