# LED - Measuring light intensity

#### theonlysnowman

what type of circuit would you draw for this? and how would you actually measure the light intensity of an LED. say you had multiple LEDs and you wanted to compare the light intensity of all of them. how would you do this with with circuit?

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#### e.chaniotakis

Well, when we needed to measure light intensity in my former lab, we adjusted the LED at some base and supplied it with the nominal voltage. Then placed a photodiode in a certain distance (so that the system would not saturate) and adapted it to and ADC and displayed results in the PC (with the appropriate software).
Of course, you will need some lamp with known intensity output for calibration!

#### theonlysnowman

Well, when we needed to measure light intensity in my former lab, we adjusted the LED at some base and supplied it with the nominal voltage. Then placed a photodiode in a certain distance (so that the system would not saturate) and adapted it to and ADC and displayed results in the PC (with the appropriate software).
Of course, you will need some lamp with known intensity output for calibration!
um i don't really understand what you said... because im only doing A level physics. i mean like would you like have a cell in series with a led, resistor (to limit the current), an ammeter, and a voltmeter in parallel with the led.

the question asked something like, if you have 3 LEDs, how would you set up an experiment to find the light intensity emitted by the diode if the current is kept the same.

in terms of A level, how would you answer this#?

#### Bobbywhy

Gold Member
Similar question in thread here in Physics Forums from eight years ago:

Anyone who wants to measure light intensity should read and learn from this introduction.
http://en.wikipedia.org/wiki/Lux

Here’s a good explanation of the units used. This is important because to communicate your objectives, measurements, and results to others you must use units to quantify your answers.
“Light measurement is done with two different set of units: the Radiometry Units and the Photometry Units.”
http://physics.tutorvista.com/light/light-measurement.html

#### sophiecentaur

Gold Member
what type of circuit would you draw for this? and how would you actually measure the light intensity of an LED. say you had multiple LEDs and you wanted to compare the light intensity of all of them. how would you do this with with circuit?
If you only need to 'compare' then you could use a camera and compare the RGB values on an image of the two LEDs side by side. Better still, you could adjust the distances you needed to place the two LEDs to get the same values and apply the inverse square law to infer the relative brightnesses. (The images of the LEDs would ideally need to cover a significant number of pixels)
This method would eliminate any problems of overall linearity of response of the camera sensor and processing. The advantage of measuring LEDs is that the spectrum does not change in the way that filament bulb spectra change with temperature.

#### Bobbywhy

Gold Member
what type of circuit would you draw for this? and how would you actually measure the light intensity of an LED. say you had multiple LEDs and you wanted to compare the light intensity of all of them. how would you do this with with circuit?
To answer your questions I suggest you check out how LED manufacturers do this:

"Almost all indicator-type LEDs are rated by their manufacturers in terms of luminous intensity in candelas, rather than light output in lumens. Luminous intensity is a function of the angle from which an LED is seen, so this value should be considered carefully when used to characterize the light output of a particular LED. Two LEDs with the same luminous flux output can have very different peak luminous intensities, if they are designed to produce different beam angles. A narrower beam angle means a higher maximum luminous intensity for the same light output."

and:

"Osram Opto Semiconductors follows the recommendations of the International Commission on Illumination (CIE) for luminous intensity measurements of LEDs as described in Publication 127-1997"
http://catalog.osram-os.com/catalogue/catalogue.do?favOid=000000010003381e026800b7&act=showBookmark

As for Standards they all use:

"The Illuminating Engineering Society of North America (IESNA) has developed a comprehensive test method for testing the absolute photometry (i.e., intensity and distribution of light) of solid state lighting (LEDs).
IES LM-79-08 “IES Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products,”
http://www.digikey.com/us/en/techzone/lighting/resources/articles/decoding-luminous-intensity-data.html [Broken]

Here are two detailed descriptions of the procedures used for LED brightness measurements:

http://www.gizmology.net/LEDs.htm
http://www.theledlight.com/technical1.html

Finally, if you could afford it here's a simple LED luminous intensity meter:

http://www.gamma-sci.com/products/intensity-meter/

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#### CWatters

Homework Helper
Gold Member
Thanks for that Bobby. Interesting reading.

