[Elecraft] K3, k4 Black knobs

[David Woolley]

Red is used because the rod cells, that are used for night vision, down 
to a point where they are almost quantum limited 
<https://www.cns.nyu.edu/csh/csh04/Articles/Rieke1998.pdf>, have a peak 
sensitivity for greenish blue, but are very insensitive to very deep 
reds, whilst they retain some sensitivity to the high frequency limit of 
the blue cone cells.  Night vision doesn't have the same spectral 
response as a monochrome camera.

Light of a colour to which they are sensitive bleaches the sensor 
pigment, reducing sensitivity.

Ideally, you don't want to use the brightest red LEDs, but rather those 
with the longest wavelength consistent with an adequate remaining cone 
cell sensitivity.

See <https://midimagic.sgc-hosting.com/huvision.htm> for typical colour 
sensitivity curves.

-- 
David Woolley

On 16/10/2020 14:31, Ken B wrote:
> Why do you use red  and not blue or green
>