Ransen's Lighting And Color Blog

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 wi ll help you: 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 wo...

Although the formula looks very complex, scarily complex basically it is actually quite simple, taken a step at a time. First a diagram: Very roughly it is the luminance from the lamps divided by the background luminance from the room's walls and ceiling.  (For an explanation of what luminance means , this book... ...wi ll help you) Have a look at this detailed explanation of the formula for UGR: Log is log10 by the way. Lb, the background luminance or cd/m2rd, L is the luminance of the luminaire.  Looking at L and Lb, glare increases with stronger lamps and lower background lighting, whereas it decreases with weaker lamps and more background illumination.  p is the Guth index, which increases with distance from direct line of sight. So as the lamp moves further from the line of site, p increases and so the glare, as measured by the UGR, decreases. Som...

VH lighting diagrams are cartesian (compare with polar diagrams). They are normally used for floodlights. They show a "hemisphere of intensities", it is assumed that no light is projected out backwards. In the real world "out backwards" would be towards the sky or the spectators of a football match. The V and H angles are explained graphically here: And here is an example: In the icon top left the flat side of the hemisphere is the one which emits light. The diagram above will be easier to understand if you also look at this annotated photometric solid: Just to explain a bit more, imagine this luminaire is at the end of a playing field, above the goal, and imagine it is pointing into the center of the field: As the V angle increases you are pointing further and further away from the goal, and closer to the center of the field. You could imagine that...

If you are working in the lighting industry sooner or later you will come across photometric diagrams and you must know how to interpret them. This blog entry page is quick introduction on how to look at a photometric diagram and get important information from it. Often photometric diagrams use the C-Gamma system. Gamma=0 points downwards towards the floor or road. Gamma=180 points upwards to the ceiling or sky. Here is a C-Gamma diagram with some of the luminous intensity “rays of light” left in: The "rays" make the diagram more confusing than it needs to be and photometric diagrams always leave out those “rays” to give you a simpler diagram as shown below: The point to remember is that the distance from the center of the diagram to one of the points on the “outline” corresponds to a luminous intensity value, often in candelas, in the given direction. These diagrams ...

You cannot assume that a LED light source emits the "same color light" in all directions. Some do and some don't. And then what does "same color light" mean? Some answers are given in IESNA document LM-79-08, with the calculation of Delta u' v'. Here are the results of measuring the spectrum of a LED light source at various angles from the horizontal: If you compare the spectrum at 90° with that at 0° you can see they are quite different, as are the u'v' values. But the u'v' values are not wildly different, so can an average be taken and then used to assign a single u'v' value to the whole light source? According to LM-79-08, within very specific limits, yes, but you must also give a number for the variation in color. Imagine you have a whole set of measurements around the light source, in all directions. In many directions there'll be little or no light at all, so they should be ignored. The samples which should b...

Photometric isocandle diagrams show what you can think of a s a " sphere of light" with isocandles plotted on its surface. They are contour maps of the intensity of the luminaire.  There are many projections of the sphere, you must use the equa l-area (also called equivalent, equiareal or authalic) projection to calculate throw and spread, which are, b y t he way, purely "graphical" calcu lations. To calculate spread: Draw a line from the C=0° G=90° point and make it tangent to the 90% isocandela contour. In the above example this happens at about 17° "East" and 62° "South" The angle at which it hits the edge of the "sphere" is the spread. It is the red diagonal line in the image above, so the spread is 27.2° To calculate throw: The throw is calculated by finding the longitude of the maximum candle intensity, then drawing a "vertical...

In floodlight VH (large area) photometries the field angle is the opening in degrees at 10% of the maximum intensity. The beam angle is the opening in degrees width at 50% of the maximum intensity. So the field angle is wider than the beam angle, it requires less intensity. As a mnemonic remember that a field (of corn for example) is much bigger than a beam of wood (in a barn on the same farm). Well. Hey. That is how I remember it! Here is an example (from OxyTech's PhotoView program):  Here's a graphical explanation of where those numbers come from (screenshots taken from OxyTech's photometric program PhotoView): In the above image the calculation for beam and field angles for H (=horizontal) has been made explicit (blue arrowed lines). See if you can roughly confirm the beam and field angles for V (in red) from the photometric polar diagram. ...