Observing Emission Spectra from Computer Screens

[Luxdot]

Hi!

I am doing some simple observations of different light sources with a simple DIY spectroscope. When I look at a computer screen I see what I believe to be an emission spectrum due to the dark spectrum with emission lines on it. Is this correct? And why does a computer screen emit an emission spectrum?

 

[Merlin3189]

I don't know the answer, nor TBH what the question is! But as you've been asking for a week, I'll offer a few comments.

First, if something emits light, it has an emission spectrum - that's just tautology isn't it?

If you are looking at an LED screen, the emissions are from LEDs (obviously) which produce bands rather than lines.

If you are looking at a TFT screen, then, in those I've seen, the light comes from a flourescent tube. You might imagine you're onto a better bet here, because fluorescent gases do emit line spectra. But the visible light comes not directly from the gas, but from the phosphors excited by the light from the gas. These again are solids, as in LEDs, so emit band spectra.

My bet would be that you are looking at an LED screen, because the LED emission bands can be quite narrow and might possibly be seen as fuzzy lines.

Hope that's of some help and that someone more knowledgeable in the area might turn up and correct me or add to the answer.

 

[sophiecentaur]

because the LED emission bands can be quite narrow and might possibly be seen as fuzzy lines.

TV displays have conflicting requirements. They need to be as bright as possible - which is easiest to achieve with broad band 'phosphors' and to have a well defined spectrum which means a narrow spectrum (none are actual spectral colours, of course.)
Things have changed a lot from CRT phosphors, through crude LCD colour, then to LED backlit and to OLED. You can understand how modern displays look so nice.
I have a cheap and cheerful spectroscope (about £30, iirc, on eBay) and it's very convenient for this sort of question. I haven't yet managed to get a good photo of the display but I can say that (on my MacBook Pro screen) the blue end has a band about 15% of the gamut, the green band is about 20% and the red has two distinct fuzzy bands about 20%. Between those, there are two dark bands of about 20%. So the phosphors cover about 60% of the spectrum. On the OLED screen of my iPhone, the three phosphors seem to a bit narrower bands with more than 50% dark.
So about half the spectrum is dark and half is' phosphor 'spectrum (pretty continuous, aamof.