Saturday, April 22, 2017

Getting tired of the claims for LED lighting (in this case Sunmatic)?

Is anybody else getting tired of the claims of LED light bulb manufacturers? Sunmatic claim that this light bulb will last 25000 hours... actually lasted less than 800, that is less than 4% of the claim. Well done SUNMATIC.

Or should that be be GPBM Nordic?

Sunmatic GPBM Nordic were wrong by 96% on the lifetime of their bulb,  which if applied to the other numbers would mean:
  1.  3.2% energy saving (not 80%)
  2. 21 lumens (not 650)
  3. Something horribly wrong with the Kelvin figure.
(I started actually measuring a bit more carefully the life of lightbulbs when several Philips light bulbs failed well before their claimed life.)

Thursday, January 26, 2017

The OXL photometric file format, with spectrum, photos, and more...

The OXL file format is an XML based format which contains the photometry (like an IESNA .IES or an Eulumdat .LDT) file format, but adds much much more. The OXL file format is open and free to use by any company. Here's an overview of what an OXL file can contain:

As you can see there must be a photometry, and to that you can any or none of the following things:

  1. PDF data sheet.
  2. Photo.
  3. Spectral data.
  4. 3D model (a simplified 3D model is ideal for Revit applications).
So when a customer wants details about your luminaire, you don't need to send him N files, just a single OXL file.

Now I think examples are easier to understand than formal definitions, so here is what the start of an OXL file looks like:

Because Eulumdat and IESNA files are so constricted, some companies "expand and re-interpret" them, with non standard "extensions" which only a few programs (mostly company internal programs) understand. These extensions often force more data into the lines than they were designed for.

Eulumdata LDT has another restriction, it is only for CG (internal and road) photometries, and cannot hold VH (external floodlight) photometries.

OXL on the other hand is flexible and future proof. Since it is an XML format your programs/programmers/personel can ignore the branches it doesn't need and only pick out the data it wants. Also you can add your own special data which may be useful to your company, but can safely be ignored by extermal lighting programs.

Here is a link to an OXL photometric file which contains a photo and a spectrum.

Here is a link to 10 example OXL files in a single zip.

Here is the OxyTech web page which explains more about the OXL file format and here is a link to a (rather dated) OXL presentation.

But above all you should remember this image which illustrates what an OXL file can contain:

Wednesday, January 4, 2017

CIE88 2004, Calculation of Tunnel Lighting Transition Zone Length

This article is a bit of extra help for this one (on CIE88 2004).

We know that we need to arrive at an internal tunnel luminance Lin, and we know we start at the formula for calculating the curve is the one given above, Ltr. 

Time and distance are related by the fixed velocity, v, of the tunnel project. If we get the time taken to go through the transition zone we can easily get the length of the transition zone.

Re-arranging the original equation...

So you can calculate d, the length of the transition zone, from these three things:
  • the project velocity
  • the threshold luminance
  • the internal luminance.
(Thanks to Bui Duc Han for correcting an error in a previous version of the steps above.)

Comparisons of UNI11095 2003, CIE88 2004, UNI11095 2011 Tunnel Lighting standards

Here are the Luminance Grids for calculating Lseq. of all three standards:

In all three standards there are 9 rings and 12 (radial) sectors.

The UNI11095 2011 grid weights the upper and lower fractional sections by their reduced area. 

The CIE88 2004 has the "tallest" grid but the uppermost and lowermost areas are not used in the calculation, as they are considered to be outside a normal person's field of view. 

The UNI11095 2003 grid did not take into account the fact that the upper and lower areas are smaller, but it did have a wider aspect ratio.

UNI11095 2011 introduced a maximum curve above the minimum curve, presumably to encourage energy saving as well as luminous uniformity:

Tuesday, January 3, 2017

How To Calculate BUG Road Lighting Glare

Though BUG (IES TM-15-11, Addendum A) is quite hard to do, it is easy to understand, it gives you numbers for the "amount of light", wasted in various non-useful directions.
  • B stands for Backlight, light thrown back away from the road.
  • U stands for Uplight, light leaking skywards and causing light pollution.
  • G stands for Glare, how much light is wasted shining directly into (a relatively distant) the driver's eye.
The output of the luminaire is divided into several sub-zones:

Light issuing from High to Very High (H and VH) zones can cause glare from a distance and is anyway wasted light. These zones, both foward and back are used for the G rating. These are the red areas in the schematic below:

Light sent to the Back away from the road and towards the sidewalk (purple in the above diagram) is not useful for a car driver. This is sometimes called Trespass light. Back Low, Back Medium and Back High zones are used when calculating the B rating.