Like many people I've been wondering how consumers can compare LED lights. It's pretty difficult as people are familiar with bulbs being specified in Watts not Lumens and the beam angle can vary a lot which also has a significant effect on apparent brightness.

For example I tried out an LED downlight that had a 120 degree beam angle compared to the 60 degrees of the halogen it was intended to replace. Initially comparing them on a one to one basis was very dissapointing. Most people would not say the LED was as bright. The spot the LED put on the wall certainly looks much dimmer and light meter confirms it. However this is entirely due to the different beam angle. When I replaced a room full of halogen (15) with LEDs the overall brightness went up and the intensity of shadows went down. The power consumption dropped from 15 * 50W = 750W to about 75W (based on the specifications, not measured).

#### sophiecentaur

Gold Member
We have hijacked this thread somewhat, I think and the OP will not be the slightest bit interested in all this boring home-owner chat. But I have to agree about the domestic LED lighting thing. The bottom line is that they just don't put out enough light yet but the manufacturers are desperate to sell some of them before they're actually suitable for the job. They are far too expensive and don't all last as long as is claimed. What do we do when some really useful LED versions come along? Do we just sling the ones we already have and which cost a fortune? Could spoil my day as I hate to sling things that are quite serviceable.

#### theonlysnowman

To answer your questions I suggest you check out how LED manufacturers do this:

"Almost all indicator-type LEDs are rated by their manufacturers in terms of luminous intensity in candelas, rather than light output in lumens. Luminous intensity is a function of the angle from which an LED is seen, so this value should be considered carefully when used to characterize the light output of a particular LED. Two LEDs with the same luminous flux output can have very different peak luminous intensities, if they are designed to produce different beam angles. A narrower beam angle means a higher maximum luminous intensity for the same light output."

and:

"Osram Opto Semiconductors follows the recommendations of the International Commission on Illumination (CIE) for luminous intensity measurements of LEDs as described in Publication 127-1997"
http://catalog.osram-os.com/catalogue/catalogue.do?favOid=000000010003381e026800b7&act=showBookmark

As for Standards they all use:

"The Illuminating Engineering Society of North America (IESNA) has developed a comprehensive test method for testing the absolute photometry (i.e., intensity and distribution of light) of solid state lighting (LEDs).
IES LM-79-08 “IES Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products,”
http://www.digikey.com/us/en/techzone/lighting/resources/articles/decoding-luminous-intensity-data.html [Broken]

Here are two detailed descriptions of the procedures used for LED brightness measurements:

http://www.gizmology.net/LEDs.htm
http://www.theledlight.com/technical1.html

Finally, if you could afford it here's a simple LED luminous intensity meter:

http://www.gamma-sci.com/products/intensity-meter/
If you only need to 'compare' then you could use a camera and compare the RGB values on an image of the two LEDs side by side. Better still, you could adjust the distances you needed to place the two LEDs to get the same values and apply the inverse square law to infer the relative brightnesses. (The images of the LEDs would ideally need to cover a significant number of pixels)
This method would eliminate any problems of overall linearity of response of the camera sensor and processing. The advantage of measuring LEDs is that the spectrum does not change in the way that filament bulb spectra change with temperature.

ok i have looked at what you've said.
so if i had a circuit like
the question asked to keep the current the same at all times, so id vary the resistance so that the current is the same for all 3 LEDs, so the energy supplied is the same. id keep the detector (which would detect the no. of photons going past it) x meters away at all times. and the room should be dark so it doesnt alter the results. use the detector to measure the no. of photons that is being emitted from LED in a given time interval. the higher the no. of photons, the higher the light intensity.d

is everything i said valid? btw i dont literally have to do the experiment, just have to say how it would be done. its 5marks.

please would anyone correct me with anything wrong with what ive mentioned.

Last edited by a moderator:

#### sophiecentaur

Gold Member
If you want the current to be the same then why not connect the LEDs in series? Simples.
Then adjust distances from detector / camera for equal.readings for each LED. A webcam could give you a picture on the monitor and the 'light meter' software could be used directly on the three images, in turn, till you get the same values

#### sophiecentaur

Gold Member
To get your five marks you need to identify all the variables and showing how they are controlled and measured. Check that you have all the variables and that you've described a method in full.

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