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I wonder - let's assume one would build some user interface using green, yellow and red or just any other available colored LED (not RGB LED) - what would be the proper (aka best) way to compare the brightness of LEDs in order to choose "equally bright" LEDs regardless of the color in respect of sensitivity to different colors by the human eye - based on thetechnical specification given for an average LED such as milliCandela, Wavelength, etc.? In particular, I am aware that two LEDs of the same mcd are in theory supposed to be equally bright - however, you rarely get different colored LEDs of exactly the same milliCandela value, so expressed differently; if there is a green LED of 45 mcd and a red LED of 23 mcd, is dimming the green LED to 50% duty cycle enough to make them equally bright? (I suppose not, but then how..?)

Thanks!

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    \$\begingroup\$ Your post is a difficult read. Break the sentences properly with full stops and try a paragraph break somewhere. What is the purpose of your enquiry? Matched lighting for the eye or for a sensor? \$\endgroup\$ – Transistor Oct 15 '17 at 13:55
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    \$\begingroup\$ candela is already weighted toward human eyes, so yes, the numbers are linear and comparable. however, there's a lot of "brightness" variance between LEDs, even in the same batch, so you'll always have to tune white balance on such arrangements. \$\endgroup\$ – dandavis Oct 15 '17 at 14:22
  • \$\begingroup\$ @dandavis You have the start of a good answer there. \$\endgroup\$ – The Photon Oct 15 '17 at 15:12
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Several factors make comparing the brightness of LEDs nontrivial.

The brightness of a light source depends on its spectrum.

The four different photoreceptors in the human eye have specific sensitivity curves over wavelength, and in aggregate the human eye is more sensitive to green wavelengths than to red or blue.

Wikipedia shows the spectral sensitivity for the human eye here:

enter image description here

Note that two different curves are shown: the green curve represents scotopic vision, which is when the eye is adapted for dark conditions, and the black curve represents photopic vision, when the eye is adapted to bright conditions, and more sensitive to color.

Because the candela is a photometric unit, it takes this spectral sensitivity into account--photometric units are based on the luminous flux of a light source, which is determined by multiplying the power spectrum of a source by a luminosity function representing the human eye's spectral sensitivity and then integrating, as opposed to radiometric units, which are based on the total radiant power of a light source, which is simply the integral of the power spectrum.

So comparing candela ratings should be apples to apples, but if you take another look at the graph, note that there is more than one black curve. These represent different curves established at different times. Because there are different curves, photometric measurements are only directly comparable if they were obtained using the same luminosity function, and unfortunately the specific function used is not often specified for indicator LEDs (it is more likely to be specified for high power LEDs intended for illumination). But for most purposes the numbers will be close enough for a rough idea.

Photometric measurements must account for angles

The candela in particular is a unit of luminous intensity, which means it indicates a certain amount of luminous flux (that is, radiant energy weighted by wavelength) within a certain solid angle. It's equivalent to lumens per steradian. This means that emitting a given amount of light over a small area will produce a higher intensity in candela than the same amount of light emitted over a larger area. It also means that the candela rating of an LED is only meaningful within a specified angle. So if you have two LEDs with the same wavelength and candela rating but different viewing angles, they should appear equally bright from directly-on, but as you increase your viewing angle, the the one with the narrower viewing angle will appear less bright than the wider one.

In addition, the intensity is not going to be constant through the entire viewing angle, It will tend to be most intense at the center, and will drop off to some degree towards the edges of the viewing angle.

Photometric measurements must account for areas

The brightness of a directly-viewed light source depends on the apparent size of that light source. A given intensity of light emitted from a point source will appear brighter than the same intensity emitted from a larger area, so a clear LED will tend to appear brighter than an otherwise equivalent diffused LED. The relevant photometric measurement here Luminance, which uses the units candela per square meter, sometimes called nits (which are in turn equivalent to lumens per steradian per square meter. Photometrics can be complicated!).

Note that the luminance will also be affected by how the LED is presented to the user. Lightpipes and lenses will change both the angle and the apparent area of emission.

Specifications must be given under known conditions

In order to obtain the specified performance of an LED, it must be used under the same conditions under which it was specified. Two LEDs of the same candela rating may have been measured at different operating currents, so if you operate them both at the same current, they will exhibit different performance. Temperature and age also play a role, but current is the main factor. Note that when driving an LED from a constant voltage (such as an MCU pin or something) via a resistor, the operating current will depend on the forward voltage of the LED which will depend on the LED chemistry. So two LEDs with similar specifications but different compositions will require different resistors to obtain the same operating current. This is fairly elementary, but it's easy to overlook when comparing LEDs of similar color.

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Apparent intensity is best judged by a wide-band photo-diode, aimed directly on axis with the LED center of axis. The maker of LED's like OPTEK supply various parameters to judge color purity and brightness in mcd.

Brightness as observed from looking down the center with a photo-diode (not your eyes, as some LED's are intense enough to damage the eyes retina) must include overall brightness, stated as mcd, which accounts for lens type and viewing angle.

As an example OPTEK makes a high efficiency white LED (OVLEW1CB9) with a brightness level of an intense 24000 mcd at just 20 mA of current, 90 mA pulsed. But that is with a water-clear lens and a tight 15\$^{o}\$ viewing angle. This LED is good for flashlights and spotlights only, as it is much too harsh for a reading lamp. Expand the viewing angle and add a smoky or tinted lens and now you have an LED that is much easier on the eyes. But that is all relative.

Pure color LED's are constantly being improved. Samsung now has 'Quantum LED's which are supposed to be more close to being true red, green and blue, but the goal is to have laser LED's become the norm where color purity is mandatory, which means TV's and monitors.

A calibrated colorometer would not give accurate readings unless it was focused on the center of the LED viewing angle. How the eye sees color is non-linear and also relative, so these LED manufactures use expensive laboratory grade colorometers from many angles to assemble their datasheets.

How the eyes see color and brightness does not count in the lab, it is a marketing issue. One can only say that a high mcd value means a bright LED, but is the brightness from the drive current, the lens type, color purity or the viewing angle? It is all 4 of these parameters, so picking just one is not accurate at all for judging an LED's true brightness.

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The voltage of LED depends on the type of semiconductor material used for the construction:

enter image description here

And this construction also determines the wavelength of its band. For example, an 840-940 nm material has an approximate voltage drop of 1.2 V. But this voltage drop is never 1.2 v if you use a high edged multimeter such as one having display count higher than 20000.

You can use the feature of voltage to detect the brightness and the relationship between brightness, voltage and wavelength. This spectra chart will offcourse provide help: enter image description here and enter image description here

You will also need to use some Arduino or Raspberry pi for reading voltage and making logic behind. Also these two documents from UCI and ACS will help you establish the relationship: UCI: Light emitting diodes, ACS

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  • \$\begingroup\$ This does not appear to answer the question. \$\endgroup\$ – ajb Sep 23 '18 at 16:55

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