The blue areas in the image above (Upper Low and Upper High) show the zones where light is wasted skywards, allowing jet passengers to have a pretty view of the city below, but not much good to road users.

It is best to have most light in the Foward Low and Forward Medium zones, which shine light onto the road surface relatively close to the luminaire. These areas are green in the diagram above. FL and FM zones are not used in the calculation, they are not zones of wasted light.

So with the idea that it is a measure of waste, high numbers are considered bad, and low numbers are considered good:

A good mnemonic would be: "It BUGs me all this waste!".
Here is another example :

In the above example 13.7% of the light is wasted in the Uplight, hence the U5 rating. The B2 and G2 ratings are not brilliant either. 
When evaluating a BUG rating the BL and UL zones are not used, because these are considered to be useful light which shines down onto the road or sidewalk. The BL and UL zones in the above example are at the bottom of the circular diagram, from 30° to 30°.

Compare the above rating to this one:

Notice how no light is wasted into the sky, none is wasted shining light far away (the zones between 60° and 90° back and front). As a result the BUG rating is much better, B1 U1 G0.

Tuesday, December 13, 2016

Imaginary Colors

I was watching QI (a BBC program of semi-serious questions and semi-serious answers, QI = Quite Interesting) and one of the questions which came up a few weeks ago was about imaginary colors. They messed up the graphics quite badly like this:

So colors between deep blue and purple-red are not supposed to exist. QI did not explain why. I've seen in some textbooks that these colors are described as mysterious or anomolous. In the textbooks  this diagram is used...

...which at least gives a bit more "explanation" of why they should not be visible. According to some people, since they are on that strange lower edge, and not on the "spectral edge" they are therefore non-existent colors.

(I found the reasoning to have the same weight as those who say "science says bumble bees should not be able to fly, but they can, so science is wrong!" It is clear to anyone with half a brain that bumble bees are not shaped like aircraft, are lighter and less dense. The science which explains bumble bee flight is going to be different to the science which explains the flight of huge passenger jets.) 

Back to colors though. There's two things wrong with this "science says we can't see mysterious colors but we can!" "reasoning":
  1. We can see colors which are not on the spectral edge, white is a good example. In other words we can see colors which are inside the shape shown above. Light yellow is another good example. So how near to the mysterious edge does a color need to be for it to become become mysterious? Clearly an arbitrary distance.
  2. We see colors because our eyes/brain (during daylight) mixes singals from three sensors (red green and blue). Our eyes don't even "know" about that mysterious edge. The diagram above is useful but it is not what goes on in the brain.

Tuesday, November 22, 2016

Explanation of the CIE88 2004 Tunnel Lighting Standard

The standard is officially called "Guide for the Lighting of Road Tunnels and Underpasses" and this article is about the 2004 version, which is currently in use. It replaces the 1990 version. 

This article is a user friendly summary of the standard and ignores some details, for example daylight screens and emergency lighting.

I'll just be looking mainly at the requirements for lighting tunnels in the daytime, a large part of which concerns the effect of the luminance of the areas surrounding the tunnel entrance.

For an explanation of what luminance means, this book will help you:

Buy Candelas Lumens and Lux as a paperback

You may have noticed that as you approach a long tunnel in bright sunlight the entrance often seems like a black hole: 

In these cases, if there was an obstacle (like a stopped car or a drunk pedestrian) just a few meters into the tunnel you would not know it until it you heard the thump!

(Note that you need to know the difference between luminous intensity, illumination and luminance to understand this article.)

The problem with tunnel lighting design is that tunnels have many variables which affect their safety. Some of the most important are:
  • The age of the driver.
  • The dryness or wetness of the road.
  • The speed of the automobile.
  • The external light conditions.
  • Atmospheric conditions (fog being an extreme example)
  • Traffic density.
  • Maintenance of the tunnel lighting.
  • The inclination of the tunnel.

It is neccessary that the driver sees into the tunnel entrance from the outside so he or she is justifiably confident about entering the tunnel. Of course the other concern is that once inside the tunnel they can understand the geometry of the tunnels easily. i.e they can see the walls and know that they are in the correct lane!

Generally lighting a tunnel at night is easier than lighting a tunnel during the day. If it is dark outside, then the change from driving outside a tunnel and driving inside a tunnel is smaller.

