# Change of light wavelength

In my research, I want to make a transparent lens where there will be a microstructure on its surface.
This lens will be used to change a wavelength of an LED lighting.
Would anyone explain a physics phenomen behind this?
Thank you.

mfb
Mentor
Do you have any reason to expect a wavelength shift?
There are multiple ways to do this, but a microstructure on a surface would be new to me.

Are you thinking of the thin films used to coat lenses to reduce reflections??

@mfb : May be there is a relation to the changing of refraction index from lens (rarer medium)to the microstructure(denser medium) so that it can also change the wavelength of light.

@technician : Yes, I'd like to coat the lens using thin film(microstucture) on its surface, but the lens I used will be a TIR Lens so it can reflect the light source fully.

mfb
Mentor
@mfb : May be there is a relation to the changing of refraction index from lens (rarer medium)to the microstructure(denser medium) so that it can also change the wavelength of light.
The wavelength in the lens is different, of course, but that is not a change of the wavelength with the usual meaning (= a change in the frequency, and a different wavelength in the same medium).

sophiecentaur
Gold Member
2020 Award
A wavelength change will not affect the frequency - which is what affects the colour you see. Historically, it was wavelength that was measured but frequency is the quantity that does not change from medium to medium.

@mfb: Does it mean the output wavelength of LED light will not change?

@sophiecentaur: I also got some reference said that the frequency will be constant/not change. But the wavelength and the wave speed change from medium to other medium.

sophiecentaur
Gold Member
2020 Award
@sophiecentaur: I also got some reference said that the frequency will be constant/not change. But the wavelength and the wave speed change from medium to other medium.
Of course the wave speed is different in different media but, if you want a colour change, you have to change the frequency. The wavelength 'inside' your optical system is not relevant to the wavelength of the light that will emerge for you to see.
Are you trying to obtain a colour change? That will not be possible unless you use doppler shift with mirrors travelling at near light-speeds!

@sophiecentaur: Yes, I want a color change but not a significant change. In example I want to change a normal blue to a blue sky. Is it still possible?

mfb
Mentor
Not with any change of the refractive index: this does not change the frequency ("color"). You need frequency doubling, higher harmonic generation, wavelength shifting materials or other fancy stuff.
It would be useful to see the planned application of that to be more specific. It might be sufficient to suppress some parts of the spectrum of the LED.

berkeman
Mentor
@sophiecentaur: Yes, I want a color change but not a significant change. In example I want to change a normal blue to a blue sky. Is it still possible?
That is normally done in LED lighting by having a number of LED sources with different wavelength outputs, and varying the amplitude of the drive current to each LED to change the overall output color that people perceive. I'm not aware of a way to do this with either a single LED source, or a single source with some optical device/lens.

I'll see if I can find a reference to the multi-wavelength LED structure that is used for (some pretty dramatic) color control...

berkeman
Mentor
The quickest reference I could find was at wikipedia:

http://en.wikipedia.org/wiki/LED_lamp

wikipedia said:
The color rendering of RGB LEDs, however, is worse than one would expect; the wavelength gap between red and green is much larger than that between green and blue, resulting in an uneven spectral density. An orange fruit, for example, does reflect some red and it does reflect some green, but not in a ratio that the human retina interprets as orange. Neglecting to poll the orange line makes most orange objects appear reddish. RGB LEDs are therefore suitable for display purposes, but less so for illumination, which prompted some manufacterers to add a fourth, amber LED, marketing the product as RGBA LED (not to be confused with the RGBA color space) or tetrachromatic white LED. It can be expected that the number of colors will be further increased to six or more, equally-tempered wavelengths.

The second method, phosphor converted LEDs (pcLEDs) uses one short-wavelength LED (usually blue, sometimes ultraviolet) in combination with a phosphor which absorbs a portion of the blue light and emits a broader spectrum of white light. (The same mechanism—the Stokes shift—is used in a fluorescent lamp emitting white light from a UV-illuminated phosphor.) The major advantage is the low production cost. The CRI (color rendering index) value can range from less than 70 to over 90, and color temperatures in the range of 2700 K (matching incandescent lamps) up to 7000 K are available. The character of the light cannot be changed dynamically. The phosphor conversion absorbs some energy, but most of the electrical energy is still wasted as heat within the LED chip itself. The low cost and adequate performance makes this the most widely used LED technology for general lighting today.

sophiecentaur