From: Peter Meilstrup <peter.meilstrup(AT)>
Date: Wed, 16 Feb 2011 16:58:17 -0800
Subject: Re: [BOB] One Watt Tail Lights Compared

On Wed, Feb 16, 2011 at 3:07 PM, William deRosset <wmderosset(AT)>wrote:

> >Archive-URL:
> >From: Peter Meilstrup <peter.meilstrup(AT)>
> >Date: Wed, 16 Feb 2011 10:13:18 -0800
> >Subject: Re: [BOB] One Watt Tail Lights Compared
> >suffice it to
> >say that I don't think pumping ever more lumens out the same size aperture
> >aimed directly at driver's eyes is actually a great strategy for
> visibility.
> Dear Peter,
> Okay, I'll bite.  Why not?
> >I could go on to talk about contrast normalization, the nonclassical
> >surround, Weber's law, the black-dominant response in V1, crowding and
> other
> >visual/neural phenomena I deal with when I'm not on the bike;
> Feel free to bring in your technical expertise.  Understanding
> conspicuity and perception helps us all out.
> Best Regards,
> Will

   "Contrast normalization" is the start of the problems. Patterns of light
impinge in the retina and go through a couple of processing steps before
anything that would count as "seeing" begins to happen. Contrast
normalization means that responses of cells in primary visual cortex appear
to be scaled to a range determined by the activity of neighboring cells. So
how much of a signal a light generates in the brain does not depend on the
brightness of the light, but more on its brightness as compared to its
immediate surroundings. Increasing the brightness of a point that is already
the most conspicuous thing in its immediate vicinity does not make it more
conspicuous, but it does damp down the response to other things nearby. With
a very bright light, you see the light, surrounded by a region in which you
are not very good at detecting anything. And this region gets larger the
brighter the light is.

"Crowding" was originally discovered in studies of reading; it's in large
part the subject of my research, so I'm going to be probably too verbose
about it. Say you put a letter in the periphery of your vision and make it
large enough to read. You can do this fine, but if you place two three
letters of the same size next to each other in the same location, you can no
longer tell what the letters are; the corners and edges belonging to the
individual letters can't get sorted out. There is what Dennis Pelli calls an
"integration field" (of which the physiological mechanism is unclear, but it
probably works after contrast normalization) which has a characteristic
size, in terms of visual angle, and it's within the integration fields that
simple image features (like corners and edges of letters) are combined into
larger groupings (like the letters themselves), and if equally salient
stimuli are put into the same integration field things tend to get jumbled.

The integration fields are sized about proportionate to their distance away
from the center of fixation, so you deal with cluttered scenes by turning
your eyes, moving the center of gaze (where integration fields are smaller)
towards different locations in turn, which is time consuming.

Take for instance looking at a clock: you can probably focus on the center
of an analog clock face and read any of the numbers around the periphery.
That's about how big the integration fields are: if the numbers were packed
closer together clocks would be slower to read. (So 24 hour dials have never
caught on.)

Now it turns out that crowding is a very general phenomenon, and crops up
any time you have to combine or compare simple visual features -- either for
recognition, or for judging the relations between separate objects; it
applies to motion as well as shape, and in particular to relative judgements
of position and motion.

So far I've described how crowding hurts you if objects are packed too close
together in peripheral vision, but it cuts the other way as well: if image
features that _need_ to be combined, compared, or put in relation to make
sense of a scene are separated by more than that critical distance, then you
can't really put them together into a coherent whole.

Take a bright light on a bike. What do you need if you are driving a car and
need to steer around a bike? You need to simply detect it, see that it's
there, but that's the easy part. You also need to see its position (in
relation to the edge of the road/lane); which direction it's moving (in
respect to the background scene); how wide it is (rider's shoulder, in
relation to your car) and so on. All of these relative spatial judgements
require not only detecting the light but also detecting other things and
judging their relation -- a job that has to be done inside the space of one
integration field.

So here's the entire problem: put contrast normalization together with the
integration fields, and you see that these relative spatial judgments become
very problematic if, due to extreme luminous intensity of the light,
everything within the integration field other than the light is
contrast-normalized away. There's nothing detectable left around the light
for drivers to make any spatial judgements about your bike with.

What are some ways to increase conspicuity or range of visibility without
invoking contrast normalization? One easy answer is to emit the light from a
larger surface area so that luminance does not have to be so high. Now on a
bike, there's a problem in that there's simply not a lot of surface area
available to display your light from. But given an excess number of lumens
like LEDs and big batteries give you nowadays, you can take a different tact
-- blast your light down and out, onto the road surface and onto surrounding
street furniture, rather than directly into drivers' eyes. This has the
effect of making your light occupy a larger space, while also illuminating
the objects in your immediate vicinity that drivers also need in order to
accurately judge your position.

You'll notice that this downward, illuminating-the-ground angle is how
Dinotte, makers of hyperpowered taillights, originally intended. They
provide a mount that is 90 degrees to the seat post, not level with the
ground. Well, they used to. Now it's an adjustable angle mount and 99% of
people will point it straight back. Argh!

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