Why is there a sharp spike of blue light in my spectrometer readings?

[Newtons Apple]

Hi! Man, I haven't posted here in a while... So I bought a cheap spectrometer and have been measuring light output of many many things. One thing is common from ALL of my readings.. Each LED or light bulb I measure has a sharp, tall narrow band of blue being put out and then a drop off of cyan/lime green and then a bell curve of the other colors. See these two images. One is from my range hood in my kitchen, the other is a 60 watt bulb in my living room.

Why is blue so removed from the rest of the bell curve? Why isn't it part of the main bell curve?


Now look below at some of my plant grow lamps. One of the lamps is UV-A freqency. When I look at the light when it's on, it's to my eyes a very light purple. When I look at the graph below though, it starts roughly at 390nm but the graph shows it as a dark blue. Shouldn't it show violet in at least some amount?

 

[DaveC426913]

I know very little about this subject but I am very suspicious of the shape of those curves. They look both smooth (which I would not expect from any light source) and similar (whoch makes me wonder if they're copies).

 

[Gleb1964]

Most "white" LEDs are including an UV-excitation source surrounded by phosphor material reemitting light in the visible range. That UV peak is coming through and you can observe it by spectrometer.

 

[tech99]

I believe that while the West was looking at band gap devices to make a white LED, Japan did it using a UV source and phosphors, and won the race. Lateral thinking and practicality.

 

[Newtons Apple]

Most "white" LEDs are including an UV-excitation source surrounded by phosphor material reemitting light in the visible range. That UV peak is coming through and you can observe it by spectrometer.

Interesting.. is this a property of LED lights in general? Will all LED's have a significantly higher UV peak towards the blue then? WHy is this needed?

 

[berkeman]

Each LED or light bulb I measure

Have you measured any incandescent light bulbs? How about a red LED?

 

[Averagesupernova]

There is plenty of info to be found on the web about the spectrum coming from LED lighting. Some may be false. Do a search.

 

[SammyS]

Hi! Man, I haven't posted here in a while... So I bought a cheap spectrometer and have been measuring light output of many many things. One thing is common from ALL of my readings.. Each LED or light bulb I measure has a sharp, tall narrow band of blue being put out and then a drop off of cyan/lime green and then a bell curve of the other colors. See these two images. One is from my range hood in my kitchen, the other is a 60 watt bulb in my living room.
View attachment 354433View attachment 354435
Why is blue so removed from the rest of the bell curve? Why isn't it part of the main bell curve?

The "60 Watt bulb" you mention apparently is a standard LED replacement bulb with an output giving approximately the illumination level as a 60 Watt incandescent bulb. To the human visual system it appears to produce White light.
Judging from the spectrum you show, the actual LED in this bulb produces a narrow band of blue light. The broad-band portion of the spectrum is due to the presence phosphors (as mentioned by @Gleb1964 above) which fluoresces.

Now look below at some of my plant grow lamps. One of the lamps is UV-A frequency. When I look at the light when it's on, it's to my eyes a very light purple. When I look at the graph below though, it starts roughly at 390nm but the graph shows it as a dark blue. Shouldn't it show violet in at least some amount?

View attachment 354436

There is a very informative Wikipedia page covering LEDs.
Link for Main LED entry: https://en.wikipedia.org/wiki/Light-emitting_diode#

Link for White LED section: https://en.wikipedia.org/wiki/Light-emitting_diode#White_LEDs

Link for Phosphor-based White LED section:
https://en.wikipedia.org/wiki/Light-emitting_diode#Phosphor-based_LEDs

Added in Edit:
The following figure is from the above Wikipedia page

The following information for figure comes from Wikimedia Commons.
Original uploader description: Spectrum of a "white" LED showing blue light directly emitted by the GaN LED and the stokes shifted yellowish light emitted by the Ce:YAG phosphor. Spectrum taken by me (Deglr6328) using an Ocean Optics HR2000 spectrometer [http://www.oceanoptics.com/] with a high-OH solarization-resistant fiber optic light guide. This spectrum is not calibrated for intensity.
Author: The original uploader was Deglr6328 at English Wikipedia.

.

 

[Tom.G]

You might check the spectrum of the Sun to eliminate possible problems with the spectrometer.

Here is what it looks like at high resolution, the sharp dips are from absorption by various elements as labeled.

Credit: https://www.oceanopticsbook.info/view/light-and-radiometry/level-2/light-from-the-sun

Cheers,
Tom

 

[Gleb1964]

Then, a spectrometer with absolute radiometric calibration using a blackbody source at a specific temperature is required.