Hi again,
Thank you to everyone who has replied to my inquiry so far.
The comments have been useful and interesting but Michael is right, what
I am mostly asking about is the limit for how bright my dial face can
be. Here's what I've discovered since I posted the question - it has
been cloudy here nearly all day so I got some time with my friend Google.
I learned that the everyday term glare is is also used in a technical
sense with the same meaning - a source of light (either an area or a
point) that is so bright it is unpleasant to look at. Apparently it
comes in two varieties: discomfort glare (unpleasant but not dangerous)
and disability glare (injury causing, such as the glare experienced if
one were to look directly at the midday sun or a powerful laser). It's
fairly easy to find material on the internet relating to disability
glare, but it doesn't seem relevant to my question. It is also fairly
easy to find material relating to discomfort glare. Unfortunately what I
found is generally related to the correct lighting levels for indoor
workplaces, and the consequences for workplace health and safety. I
haven't found anything about glare caused by a bright outdoors surface.
The stuff on indoor workplace glare suggests that glare becomes a
problem when there is a very bright area in one part of our visual
field, with much dimmer areas around it - for example a window on a
bright sky bringing light into an inadequately illuminated office. The
solution is to add more lighting to the office so that the internal
surfaces are as bright as the window. So, I have found lots of stuff on
minimum levels of background luminance (or brightness), but not much on
maximum levels for objects being observed.
I did a bit of experimentation in the few cloudless minutes that we had
in Vancouver today. I played around with paint sample cards for various
reds, blues, greens, browns and greys, ranging from very dark to very
pale, almost white.
Michael was right: much to my surprise, even a near-black matte surface
will catch a shadow! On the other hand, it needs direct sunlight. The
shadow on my darkest cards becomes very difficult to make out if the sun
is covered by thin cloud or haze - a situation when the pale cards will
still easily show a shadow (albeit slightly blurred).
My brief testing so far suggests that hue (what we ordinarily call
colour) and saturation (colour versus grayness) are unimportant to how
well a shadow forms, whereas luminance (the amount of incoming light
that is radiated back towards the observer) and specularity (matte
versus mirrored) seem more important. The issue is that luminous or
specular surfaces cause glare.
In practice I judged that for each of my sets of cards, only the darkest
ones would not work as a dial face in practice. I also judged that the
lightest ones would be too bright. Each colour set is 8 cards, so that
leaves 3 or 4 cards in the middle that all looked as if they work work
out fine.
I was doing my testing at about 9:30 in the morning in the late winter,
so the sun was relatively low in the sky. Perhaps those middle cards
would be too bright in the midday summer sun - I got no feeling for
that. As well, the cards are only 4 cm square. My dial would be a
hundred times the area. I don't know if that would affect it.
That's all for now. I'll keep you posted as I discover more.
Steve
On 2017-02-25 6:25 PM, Michael Ossipoff wrote:
It seems to me that Steve's question has been mostly disregarded
rather than answered.
Not having experience with translucent dial-faces, I didn't know about
their lack of accuracy, and I certainly can't disagree with what two
people have said about that.
It means that the advantage of a translucent dial, for omnidirectional
reading, comes with a disadvantage of less precise accuracy.
But of course a high-mounted dial intended for relatively distant
reading might not be as concerned with fine accuracy as with
omnidirectional viewing. And so translucent dials for all-directions
viewing certainly aren't ruled-out.
Steve's main question was about the choice of dial-face hue,
saturation and brilliance, for easy and safe dial-reading. It seems to
me that Steve's question has been mostly disregarded and discounted
rather than answered.
I lied.
I said that I can't speak from experience on that matter.
But my experience with a few paper-on-cardboard tablet-dials is
sufficient to say this:
From my experience, I can say that you definitely don't want a white
dial-face.
As I said, my first dial had a white dial-face. After that, I switched
to brown, which was a big improvement in usability.
I suggest brown instead of white.
Someone implied that, the more contrast (between light and shadow),
the better. Not so, when the dial-face is too white to look at in
bright sunlight.
As for gray: Gray reflects the visible wavelengths in a relatively
equal mix, resulting in no perceived hue. If some hues are (at least
relatively) to be avoided, then obviously gray isn't what you want.
At each end of the visible spectrum, there is, of course, radiation
that isn't visible.
Infrared (IR) and ultraviolet (UV).
One possible disadvantage of that is that, when you don't perceive it
or its intensity, then of course you could conceivably get a dangerous
amount (accutely or cumulatively) without any perception of it.
For example, never look at the sun when, due to a haze, or due to the
sun being low in the sky, the sun doesn't look bright. You don't have
any perception of how ingtense the UV or IR is. It could burn your eye
without any feeling of discomfort. (I don't know which of those is
more dangerous, but there have been official warnings to never look at
the sun when it seems less bright due to haze or low altitude.)
Aside from that, there's been evidence that, when people spend a lot
of time outdoors, in bright sunny climate, then many years of exposure
to the bright blue light can cause some long-term cumulative damage.
So maybe blue isn't the most desirable hue.
Yellow, beings the complement of blue, looks yellow because it absorbs
blue, removes blue from the light that it reflects.
Also, yellow isn't particularly close to either end of the visible
spectrum.
Brown is defined as:
"Any of a group of colors between red and yellow in hue, of medium to
low brilliance, and of moderate to low saturation."
Then, dark brown would be brown with particularly low brilliance--a
desirable attribute for a sundial-face. Might that be the best color
for a dial-face?
Tan is defined as:
"Light yellowish brown."
...suggesting more brilliance than brown (but surely a lot less than
white), and enough saturation to be perceived as yellow, which seems a
good thing.
Brown, especially dark brown, or maybe tan, sound like acceptable
colors for a dial-face.
By the way, beige is defined as:
"A variable color averaging light grayish yellowish brown."
Sounds like tan, but with distinct grayness, lower saturation, making
it probably less desirable.
In my previous post I said that I bought brown construction-paper, but
didn't use it, and, instead, just marked the hour-lines on the
corrugated cardboard instead of using paper. Actually, I probably did
use the brown construction-paper. It looks better of course, and it
allowed me to conveniently use a carbon-paper template that I'd
prepared for drawing the hour-lines.
Maybe the plain cardboard dial-face would have easier construction in
one way, and less easy construction in another way. Maybe I tried one
all-cardboard dial. It was a long time ago.
Michael Ossipoff
On Fri, Feb 24, 2017 at 1:04 PM, Steve Lelievre
<steve.lelievre.can...@gmail.com
<mailto:steve.lelievre.can...@gmail.com>> wrote:
Fellow sundiallers,
I’m planning to make my next sundial from outdoor grade UV
resistant plastic sheeting. These come in a range of colours and I
want to choose one that works well for a sundial. Assuming I get
the material grit-blasted or somehow treated so that it not shiny,
and leaving aesthetic considerations aside, what light-related
attributes should I be looking for?
As anyone who has played with paper sundials knows, a white
surface is hard to look at in full sun, even if non-shiny; black
would not show any shadow. I need something in between: light
enough to catch a shadow, but dark enough to avoid glare in full
sun. I assume that latitude has a bearing on this, as the midday
sun illuminates more strongly as we approach the equator. In my
case, the design latitude is around 45 N. My dial will be about
25cm in diameter.
Are there any conventions or empirical guidelines, or even
practical experience, to help me choose?
Which properties matter? I quick read of Wikipedia suggests
colours seem to involve hue, saturation or luminosity (or parallel
concepts in other classifications).
Cheers,
Steve
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