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Building an LED light/flash
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mawyatt



Joined: 22 Aug 2013
Posts: 2452
Location: Clearwater

PostPosted: Fri Mar 27, 2020 10:14 am    Post subject: Reply with quote

Saul wrote:
mawyatt wrote:
...pn diode junction can be used with a DVM as a crude light meter ...

Yep, during soviet occupation we had very limited electronic part supply - so , instead sensors we used Russian transistors, filed their cap tops off...it was better than photo resistors (which you couldn't get also).


Interesting about the Russian transistors!!

The old (~1955) Raytheon CK722 plastic germanium bipolar transistor was built with a plastic that wasn't opaque. Remember shinning a flashlight on them and watching the collector current jump around.

BTW just did a quick evaluation of the Godox Vidoe light.

https://www.photomacrography.net/forum/viewtopic.php?t=41353

Best,
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mjkzz



Joined: 01 Jul 2015
Posts: 1237
Location: California/Shenzhen

PostPosted: Fri Mar 27, 2020 3:41 pm    Post subject: Reply with quote

Saul wrote:
mjkzz wrote:
...as relative meter, ie, I do not trust its absolute reading, but relative one...

Yep, correct, I'm doing like this way already, using Iphone app, and it gives pretty good idea
Quote:
...and want to build your own light meter...

No way, no time for that Smile
Quote:
Or just use a light meter for photography, like Sekonic L-308S...

In this case I have to find my Minolta meter, which is somewhere ...

Looks like, I have to stay with my Iphone app ...


sure, that definitely works.

my post was meant for generic readers and those who does not seem to know how to do it correctly.
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mawyatt



Joined: 22 Aug 2013
Posts: 2452
Location: Clearwater

PostPosted: Sun Mar 29, 2020 6:09 am    Post subject: Reply with quote

Saul wrote:
mawyatt wrote:
Saul wrote:
mawyatt wrote:
...We did a quick "hands on" test here that required 1.6 seconds exposure of a 220 lumen LED illumination for proper exposure, so for a 10ms exposure would require 35200 lumens...

Hi Mike,
How are you measuring lumens ?


Hi Saul,

This is from the specification on the light, then somewhat confirmed by the actual LED that's used, a Cree XPE. Which has a maximum 307 lumen specification with a 122 lumen/watt maximum efficacy. The LED has a drop of 3.3 volts at 700ma, and the light power supply is rated at 4 volts 750ma. So things seem reasonable for an LED current of 550~670ma @ ~3.3 volts to produce 220 lumens.

Best,

Is there some simple way to measure lm (physically), not from numbers/specs ? I saw a lot 2nd market unknown leds, mounted on the brand bases ...


Hi Saul,

See this post:

https://www.photomacrography.net/forum/viewtopic.php?t=41353

We used a Sekonic L-308DC light sensor with the "Lumidisc" installed to allow direct Lux measurements. The LED fixture was placed inside a standard Bowens type bright aluminum reflector for strobes, such that the reflector better defined the illumination edges but didn't significantly reduce the main lobe of the illumination. The idea is to slightly confine the illumination for a better edge to set the proper height.

The circular illumination pattern was projected in a dark room to the floor by attaching the LED fixture on a standard photo light mount until the circular pattern had a ~1.13 meter diameter was projected onto the floor.

Since Lux and lumens are related at 1 Lux equals 1 lumen per square meter, then with an illumination pattern diameter of 2(square root(1/pi)), {area = pi R^2, so D = 2* square root(area/pi)}, then they are equal in magnitude in a uniform pattern with 1 square meter area.

The light sensor was placed on the floor in the center of the circular illumination pattern, we moved the sensor around to check how uniform the illumination was. Ambient was measured and subtracted for the readings.

The result was the IKEA light with the Cree XPE 220 lumen LED produced readings of 200~240 around the center consistent with a 220 lumen output and the Godox SL-150W 16000 lumen produced readings of 17000~19000 consistent with a 16000 lumen specification. Also a direct comparison was made between the two (see link) at a 1 meter distance above the light sensor and this produced an estimated SL-150W output of 18,333 lumens using an "assumed" Cree XPE of 220 lumens.

A more diligent effort, which I'm not going to bother with, would measure the illumination at multiple positions, then do an averaging to estimate the uniform illumination, then divide by the attenuation of the aluminum reflector. Bright aluminum has a reflectivity of ~90% in visible, so one could estimate the overall efficiency of the reflector. Since the averaging tends to reduce the result and dividing by the reflector attenuation increases the result, the two tend to somewhat cancel in effect.

Anyway, this hands on measurement technique allows a rough estimate of the lumen output using a calibrated light sensor to measure the output intensity in Lux.

Hope this helps.

Best,
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