objective measurement of LED luminosity

[gregc]

several threads have asked about LED intensity.

i wonder if a CDS cell can be used to objectively measure LED intensity.   Looks like they are sensitive to visible light and roughly similar to human eye sensitivity.

 

 

"In the dark, their resistance is very high".  The voltage measured across a 10K resistor in series with a CDS cell,  across a known voltage source and the 10K tied to ground would provide a measurement that is increases with light intensity. 

the resistance of a resistors in series with various LEDs could be adjusted to obtain roughly the same voltage measured across the 10K resistor.

 

[RR_Mel]

Greg

 

I bought a (cheapo) light meter on advice from my daughter who is a professional photographer to improve my picture taking abilities.  Unfortunately you have to be a photographer to use one.

 

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=MT30+Luminometer+Luxmeter&_sacat=0

 

I tried it for measuring LED brightness and it works very good as long as the LEDs are similar, i.e.; warm white to warm white comparison.

 

Doesn’t do well for measuring the brightness for tri-color LEDs as far as equalizing the brightness between colors to my eyes.

 

 

 

Mel

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

[wvg_ca]

only -roughly- ...

there is no way a cds cell can measure angle [of the emitter] with any certainty ...

it is limited to the angle obtained in the setup ...

 

[rrinker]

 It may be able to tell the relative light intensity, but that's almost meaningless when it comes to human perception. The human eye sees certain wavelnghts better than others. Most greeen LEDs, for example, are way less bright even with the same current than a similar red LED, but the green appears MUCH brighter. The curve for the CdS LOOKS similar to the human eye response, but the scale of the two charts is massively different - I'm not so sure the CdS cell is that identical to the human eye. 

So while a light meter (and the data sheet) may say one LED is 60mcd and another is only 20mcd, to the human eye, the 20mcd might appear brighter, depending on the color. 

                                      --Randy

[gregc]

rrinker

The human eye sees certain wavelnghts better than others

the spectrum on the right is for the human eye

 The image on the left indicates that the CDS cell also favors the center of the visibnle spectrum

[rrinker]

 On reading the article the chart comes from, it does say the CdS cell matches the response of the human eye - hmmm. Though it still does respond to frequencies outside the human range, with a steep dropoff. Perhaps a little filtering to eliminate the effects of non-visible (to humans) light would make it a near perfect match.

                                  --Randy

 

[gmpullman]

https://tinyurl.com/s9dqxkm

 

 

IMG_1487 by Edmund, on Flickr

 

IMG_1474 by Edmund, on Flickr

And if you were wondering what a footcandle is:

GE_Footcandle by Edmund, on Flickr

Cheers, Ed