NY wrote
James Wilkinson Sword wrote

As far as I am aware most approaches revolve around the doping of the
part which emits light. Normally more than one led is used each giving
off a fairly narrow range of colour, and then combining them to make a
perceptually white light.

I have also seen some that refine this a bit with internal filtering,
but essentially, as far as I am aware, nobody has made a single led
element that has a sufficiently wide band output to give white on its
own.

We've just got a couple of Philips Hue bulbs which can be set to a variety
of different colours (as well as being dimmable), presumably by varying
the proportions of different coloured LEDs.

I'm in the process of changing over to Hues entirely.

It was my wife's idea and they are a bit of a gimmick. I tend to use the
one in my study mainly on fairly neutral white (ranging between warm white
and daylight)

I prefer the cool white end myself. Surprised me a bit because
I used to use PAR38 floods inside the house because I preferred
the light from those to the long tube fluoros with daylight tubes.

but my wife likes to set hers to lurid purple or red as background
lighting when she reads on her Kindle (which produces its own light, so
the colour of the room lighting doesn't affect the colour of the text).

Urk don't do that at all myself.

We got them partly to test the technology, controlling by mobile phone app
or Alexa voice recognition;

I did it mainly to move to complete automation of the lights
and other stuff like the electric blanket etc. Ideally I would
prefer to operate without having to ever use light switches
at all and have the system work out when someone is in a
particular location and decide if light is required there etc.

Not totally automatable probably unless I can work out some
time based rules too, because I prefer to have full light when
I am actually eating, but not when using the computer or
watching video and that's all done in the one place.

Not yet clear if time rules added to presence detection
will work. It might because I do eat at a very fixed time
for the evening meal and don't need full light for my
very minimal breakfast which is just a huge great slab
of toast, as thick as will still fit in the toaster.

if we were to get any more we'd go for much cheaper fixed-colour ones.

I did start with that starter kit but hate the very orangy color of those.

Don't mind so much in the bedroom, but hated it for the main
armchair I do most stuff from, including eating the evening meal.

Didn't like it in the kitchen either so I got some of the fully color temp
controllable ones to replace those. The fixed color temp ones will be
fine in the less often used places like the room full of brewed beer and
distilled spirits etc in massive great piles of our milk crates with the
beer mostly in full sized beer bottles we call long necks.

What is interesting is that even the whitest light, which appears slightly
blue to the eye,

Doesn't to me for some reason. With the Light strip Plus that I have
two of now in the kitchen, I can't find the white that just has the big
white leds on, the other two colored leds are on at a low level too.

I've only been driving them with the apps so far tho, not by programming.

shows on a digital camera as being a little warmer than full sunlight
(about 5000K). A "daylight" CFL alongside it (OK, I was curious so I did a
comparison!) looks slightly warmer by eye but the camera sees it as being
cooler than the LED.

This goes to show that the eye is a very poor indicator of colour
temperature, and that it has a lot of auto-
adjustment built into the eye/brain mechanism.

Of course, if the digital camera is set to auto-white-balance rather than
a preset "tungsten", "fluorescent", "sunlight", "shade", it adjusts too.

Some time I'll have to try some test photos using the same Hue bulb on
various colours and brightnesses, and see what the colour rendition of
real subjects is like in various conditions.

When I want to put stuff on one of the facebook buy sell swap
groups or ebay etc I normally just put on a table outside in the sun.

Would be more convenient to have a dedicated
table inside with a Hue or two I spose.

When I've done it in the past, using sunlight, shade, tungsten bulb, warm
white fluorescent tube, daylight CFL and daylight LED GU10, the camera's
auto-white has made them all look fairly similar in terms of overall
colour cast, but red objects tend to be a bit darker and less vibrant with
CFL and LED.

I dread to think what the spectrum of some of these bulbs is like,

Yeah, bet its really weird.

but I bet there are a lot of holes in the spectrum compared with a
black-body radiator like a tungsten bulb or the sun (ignoring very small
gaps in sunlight due to absorbtion lines of the atmosphere). I dimmed a
tungsten bulb from full brightness to barely lit, using a conventional
thyristor dimmer, and the camera's auto-white and auto-exposure made all
the test photos look pretty well identical, which is how it *ought* to be
with LEDs and CFLs.

I presume LEDs vary their brightness by varying the duty cycle of a square
wave

IMO its much more likely to be done by varying the
constant current but I don't know that for a fact. It
would be easy to check with the Lightstrip Plus using
a CRO, must dig mine out and see what they do.

(and maybe even varying its frequency too).

Dunno what the point of varying the frequency would be.

I've always wondered why this doesn't causing any banding or beating when
those lights are used as studio lights in a TV studio.

Yeah, that's the other reason for varying the current and not doing PWM
with those, That inevitably going to be a major problem with photographing
stuff, even for domestic situations rather than professional with digital
cameras.