Creating White LEDs: Phosphor vs RGB

[Newtons Apple]

Hi, so I've been just obsessing over color and light for a while... but I'm trying to understand something. To my knowledge there's two many methods of creating a white LED.

1. Taking a blue LED and putting some sort of phosphor like zinc on top of it, and as the blue light passes through, it mixes with the phosphor to create a white light.
2. Mixing a red, green and blue led together to produce a white light.

Now my main question is...are these two whites the same? Sure they both will look white to us... but are they the 'same' white? If we measure with a spectrometer both methods, will we see the same output?

I've gotten into growing plants recently and they use different color light for their needs such as creating glucose, rooting, flowering etc. They can do this naturally with the light from the sun which is a full spectrum. Is the Blue LED method going to provide the same thing? Even though it's technically just blue? Does it turning white from the phosphor able to create the full spectrum of light?

Like wise if we take a prism and hold it up to the Blue LED method, will we only get blue out of the spectrum? Or will it show the full rainbow?

 

[Baluncore]

A blue LED can be used to excite a blend of phosphors, that will generate the required white. There can be many wavelengths present in the phosphors, but we only need three to satisfy human colour vision. The colour of a phosphor LED is dependent on the chemical phosphor, not on the current.

Three LEDs, R, G, and B, can be used to make a variable colour light. But that requires the current be regulated to three elements in each LED.

Both LEDs and phosphors radiate light having narrow wavelength bands. We cannot see that because we have crude RGB eyes.

A prism or grating is needed to see the separated narrow bands of colour being radiated. You might start by looking at a reflection of the light source, from the back of a CD or DVD, which is effectively a grating. Sunlight and filament lamps will have a continuous spectrum, without gaps between the colours.

 

[sophiecentaur]

.are these two whites the same?

It all depends on the details and the analysis in the eye. There are an infinite number of different combinations of individual colours that can produce whites. There are many different standard 'whites' which are used to illuminate a scene. In the old days, a tungsten filament was a convenient source but RGB sources can be used to achieve the same effect. (But not just any old R,G and B.)

TV display manufactures have been working hard over the decades to produce the blisteringly good TV displays that you can see these days. When you see old TV recorded programs from the 60s and later, they are soft, glubby and brown. We thought they were brilliant pictures at the time.

 

[sophiecentaur]

We cannot see that because we have crude RGB eyes.

If we really needed our eyes to behave like spectrometers then we would have evolved that way. We survive very well with just three analysis curves for colour vision. "Crude" is a bit of a harsh description.

 

[hutchphd]

Now my main question is...are these two whites the same? Sure they both will look white to us... but are they the 'same' white? If we measure with a spectrometer both methods, will we see the same output?

No, they will be identifiably different.

I've gotten into growing plants recently and they use different color light for their needs such as creating glucose, rooting, flowering etc.

The wavelength requirements are largely determined by the absorbance of Chlorophyl. I think there is an absorbance band in the red and one in the blue (please look it up !) The flowering, etc depends upon the daily duration and timing of the lighting.

 

[Baluncore]

The flowering, etc depends upon the daily duration and timing of the lighting.

There are some (often hydroponically grown) plants that provide a higher yield, when they are grown at high altitude, or where there is a higher UV content.