During daylight hours the problem is that while the eye can adapt from light (outside the tunnel) to dark (inside the tunnel), it cannot adapt very quickly. A slow moving car gives the eye more time to adapt than a fast moving car. That's one reason not to exceed the speed limit when entering a tunnel!

The obstacle

The CIE88 2004 standard has taken the position that it is impossible to define a level of visibility for all sorts of objects you might find inside a tunnel. Some tunnels contain only cars, some tunnels contain pedestrians and cyclists. Some cyclists don't switch on their lights! So the standard takes a "standard object" which must be visible at the entrance to the tunnel. This standard object is is a 0.2m x 0.2m square having a reflectance of 0.2: 

The object or obstacle is placed at the entrance to the tunnel, standing on the road surface, and the "stopping distance" is such that when the driver sees the object he must be able to stop in time without crashing into it.

Long and short tunnels

There are officially defined long and short tunnels, and in general short tunnels are those where the driver can easily see the exit of the tunnel from the entrance. This means that some "short in length" tunnels are treated as long ones when the short tunnel bends and the driver cannot see the exit from the entrance.

So there are three cases
  1. Geometrically long tunnels.
  2. Optically long tunnels (they may be geometrically short, but they're bent).
  3. Short tunnels (short and straight)
The sort of lighting level which needs to be applied is determined by the decision graph shown below. You start at the top answering the questions as you go downwards until you come to one of the three answers:

(I often wonder what we are expected to do if the answer to question 4 is 0.3, i.e. medium wall reflectance.)

The threshold zone lighting level is the lighting level at the start of the tunnel.

Explanation of terms used in CIE88 2004 standard.

Design speed: The speed for which the tunnel is laid out. It is often the same as the maximum speed allowed just outside the tunnel.

Reference point: A point in the center of the approaching lanes 1.5m above the road surface and outside the tunnel at the stopping distance away.

Stopping distance: The distance neccessary to safely stop the vehicle moving at the design speed. It is composed of the distance taken for the reaction (of the driver) time and the distance taken for the braking time.

Vertical Luminance, Ev: Vertical luminance is simply the luminance of a vertical plane, the normal to the plane is horizontal.

Contrast Revealing Coefficient, qc : The ratio between the luminance of the road surface and the vertical luminance at a given position: qc = Lr/Ev , illustrated here:

Symmetric Lighting: When the luminaire throws light equally backwards against the traffic flow and forwards with the traffic flow. In the example below the photometric solid has "wings" which are symmetrical along the traffic flow.

Counter-beam Lighting (CBL): When the luminaire throws light "backwards" into the flow of the traffic. 

Pro-beam Lighting: When the luminaire throws light along the flow of the traffic. 

The luminance curve for tunnels

A very important graph shows how the the luminance should change as the car moves into, through and out of the tunnel:

In general for long tunnels the interior zone is much longer than shown above. I've contracted the interior zone so the interesting entrance and exit luminances are show clearer. Remember that the above graph is a graph of luminace against distance.
Let's look at a more detailed version of the first half:

Lseq, Lth, Ltr and Lin are all luminances, hence the "L". They are explained in more detail below. Luminance is roughly the apparent brightness, what the eye percieves, not to be confused with illumination or luminous intensity.

Note that the Access Zone and the Stopping Distance (SD) are the same. The access zone is the section of road before the tunnel entrance, starting outside the tunnel, at the stopping distance from the tunnel entrance. So Lseq is the luminance in that section of "open" road. Notice that luminance falls once we get near the tunnel, because the tunnel mouth will start to dominate the visual field. This is shown graphically here:

Consider the three images, as the tunnel gets closer the "average brightness" percieved by the eye goes down. However well lit, in the daytime, the tunnel always has a luminance lower than the external environment.

Once the car is inside the tunnel it is in the "Threshold Zone", called this because the car is on the threshold between external road and the tunnel proper. As shown above the length of the threshold zone should be at least the stopping distance (SD).

Right after the Threshold Zone is the Transition Zone, where the luminance will fall to a (more or less) fixed value which most of the tunnel will have.

The Interior Zone has the fixed luminance value which will last until the car gets to the Exit Zone.  

The Exit Zone is often where external daylight illuminates the last part of the tunnel, and where the driver sees the external, brighter, landscape dominate his or her visual field.
The values Lth etc are generally taken to be minimum, and tunnel lighting should be at these minimums of above them.

Percieved contrast.

