On Wed, 26 Apr 2017 13:45:18 +0100, NY wrote:

"Dave Plowman (News)" wrote in message
...
In article ,
NY wrote:
What is interesting is that even the whitest light, which appears
slightly blue to the eye, 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.

Colour temperature to the eye is subjective. The real problem with many of
this sort of light source is they ain't continuous or smooth over the
visible light spectrum. Which can make colours - like paint - appear a
different colour (or shade) than in daylight, or halogen.

This may not matter much in a domestic living setting, but certainly can
in a workshop, etc. Or even a kitchen.

Or if you are viewing certain precious stones which change colour
dramatically under sunlight or tungsten or one of the many CFL or LED
lights.

The other thing you have to be careful of is pulsed lights with very short
persistence. I first learned this from my grandpa who had a lathe for making
model engineering models. He had his garage workshop illuminated with
fluorescent strip lights but he had a tungsten bulb that he could shine on
the work. He showed me how important that light was by running the lathe a
certain speeds which were an exact multiple of the mains frequency. Under
the fluorescents along, the work appeared to be stationary (and therefore
safe to touch); under the tungsten light you could see enough blur to make
it obvious that it was spinning and therefore dangerous to touch. And he was
catering for that moment of inattention; normally when you have your brain
devoted to the task, it's blindingly obvious that if you can hear the motor,
the chuck is spinning.

I gather that in situations where pulsed light (eg fluorescent or LED) is
used as the only light in engineering works, they have circuitry which
throws in occasional "extra random heartbeats" into the mains-fed lights,
which is enough to give some blur or jitter on the work in the lathe to make
it clear that it is spinning, even though dead-regular mains at 50 Hz would
freeze it stroboscopically. I heard of this when someone was filming a video
in an engineering works and got all sorts of flicker even though the camera
was set to a flicker-free 50 Hz refresh. He had to get H&S to sanction
temporarily disabling this safety feature during filming because it was
noticeable even though most of the light came from the filming lights.


My wife has an LED desktop lamp with LEDs that are supposed to give better
colour matching more like daylight or tungsten (ie with fewer peaks and
troughs). But it does have an annoying side-effect. I don't know what
frequency they pulse the LEDs at, but occasionally if you move your eyes
rapidly from one thing to another you can see a trail of sharp images,
especially if it's dimmed so there's probably more space and less mark in
each cycle of the lights. It's like you see with some car tail lights. or
with the "red/green man" signs on the pole of pedestrian lights - the large
bright lights that you see from the opposite side of the crossing are fine,
but the little telltale light on the pole beside the button has bad flicker
that is visible out of the corner of your eye.

A decent LED light doesn't pulse.

--
If you are having sex with TWO women and ONE more woman walks in, what do you have?
Divorce proceedings, most likely.