The percieved contrast is defined like this:

So it is the relationship between the luminance of the object and the luminance of the road. Obviously we'd like them to be different, if they are the same the contrast is 0! Remember that the object is a 0.2m square with reflectance (rho) of 0.2.

Now Lop and Lrp (used in the equation above) are defined as a sum of other luminances passing through mediums of varying transparency (transmittance). The windshield for example will reduce the luminance of the object because it does not transmit all the light which hits it. And the atmosphere too is not completely transparent. 

Lop and Lrp are calculated like this:

For example the atmosphere between you and the obstacle has a luminance (very small usually, unless you are in brightly lit fog) and this lumiinance is attenuated by tws, the transmittance of the windscreen.

Lseq is important. It is called the Equivalent Veiling Luminance. When light enters your eyeball it bounces around and gives a veil of light over the ordinary clean image. Lseq is considered to come from all the objects around a 2° cone of vision. The driver should be concentrating on that 2° cone, but the veiling luminance will reduce the contrast of what he sees.

It is not explicitly stated in the standard but I assume that the 2° cone of vision goes to the high resolution part of the retina. Other parts of the retina are medium or low resolution.

Compare perceived contrast with intrinsic contrast. The latter is the contrast when the you are very close to the object, in other words when there is no atmospheric or glare effects. Percieved contrast is different from intrinsic contrast because you are far from the object and light from other sources enters your eye, and the atmosphere between you and the object also reduced the contrast.

Lighting in the threshold zone.

You must be able to see other road uses in the dark threshold zone while you are driving outside the tunnel and are at the stopping distance away from the tunnel entrance. Obviously we are trying to avoid the "black hole" effect. Mathematically the percieved contrast should be at, or higher than, a given minimum.

Lth is the luminance in the first part of the tunnel, and is the horizontal section of the threshold zone (after the tunnel entrance in the graphs above). Lth is calculated like this:

Cm is the minimum percieved contrast required percieved contrast required. Rho is the reflectance of the obstacle (often set at 0.2) and qc is the contrast revealing coefficient. Generally all these numbers are given to us, except for Lseq...
So Lseq is the luminance created inside the 2° cone by light outside of the 2° cone. So this surrounding light veils what you are looking at, reducing the contrast.
How is Lseq calculated? You can either actually go to the tunnel and measure it with appropriate instruments, or use a graphical method explained below.
A polar grid is superimposed on the view of the tunnel entrance and its surroundings. Here is the grid :

You can understand it better if you see it over a photo:

The 2° cone is shown by the inner circle with the X in the middle. Inside your eye light from the other sectors invade that inner disk (on the retina of your eye) and reduces visibility there. The grid helps us get an idea of the luminance surrounding the cone.

Each area has been calculated to have the same influence on the 2° cone as all the others. Larger areas at the edge of vision have the same effect as smaller areas near the center of vision, given the same luminance in both areas.
The image above is actually a screenshot from a program which will sum the areas in the correct portions for you, giving you a value for Lseq. The program is LITESTAR 4D Tunnel Plus from OxyTech. Here is a fuller screenshot:

(For different standards there are slightly different radial grids, click here for a comparison of UNI11095 2003, CIE88 2004 and UNI11095 2011)

The standard requires that each quadrilateral is assigned a percentage of Sky, Road, Rocks, Building, Snow, Vegetation and Tunnel mouth. And each type of area is assigned a luminance.

Different areas occupy different amounts of the "quadrilaterals". Here is an example:

For example Sky is 8 kcd/squ-m in the example shown below. The luminances change with season and hour of course.

Lseq is a weighted sum of all the quadrilateral areas. The weights are the percentages of area type (Sky, Vegetation etc.) present in the area.

Back to that horrid looking formula:

We calculate Lseq with the grid, and all the other values in the equation are known to us. The threshold zone's constant luminance of Lth should last half the stopping distance, and then fall linearly to 40% of Lth. At which point we move into the transition zone...

The transition zone length and luminance.

The transition zone is the last zone before we hit the internal lighting zone of the tunnel. Here is a closeup of the move from threshold zone to transition zone. 

In the transition zone a new formula takes over, as shown above. The numbers are arranged so that at t=0 (0 meters into the transition zone) Ltr is almost exactly 0.4, thus taking over from the 40% linear fall in the second half of the threshold zone.

How do we calculate the transition zone length? Contact me for a detailed explanation.
How do we calculate the stopping distance? Contact me for a detailed explanation.